Influenza

Clinical Snapshot

Influenza is an acute viral illness of the respiratory tract caused by the influenza A or influenza B virus, with a typical Canadian season running from mid-October to the end of April.^[1] Illness ranges from mild to severe, and complications include bacterial pneumonia, worsening of underlying chronic cardiac or respiratory conditions, and hospitalization or death in high-risk individuals. Influenza infection is also associated with a significantly increased short-term risk of myocardial infarction and stroke in the days following acute infection, particularly in the first week — a clinically important reason vaccination and prompt treatment matter for patients with cardiovascular disease.^[2] An infected adult can spread influenza starting about 1 day before symptom onset and remains contagious for approximately 5 days after symptoms begin; children and immunocompromised individuals may remain contagious longer.^[1]

Influenza overlaps substantially in presentation with COVID-19, RSV, and other circulating respiratory viruses, all of which can produce "influenza-like illness" (ILI) — complicating recognition and testing decisions for both clinicians and public health surveillance. Two complementary strategies define modern influenza management: annual vaccination as the primary population-level prevention strategy, and, for a specific subset of patients defined by risk and timing, early antiviral treatment or post-exposure chemoprophylaxis.

Recognize the Condition

Influenza-like illness (ILI) case definition

Acute onset of respiratory illness with fever and cough, plus 1 or more of: sore throat, headache, anorexia, arthralgia, myalgia, or extreme exhaustion/weakness. Gastrointestinal symptoms (nausea, vomiting, diarrhea) may be present in children under 5 years of age. Fever may not be prominent in children under 5 or adults 65 and older, so its absence does not rule out influenza in these groups.

A clinically useful predictor

Among healthy adolescents and young adults not requiring hospitalization, abrupt onset of fever 38.5°C or higher together with a dry cough is a strong predictor of influenza when influenza is known to be circulating locally or the patient has recently travelled to an area of active transmission. This pattern is markedly less predictive in patients with multiple chronic conditions, older adults, and young children — use the age-specific presentations below for those groups instead.

How presentation varies by age

Young children (under 5 years)

May present with a nonspecific febrile illness, or with a respiratory illness resembling croup, bronchitis, or occasionally bronchiolitis, rather than the classic adult picture. Gastrointestinal symptoms (nausea, vomiting, diarrhea) may accompany or precede respiratory symptoms. Fever may be the dominant or only clue.

Healthy adolescents and adults

The classic pattern: abrupt-onset high fever, dry cough, and prominent myalgia/arthralgia and exhaustion, typically lasting 4-7 days.

Older adults (65 years and older)

Atypical presentation is common — extreme exhaustion/weakness with a low-grade or absent fever. Maintain a high index of suspicion in this group even without a classic febrile picture, since delayed recognition delays both antiviral treatment and complication monitoring.

Risk Factors

Factors that increase exposure or transmission risk

Close contact with a person who has confirmed or suspected influenza, including within a household.
Congregate or institutional settings — long-term care facilities, nursing homes, childcare settings, cruise ships, and other closed or relatively closed environments.
Lack of current-season vaccination.
Travel to a region with ongoing influenza transmission, including tropical regions (year-round circulation) or the Southern hemisphere during its own influenza season (April-October).
Occupational exposure, including healthcare work and direct contact with avian-influenza-infected poultry during culling operations.

Groups at high risk for influenza-related complications

Vaccination priority and antiviral-treatment eligibility both draw on this list.

Chronic health conditions (adults and children)

Cardiac or respiratory disorders, including asthma, bronchopulmonary dysplasia, and cystic fibrosis.
Diabetes mellitus or another metabolic disorder.
Cancer or another immune-compromising condition, from either the underlying disease or its treatment.
Renal disease.
Anemia or hemoglobinopathy.
Neurologic or neurodevelopmental conditions — in children: seizure disorders, febrile seizures, and isolated developmental delay; in adults: neuromuscular, neurovascular, neurodegenerative, or neurodevelopmental conditions and seizure disorders (excluding migraine and neuropsychiatric conditions without a neurologic component).
Morbid obesity (BMI ≥40 kg/m²).
Children and adolescents (6 months-18 years) requiring chronic ASA therapy.

Age and life-stage risk groups

All individuals 65 years of age and older.
All children 6-59 months of age.
All pregnant individuals, in any trimester.
Residents of nursing homes and other long-term care facilities, at any age.
Indigenous peoples, who face increased vulnerability to severe influenza outcomes linked to social inequities including housing, health-care access, and remote or isolated living locations.

People capable of transmitting influenza to those at high risk

Annual vaccination is also recommended for this group specifically to protect the high-risk individuals around them:^[3]

Health-care workers and other care providers in all practice settings, including volunteers and housekeeping staff.
Household contacts (adults and children) of high-risk individuals, including infants under 6 months and members of a household expecting a newborn during influenza season.
People who provide regular child care to children 0-59 months of age, in or out of the home.
People who provide services within closed or relatively closed settings to high-risk individuals (for example, crew on a ship).
People who provide essential community services (for example, first responders).

Red Flags & Refer

Escalate for urgent assessment or hospitalization for any of the following:

Respiratory distress — shortness of breath at rest or with minimal exertion, rapid or laboured breathing, or oxygen desaturation.
Signs suggesting bacterial pneumonia — new or worsening productive cough, pleuritic chest pain, new focal crackles, or a new area of consolidation.
Symptoms that improve and then worsen again (a biphasic pattern) — this is the classic presentation of bacterial superinfection following an initial viral illness, not a normal part of influenza's course.
Signs of dehydration — poor oral intake, markedly decreased urine output, or lethargy, especially in young children and older adults.
Altered mental status, lethargy, difficulty waking, or a new seizure — particularly concerning in children, where it may indicate influenza-associated encephalopathy.
Chest pain, palpitations, or new or worsening cardiovascular symptoms, given the known short-term rise in myocardial infarction and stroke risk after influenza infection.
Illness that is progressive, severe, or complicated regardless of the patient's previous health status, or illness serious enough to require hospitalization.
Fever in an infant under 3 months of age — this needs a standard febrile-infant workup, not a presumed-viral approach.
Cyanosis, or any other sign of hemodynamic or respiratory compromise.
Suspected influenza outbreak in a long-term care facility or other institutional setting — this needs immediate notification of the facility's infection prevention and control lead and the local public health unit, not case-by-case outpatient management.

Assessment & Diagnosis

Differentiating influenza from COVID-19, RSV, and the common cold

All 4 can present as "influenza-like illness" and co-circulate in the same season — use this pattern alongside local circulation data and, where management depends on the result, laboratory testing.

Parameter	Influenza	COVID-19	RSV	Common cold
Onset	Abrupt, over hours	Variable, often more gradual	Gradual	Gradual
Fever	Common, often high and abrupt	Common, variable severity	Variable; more common in infants and young children	Uncommon; low-grade if present
Cough	Dry, prominent	Dry or productive, prominent	Prominent; may include wheeze, especially bronchiolitis in infants	Mild, if present
Myalgia / fatigue	Prominent, often severe	Variable, can be prominent	Uncommon in isolation	Mild or absent
Sore throat	Common	Common	Less prominent	Common, often the presenting symptom
Nasal congestion / rhinorrhea	Present but not dominant	Present but not dominant	Prominent, especially in infants	Dominant symptom
Loss of smell or taste	Uncommon	Can occur	Uncommon	Uncommon
Gastrointestinal symptoms	Can occur, especially in children under 5	Can occur	Uncommon	Uncommon
Lower respiratory involvement in young children	Croup, bronchitis, occasionally bronchiolitis	Variable	Classic cause of bronchiolitis in infants	Rare
Typical course	Days to about a week, often abrupt improvement	Variable, days to weeks	Days to about a week; can be prolonged in infants	Self-limited, days
Confirmatory testing	NAAT/PCR or rapid antigen	NAAT/PCR or rapid antigen	NAAT/PCR or rapid antigen	Not routinely tested — clinical diagnosis

History

Onset timing and symptom duration — this determines whether the patient falls within, or how far outside, the 48-hour antiviral treatment window.
Symptom pattern: abrupt versus gradual onset, fever height, cough character, myalgia severity, and gastrointestinal symptoms.
Exposure and travel history, and whether influenza is known to be circulating locally.
Current-season vaccination status.
Comorbidities and pregnancy status, to determine whether the patient is in a high-risk-for-complications group.

When testing changes management

Beyond formal surveillance, widespread laboratory testing for influenza in the community is not necessary or practical for an otherwise straightforward, low-risk presentation — clinical diagnosis is sufficient to guide supportive care. Testing meaningfully changes management in these situations:^[4]

Outbreak or cluster settings — long-term care, other institutional settings, or schools — where confirming the causative virus determines whether influenza-specific chemoprophylaxis and outbreak control measures should be triggered. As a general threshold, obtain nasopharyngeal swabs when 2 or more cases of ILI occur within a 72-hour period in a long-term care facility, sampling the initial cases plus the next 3-5 individuals who present with ILI; if a nasopharyngeal swab is not possible, a combined throat-and-nares swab is a suitable alternative. Protocols vary by jurisdiction — confirm the local public health unit's specific criteria.^[5]
Hospitalized patients — obtain a nucleic acid amplification test (NAAT/PCR), the most sensitive method available, with a panel that at minimum detects influenza, RSV, and SARS-CoV-2; broader multiplex panels can be considered in severe (e.g., ICU) or immunocompromising presentations.
A patient near the antiviral-eligibility threshold — for example, an ambiguous presentation in a high-risk patient where confirming influenza (versus COVID-19 or another respiratory virus) changes whether an antiviral is started, or which isolation precautions apply.

Collect specimens within 5 days of symptom onset, preferably within 48 hours; sampling beyond 5 days can still be considered in patients with persisting or worsening symptoms regardless of age, in young children or older adults, and in immunocompromised patients. False-negative nasopharyngeal swabs can occur with progressive disease — collect a lower respiratory tract specimen in that situation. Gargle tests have not been validated for influenza.

Treatment Targets

Relieve influenza symptoms and shorten the duration of illness.
Reduce influenza-related complications — bacterial pneumonia, exacerbation of chronic cardiac or respiratory disease, and cardiovascular events.
Reduce influenza-related hospitalization and death, particularly in high-risk patients.
Offer post-exposure chemoprophylaxis to high-risk contacts and in institutional outbreak settings.
Achieve annual, appropriately timed vaccination as the primary population-level prevention strategy.
Promote antimicrobial stewardship by avoiding unnecessary antibacterial therapy for uncomplicated viral illness.

Choose Initial Treatment

Direct treatment rule

Who to treat with an antiviral

Do not routinely treat healthy children and adults with an antiviral for uncomplicated influenza — the benefit (a modest reduction in symptom duration) does not justify routine use in this group.^[4]

Treat anyone in a high-risk-for-complications group (see Risk Factors) who has laboratory-confirmed or strongly suspected influenza, and anyone 65 years of age or older when influenza is either laboratory-confirmed or strongly suspected based on symptoms plus known local circulation.

Treat regardless of symptom duration when hospitalization is required, or the illness is progressive, severe, or complicated irrespective of previous health status — start the antiviral immediately at any point in the illness.

Timing

1st line start treatment within 48 hours of symptom onset for the greatest benefit.

Beyond 48 hours still initiate treatment when the illness requires hospitalization, is progressive/severe/complicated regardless of previous health status, or the patient belongs to a high-risk-for-complications group — the 48-hour window applies to routine treatment of an otherwise uncomplicated, low-risk illness, not to these situations.^[4]

Which antiviral

1st line oseltamivir — oral neuraminidase inhibitor, the most widely used and best-studied option, approved from 1 year of age for treatment.

Alternative zanamivir — inhaled neuraminidase inhibitor, approved from 7 years of age; use when oral administration is not appropriate, or for a suspected oseltamivir-resistant virus. Avoid in patients with asthma or COPD given bronchospasm risk.

Special situation baloxavir marboxil — single-dose oral cap-dependent endonuclease inhibitor for uncomplicated influenza in patients 12 years and older weighing at least 40 kg, symptomatic no more than 48 hours. Its role remains deliberately limited in Canada: protection-from-severe-disease data are lacking, treatment-emergent resistance is a concern (particularly in immunocompromised patients), and cost/availability are less favourable than oseltamivir — consider reserving it rather than using it as a routine first-line choice.^[4]

Do not use amantadine — current circulating influenza strains are resistant, so it is no longer recommended for treatment or prophylaxis.

Do not increase the neuraminidase-inhibitor dose for severely ill patients — higher-than-standard oseltamivir dosing has not shown greater effectiveness than the standard dose across multiple studied seasons.^[4]

Chemoprophylaxis

Long-term care or institutional outbreak start antiviral chemoprophylaxis (oseltamivir preferred) in all residents not yet ill, regardless of their vaccination status, plus nonimmunized health-care providers, as soon as possible after the outbreak is identified.

Individual high-risk contact consider post-exposure chemoprophylaxis case-by-case for an unvaccinated high-risk contact, or a close contact of a severely immunocompromised individual (e.g., a transplant recipient) where transmission would carry serious consequences — consult local public health or a specialist as needed rather than prescribing routinely.

Vaccination

Everyone 6 months of age and older without a contraindication should receive annual influenza vaccination, using any age-appropriate trivalent inactivated influenza vaccine (IIV3) or, for ages 2-17, the live attenuated influenza vaccine (LAIV) as a needle-free alternative.^[3]

65 years of age and older preferentially offer a high-dose, adjuvanted, or recombinant influenza vaccine when available, given improved immunogenicity in this age group; use any age-appropriate vaccine if none of these preferred formulations is available.

Give the vaccine October to mid-November for the best-timed protection through the Canadian influenza season (mid-October to end of April) — vaccination remains worthwhile later in the season too, since influenza activity has often not yet peaked going into the winter holidays.

Symptom relief and antimicrobial stewardship

Use acetaminophen or ibuprofen for fever, headache, and myalgia in every patient regardless of antiviral eligibility.

Do not prescribe antibiotics for uncomplicated influenza — it is a viral illness. Reserve antibacterial therapy for confirmed or strongly suspected bacterial superinfection (for example, worsening after initial improvement, or a new focal pneumonia).

Non-Pharmacologic Treatment

Rest and adequate fluid intake; stay home from work or school while symptomatic, and avoid visiting hospitals, nursing homes, or high-risk individuals until acute symptoms resolve.
Respiratory hygiene — cough or sneeze into a tissue (or a sleeve if no tissue is available) and discard used tissues immediately; wash hands often, using hand sanitizer when soap and water are unavailable.
Wear a well-fitting mask when sick and around others, especially in crowded indoor public places and around people at high risk for complications.
Counsel patients using combination cough/cold products not to also take separate acetaminophen or ibuprofen without checking ingredients first — many combination products already contain one of these, and doubling up risks an overdose.
Reassure patients that antiviral medication is not a substitute for the influenza vaccine — the two strategies are complementary, not interchangeable.

Long-term care / institutional outbreak control measures

Implement once ≥2 cases of ILI occur within 72 hours, alongside the facility's infection prevention and control (IPAC) protocol and local public health guidance.

Precautions and PPE

Implement droplet precautions for all symptomatic residents.
Use appropriate personal protective equipment — gloves, gowns, medical masks, N95 or equivalent respirators, and eye protection as indicated.
Confine symptomatic residents to their rooms until acute symptoms resolve.

Staffing and environment

Enhance environmental cleaning frequency.
Where possible, symptomatic residents should be cared for only by staff who received their annual influenza vaccination, and by staff not also caring for well residents; if this is not possible, care for well residents first with strict hand hygiene between each patient.
Staff with ILI symptoms should not work until acute symptoms resolve.

Limiting spread

Limit gatherings or activities that bring residents from different areas of the facility into contact with one another.
Limit visitors when possible, and exclude any visitor with ILI symptoms.
Promote and reinforce proper hand hygiene among staff, ambulatory residents, and visitors.

Pharmacologic Treatment Options

Drug / intervention	Role	Dose / use	Important adverse effects	Teaching points
Oseltamivir Preferred	1st-line oral antiviral, treatment and post-exposure/outbreak prophylaxis	Treatment: 75 mg PO BID x 5 days (adult, ClCr >60 mL/min); dose-adjust for renal impairment. Prophylaxis: 75 mg PO once daily (ClCr >60 mL/min)	Nausea, vomiting, headache	Take with food to improve GI tolerability. Start within 48h of symptom onset when possible; still start later if hospitalized, high-risk, or severe/progressive illness.
Zanamivir Alternative	Inhaled antiviral when oral therapy is unsuitable, or for suspected oseltamivir-resistant virus	Treatment (≥7y): 10 mg (2 inhalations) BID x 5 days. Prophylaxis (≥7y): 10 mg (2 inhalations) once daily	Bronchospasm (especially with underlying respiratory disease), headache, GI upset	Avoid in asthma or COPD. Administer only via the supplied Diskhaler device — never reconstitute or use with a nebulizer.
Baloxavir marboxil Special situation	Single-dose alternative for uncomplicated influenza, symptomatic ≤48h	40 mg PO single dose (40 to under 80 kg); 80 mg PO single dose (≥80 kg). Age ≥12y and weight ≥40 kg only	Nausea, vomiting, diarrhea, headache	Avoid administering with polyvalent-cation products (dairy, multivitamins) — reduced absorption. Consider reserving given limited severe-disease data and resistance-emergence concerns.
Acetaminophen Preferred	Symptomatic relief of fever, headache, and myalgia	Per product dosing; use the lowest effective dose for the shortest duration needed	Hepatotoxicity at supratherapeutic doses	Preferred antipyretic/analgesic in pregnancy. Check patients are not also taking a combination cough/cold product containing acetaminophen.
Ibuprofen Alternative	Symptomatic relief of fever, headache, and myalgia	Per product dosing	GI upset, renal effects	Avoid after 20 weeks' gestation. ASA is contraindicated in children and adolescents with a viral illness due to Reye syndrome risk — use acetaminophen or ibuprofen instead.
Influenza vaccine, trivalent inactivated (IIV3) Preferred	Primary prevention, all individuals ≥6 months without contraindication	0.5 mL IM annually (0.25 mL an option for ages 6-35 months, though NACI recommends the full 0.5 mL dose)	Injection-site soreness, mild flu-like symptoms	For adults ≥65y, preferentially use a high-dose, adjuvanted, or recombinant formulation if available — see Drug Tables. Egg allergy, including a past severe reaction, is not a contraindication to any influenza vaccine.
Influenza vaccine, live attenuated (LAIV) Alternative	Needle-free option for ages 2-17 without a contraindication	0.2 mL intranasally (1 spray per nostril) annually	Nasal congestion, rhinorrhea; increased wheezing reported in children under 24 months given this vaccine	Contraindicated in pregnancy, immunocompromise, severe asthma or recent medically attended wheeze, and chronic ASA use under 18y.

Special Populations

Pregnancy: pregnant individuals face increased risk of influenza-related complications, greatest in the third trimester, even without other comorbidities. Vaccinate in any trimester with an inactivated or recombinant vaccine (safe and effective); do not use LAIV in pregnancy, though it may be used while breastfeeding. Maternal vaccination extends protection against laboratory-confirmed influenza to the infant through about 6 months of age. For fever or pain, acetaminophen at the lowest effective dose for the shortest duration needed is the recommended treatment in pregnancy, per the 2025 Health Canada and Society of Obstetricians and Gynaecologists of Canada position, with no conclusive evidence linking appropriate use to autism or other neurodevelopmental disorders.^[6] NSAIDs (e.g., ibuprofen) are not recommended after 20 weeks' gestation due to a potential reduction in amniotic fluid, and specifically avoid them in the third trimester given the risk of premature closure of the ductus arteriosus. For antiviral treatment, oseltamivir is recommended over zanamivir given more available safety data and more reliable systemic drug delivery — inhaled zanamivir's distribution may be affected by the gravid uterus, particularly later in pregnancy; start treatment early. There is no evidence supporting baloxavir use in pregnancy — avoid it in this population.^[4]

Breastfeeding

Both oseltamivir and zanamivir are considered compatible with breastfeeding. LAIV is compatible with breastfeeding (unlike pregnancy). Routine annual vaccination is recommended while breastfeeding.

Pediatrics

Neuraminidase inhibitors are not approved by Health Canada for routine treatment of mild influenza in infants under 1 year of age, and data in this age group are limited.
For healthy children 1 year of age and older with mild illness, antivirals are not routinely recommended.
If mild illness occurs with a risk factor for severe disease in a child ≥1 year, treat within 48 hours with oseltamivir or age-appropriate zanamivir.
For hospitalized children, or children with severe, complicated, or progressive illness, start antivirals immediately regardless of how long symptoms have been present.^[7]
Antivirals are not typically recommended for asymptomatic children with known influenza exposure; presumptive treatment is reserved for exceptional circumstances (e.g., a close contact of a severely immunocompromised household member) under specialist guidance.
ASA is contraindicated in children and adolescents with a suspected viral illness because of Reye syndrome risk.
LAIV is not recommended under 2 years of age (increased wheezing risk), or in children with the chronic conditions listed under Risk Factors, severe asthma, active wheeze, immunocompromise, chronic ASA use, or pregnancy.
Children 6 months to under 9 years receiving influenza vaccine for the first time need 2 doses at least 4 weeks apart; those with at least 1 prior dose in a previous season need only 1 dose per season thereafter.

Older adults (65 years and older)

Atypical, afebrile presentation is common, so maintain a high index of suspicion. Age-related immunosenescence reduces the immune response to standard-dose vaccine — preferentially offer a high-dose, adjuvanted, or recombinant vaccine when available. Age 65 or older alone (even without another comorbidity) is sufficient reason to consider antiviral treatment for laboratory-confirmed or strongly suspected influenza.

Renal impairment

Oseltamivir requires dose adjustment across a series of creatinine clearance strata, down to specific hemodialysis and peritoneal dialysis regimens — see Drug Tables for the full schedule.^[8] Zanamivir and baloxavir do not require renal dose adjustment (baloxavir has not been formally studied below ClCr 50 mL/min, but reduced excretion is not expected to be clinically significant).

Hepatic impairment

None of oseltamivir, zanamivir, or baloxavir has been studied in severe hepatic impairment, and no dose adjustment has been established for any of the three — use clinical judgment and caution.

Immunocompromised patients

Vaccine immune response may be reduced, but routine annual vaccination is still recommended given elevated influenza risk in this group. Treat any suspected influenza promptly regardless of severity or how long symptoms have been present. Some immunocompromised patients have prolonged viral replication with sustained or recurring ILI symptoms despite a complete antiviral course — extending oseltamivir treatment to 10 days is a reasonable, individualized option using clinical judgment; prolonged shedding despite therapy should prompt a request for antiviral resistance testing through the local diagnostic or public health laboratory.^[4] LAIV is contraindicated. Because of a theoretical risk of transmitting vaccine virus, an LAIV recipient should avoid close contact with a severely immunocompromised individual (for example, a stem-cell transplant recipient requiring isolation) for at least 2 weeks after their own LAIV dose.

Egg allergy

Egg-allergic adults and children, including those with a past severe reaction, may receive any available influenza vaccine at the full dose without prior vaccine skin testing and without any special consideration for the immunization setting.^[3]

History of Guillain-Barré syndrome

Avoid subsequent influenza vaccination in a patient who developed Guillain-Barré syndrome without another known cause within 6 weeks of a previous influenza vaccine dose; discuss individualized risk and benefit for future seasons.

Monitoring & Follow-Up

Reassess if symptoms fail to improve, or worsen, after about 72 hours of appropriate management.
Watch specifically for a biphasic pattern — improvement followed by renewed or worsening symptoms — which suggests bacterial superinfection rather than the normal course of viral illness.
In an immunocompromised patient on extended antiviral therapy, monitor the clinical course closely and pursue antiviral resistance testing if symptoms or viral shedding persist despite a complete course.
For long-term care or institutional chemoprophylaxis, continue active surveillance for new ILI cases to confirm the prophylaxis is controlling the outbreak. If laboratory-confirmed cases continue to occur more than 96 hours after chemoprophylaxis was started, consider prophylaxis failure and contact the local public health unit for guidance.^[4]
Confirm and document current-season vaccination status at every applicable visit throughout the influenza season — it is not too late to benefit from vaccination even after the season has started.
Routine test-of-cure is not needed after an uncomplicated, resolved illness.

Treatment Failure / Next Step

Confirm the antiviral was started promptly, at the correct dose, with any needed renal adjustment correctly applied, and taken for the full prescribed duration.
Reassess for an alternative or superimposed diagnosis — bacterial pneumonia, another respiratory virus, or a non-infectious cause — rather than assuming the antiviral has simply failed; obtain testing as indicated.
If symptoms worsen after an initial period of improvement, evaluate specifically for bacterial superinfection (pneumonia, otitis media, sinusitis) and treat accordingly.
Confirm no interaction interfered with absorption — for example, baloxavir taken together with dairy products, multivitamins, or antacids.
In an immunocompromised patient with prolonged or recurrent symptoms despite a complete antiviral course, request antiviral resistance testing through the local public health or diagnostic laboratory and consult infectious diseases; extending oseltamivir to 10 days may be reasonable under specialist guidance.
For a suspected long-term care or institutional outbreak with new laboratory-confirmed cases more than 96 hours after chemoprophylaxis was started, consider prophylaxis failure, request resistance testing, and contact the local public health unit.

Common Errors to Avoid

Treating a healthy, non-high-risk patient with routine antivirals for mild, uncomplicated influenza.
Withholding antiviral treatment from a high-risk, hospitalized, or severely ill patient solely because more than 48 hours have passed since symptom onset.
Increasing the neuraminidase-inhibitor dose in severe illness expecting greater benefit — not supported by current evidence.
Using amantadine — resistance is essentially universal among circulating strains.
Reconstituting or nebulizing zanamivir, or using it in a patient with asthma or COPD.
Giving baloxavir together with dairy products, multivitamins, or other polyvalent-cation products.
Prescribing ASA to a child or adolescent with suspected influenza or another viral illness — Reye syndrome risk.
Requiring a skin test or special precautions for an egg-allergic patient before giving an influenza vaccine — not necessary for any available product.
Using LAIV in a pregnant patient, an immunocompromised patient, or a child under 2 years of age.
Treating a negative rapid antigen test as ruling out influenza when management genuinely depends on the result — confirm with NAAT/PCR instead.
Prescribing antibiotics empirically for uncomplicated viral influenza.
Missing an emerging long-term care or institutional ILI cluster that should trigger outbreak testing and chemoprophylaxis, instead managing each case individually.

Clinical Decision Algorithm

Treatment of uncomplicated influenza-like illness in the community

ILI symptoms presenting in the community setting

Assess severity: hospitalization required, or illness progressive, severe, or complicated?

Yes

Treat with an antiviral immediately, regardless of time since onset; assess need for hospitalization and specialist input

No — uncomplicated illness

Assess for high-risk-for-complications status (chronic conditions, age under 5 or ≥65, pregnancy/≤4wk postpartum, immunocompromise, LTC residence, Indigenous status)

High-risk, or age ≥65y with confirmed/strongly suspected influenza

Antiviral treatment indicated — oseltamivir preferred

Within 48h of symptom onset

Start oseltamivir (or zanamivir/baloxavir per Choose Initial Treatment) now for greatest benefit

Beyond 48h

Still start treatment — high-risk status alone justifies treatment outside the 48h window

Healthy, not high-risk

Antiviral treatment not routinely recommended — supportive care: antipyretics/analgesics, fluids, rest, respiratory hygiene

Managing a suspected influenza outbreak in long-term care

2 or more cases of influenza-like illness within a 72-hour period in a long-term care or other institutional setting

Notify the facility's IPAC lead and the local public health unit immediately; obtain nasopharyngeal swabs from the initial cases and the next 3-5 ILI presentations

Implement outbreak control measures: droplet precautions, confine symptomatic residents to their room, dedicated staffing where possible, enhanced cleaning, restrict visitors and gatherings, exclude symptomatic staff

Outbreak confirmed as influenza A or B

Start antiviral chemoprophylaxis (oseltamivir preferred) in all residents not yet ill — regardless of vaccination status — plus nonimmunized staff, as soon as possible

Continue active surveillance for new ILI cases to confirm prophylaxis effectiveness

New lab-confirmed cases continuing beyond 96h after starting prophylaxis: consider prophylaxis failure — contact local public health, consider resistance testing

Outbreak due to another pathogen (RSV, SARS-CoV-2, etc.)

Manage per pathogen-specific outbreak protocol — influenza-specific chemoprophylaxis is not indicated

Drug Tables

How to use this table: Doses are adult doses unless otherwise specified and require patient-specific confirmation, including renal/hepatic adjustment. Select any drug name to open its complete Pharmasist Drug Database page.

Antivirals — neuraminidase inhibitors

Effective for treatment and prophylaxis of influenza A and B. Treatment is best started within 48 hours of symptom onset, but should still be started later if the illness requires hospitalization, is progressive/severe/complicated, or the patient is in a high-risk-for-complications group.

Drug	Dosage	Adverse effects	Comments
Oseltamivir	Treatment, adults/adolescents ≥13y, ClCr >60 mL/min: 75 mg PO BID x 5 days Treatment, ClCr 31-60 mL/min: 75 mg once daily or 30 mg PO BID x 5 days Treatment, ClCr 10-30 mL/min: 30 mg PO once daily x 5 days Treatment, ClCr under 10 mL/min (not on dialysis): Single 75 mg PO dose Treatment, low-flux hemodialysis: 30 mg PO after each dialysis session; stop after 5 days Treatment, high-flux hemodialysis: 75 mg PO after each dialysis session; stop after 5 days Treatment, CAPD dialysis: Single 30 mg PO dose once weekly Treatment, CRRT high-flux dialysis: 30 mg daily or 75 mg PO Q48H x 5 days Treatment, children 1-12y, ≥1y and ≤15kg: 30 mg PO BID x 5 days Treatment, children 15-23kg: 45 mg PO BID x 5 days Treatment, children 24-40kg: 60 mg PO BID x 5 days Treatment, children >40kg: 75 mg PO BID x 5 days Prophylaxis, adults/adolescents ≥13y, ClCr >60 mL/min: 75 mg PO once daily Prophylaxis, ClCr 31-60 mL/min: 75 mg Q48H or 30 mg PO once daily Prophylaxis, ClCr 10-30 mL/min: 30 mg PO Q48H Prophylaxis, low-flux hemodialysis: 30 mg PO after alternate dialysis sessions Prophylaxis, CAPD dialysis: 30 mg PO once weekly Prophylaxis, children 1-12y (same weight bands as treatment): Once daily at the corresponding treatment weight-band dose; duration variable — continue for the duration of the outbreak Immunocompromised, prolonged/severe illness (specialist-directed): Extending to 10 days total may be reasonable using clinical judgment	Nausea, vomiting, headache	Available as 30, 45, and 75 mg capsules and a 6 mg/mL suspension. If suspension is unavailable, capsules may be opened and mixed with a sweetened liquid such as regular or sugar-free chocolate syrup. Not approved by Health Canada for routine treatment under 1 year of age.
Zanamivir	Treatment (≥7y): 10 mg (2 inhalations) PO inhaled BID x 5 days Prophylaxis (≥7y): 10 mg (2 inhalations) inhaled once daily x 10 days (rarely used for this indication)	Headache, dizziness, GI upset, cough; bronchospasm reported, especially with underlying respiratory disease	Administer only via the supplied Diskhaler device — do not reconstitute or use for nebulization. Do not use in patients with asthma or COPD given serious bronchospasm risk. No renal dose adjustment required.

Antivirals — cap-dependent endonuclease inhibitor

Now marketed in Canada. Role remains deliberately limited given less severe-disease outcome data, treatment-emergent resistance concerns (particularly in immunocompromised patients), and cost/availability relative to oseltamivir — consider reserving rather than using as routine 1st line.

Drug	Dosage	Adverse effects	Comments
Baloxavir marboxil	Treatment, 40 to under 80kg (age ≥12y): 40 mg PO single dose Treatment, ≥80kg (age ≥12y): 80 mg PO single dose	Nausea, vomiting, diarrhea, headache	Swallow tablets whole; do not crush or split. Avoid administration with polyvalent-cation products (dairy products, multivitamins) — reduced absorption. Not studied in pregnancy — avoid. Not established in renal/hepatic impairment or below age 12 / weight 40kg.

Not routinely available in Canada: peramivir (brand Rapivab) is authorized by Health Canada for uncomplicated influenza treatment in adults ≥18y symptomatic ≤48h, but is not currently marketed — it requires Health Canada Special Access Program approval and delays in access should be expected; consider only when enteral access/absorption is unreliable or marketed antivirals are contraindicated. Intravenous zanamivir (brand Dectova) is not authorized by Health Canada at all. Amantadine is omitted from current recommendations given near-universal resistance in circulating strains.

Vaccines

Any age-appropriate vaccine may be used for individuals ≥6 months without contraindications; availability of specific formulations varies by province/territory and season. Contraindicated in anyone with a history of anaphylaxis to a previous dose or any vaccine component. Egg allergy, including a past severe reaction, is not a contraindication to any influenza vaccine and does not require prior skin testing or special precautions.

Drug	Dosage	Adverse effects	Comments
Influenza vaccine, trivalent inactivated, split-virion/subunit (IIV3)	All individuals ≥6 months: 0.5 mL IM single dose Children 6-35 months: 0.25 mL or 0.5 mL IM; NACI recommends the full 0.5 mL dose First-time recipients 6mo-under 9y: 2 doses ≥4 weeks apart in the first season vaccinated	Injection-site soreness; less commonly mild flu-like symptoms (myalgia, malaise, low-grade fever); rare oculorespiratory syndrome (bilateral red eyes plus a respiratory symptom within 24h, resolving within 48h — not an allergic reaction and not a contraindication to future vaccination, unless accompanied by lower respiratory symptoms)	Standard-dose option for the general population ≥6 months.
Influenza vaccine, high-dose / adjuvanted / recombinant (preferentially recommended ≥65y)	Adults ≥65y: 0.5 mL IM single dose	Injection-site soreness (adjuvanted formulations may have more noticeable local reactions, still considered mild); less commonly mild flu-like symptoms	Distinct, higher-immunogenicity products from the standard-dose vaccine above — not yet catalogued as separate Drug Database monographs, so no direct product link is given here; do not substitute the standard-dose vaccine link for these. Preferentially offer one of these formulations to adults ≥65y when available; recombinant formulations are produced without egg or influenza virus and may also be considered for egg-allergic patients, though this is not a requirement given current NACI egg-allergy guidance. Use any age-appropriate vaccine if none of these preferred formulations is available.
Influenza vaccine, live attenuated (LAIV)	Ages 2-59y: 0.2 mL intranasally (1 spray/0.1 mL per nostril) single dose First-time recipients 2y-under 9y: 2 doses ≥4 weeks apart in the first season vaccinated	Nasal congestion, rhinorrhea; increased wheezing reported in children under 24 months given this vaccine (not observed in older children/adults)	Needle-free option, not preferentially recommended for healthy children 2-17y but an option if injectable vaccine is unavailable or for needle-phobic patients without contraindications. Contraindicated in pregnancy, immunocompromise, severe asthma or medically attended wheezing in the past 7 days (stable asthma is not a contraindication), and chronic ASA use under 18y. LAIV recipients should avoid close contact with severely immunocompromised individuals for at least 2 weeks post-vaccination.

Analgesics / antipyretics

Use for symptomatic relief of fever, headache, and myalgia in every patient regardless of antiviral eligibility. Caution patients about concurrent use of combination cough/cold products already containing acetaminophen or ibuprofen, to avoid inadvertent overdose.

Drug	Dosage	Adverse effects	Comments
Acetaminophen	Dosing: Per product labelling	Hepatotoxicity at supratherapeutic doses	Preferred antipyretic/analgesic in pregnancy — lowest effective dose, shortest duration needed.
Ibuprofen	Dosing: Per product labelling	GI upset, renal effects	Avoid after 20 weeks' gestation (amniotic fluid reduction risk) and specifically in the third trimester (premature ductus arteriosus closure risk).
ASA (acetylsalicylic acid)	Dosing: Adult use only — not for children/adolescents with suspected influenza	GI upset, bleeding	Do not use in children or adolescents with a suspected viral illness such as influenza — Reye syndrome risk. Not an antipyretic choice for this condition in this age group under any circumstance.

References

Flu (influenza): for health professionals — Public Health Agency of Canada. Government of Canada, last reviewed 2024
Acute myocardial infarction after laboratory-confirmed influenza infection — New England Journal of Medicine (Kwong JC, Schwartz KL, Campitelli MA, et al.). N Engl J Med 2018;378(4):345-353
An Advisory Committee Statement (ACS). National Advisory Committee on Immunization (NACI): statement on seasonal influenza vaccines for 2026-2027 — Public Health Agency of Canada. Government of Canada, 2026
AMMI Canada seasonal influenza guideline update 2025-26: some questions answered — Journal of the Association of Medical Microbiology and Infectious Disease Canada (Harrison R, Reid EK, Allen UD, Papenburg J, Skowronski DM, Mubareka S, Hatchette T, Evans GA). J Assoc Med Microbiol Infect Dis Can 2026;11(1)
Guidance: infection prevention and control measures for healthcare workers in acute care and long-term care settings — Public Health Agency of Canada. Government of Canada
Acetaminophen is a recommended treatment for fever and pain during pregnancy — Health Canada / Government of Canada. Government of Canada, September 2025
The use of antiviral drugs for influenza: guidance for practitioners — Canadian Paediatric Society, Infectious Diseases and Immunization Committee (Allen UD). Canadian Paediatric Society, 2024
Use of antiviral drugs for seasonal influenza: foundation document for practitioners – update 2019 — Journal of the Association of Medical Microbiology and Infectious Disease Canada (Aoki FY, Allen UD, Mubareka S, Papenburg J, Stiver HG, Evans GA). J Assoc Med Microbiol Infect Dis Can 2019;4(2):60-82

Professional reference only. Apply clinical judgment and verify patient-specific decisions against current authoritative guidance and product information.