Influenza

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.

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.

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.

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).

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.

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.

• 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.

• 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.

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.

• 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.

1. Confirm the antiviral was started promptly, at the correct dose, with any needed renal adjustment correctly applied, and taken for the full prescribed duration.
2. 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.
3. If symptoms worsen after an initial period of improvement, evaluate specifically for bacterial superinfection (pneumonia, otitis media, sinusitis) and treat accordingly.
4. Confirm no interaction interfered with absorption — for example, baloxavir taken together with dairy products, multivitamins, or antacids.
5. 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.
6. 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.

• 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.

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.

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.

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.

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.