COVID-19

COVID-19 is the illness caused by infection with SARS-CoV-2, a coronavirus that continues to circulate as an endemic, seasonally fluctuating community respiratory virus rather than the acute pandemic-phase threat it once was.^[1] For most people it causes a self-limited upper respiratory illness that resolves with supportive care alone. Specific groups — older adults, pregnant patients, and people with significant immunocompromise or certain chronic diseases — remain at meaningfully higher risk of progression to severe disease, hospitalization, or death, and this is where pharmacist involvement has the most concrete impact: identifying eligible high-risk patients and getting them onto oral or IV antiviral therapy within a narrow days-long window from symptom onset, screening the resulting drug-interaction risk (nirmatrelvir/ritonavir is a potent CYP3A inhibitor with a long list of contraindicated and major-interaction co-medications), and keeping patients current on an annually updated vaccine program.^[2]

Typical symptoms, by system

Symptom combination and severity vary widely between patients and, taken alone, are not reliably distinguishable from other circulating respiratory viruses — see the differential comparison table at the top of Assessment & Diagnosis.^[1] Reported symptoms span multiple systems:

• Respiratory: sputum, cough (may persist for weeks after other symptoms resolve), wheeze, shortness of breath or breathlessness while walking or talking.
• Thermoregulatory: fever, chills, rigors.
• Musculoskeletal: arthralgia, myalgia.
• Gastrointestinal: abdominal pain, nausea, vomiting, diarrhea, anorexia.
• Neurologic: fatigue, malaise, confusion or "brain fog," memory loss, delirium (especially in older adults).
• Cardiac: chest pain, palpitations, or new arrhythmia.
• Ears, nose, throat: anosmia or dysgeusia (loss of smell/taste — a relatively distinctive early feature compared with most other respiratory viruses), hoarse voice, ear pain.
• Psychiatric: new or worsened anxiety, depression, insomnia.

Typical course

Symptoms usually peak around day 4 of illness and begin to improve soon after, with most patients recovering fully within 7-10 days, although cough can persist longer than other symptoms. A patient who is not improving — or is actively worsening — by day 4-5 of symptom onset needs in-person reassessment rather than continued self-management alone.

Possible complications

• Acute respiratory distress syndrome and respiratory failure.
• Septic shock.
• Coagulation disorders (thromboembolic events).
• Cardiac complications, including myocarditis and new arrhythmia.
• Multisystem inflammatory syndrome in children (MIS-C) — see below.
• Post-acute sequelae of COVID-19 (PASC, "long COVID") — see Monitoring & Follow-up.
• Death, occurring disproportionately in unvaccinated or severely high-risk patients who did not receive timely treatment.

Multisystem inflammatory syndrome in children (MIS-C)

An uncommon complication that appears 2-6 weeks after infection. Suspect it with fever lasting 3 or more days plus any combination of gastrointestinal symptoms (pain, vomiting, diarrhea), fatigue, headache, dizziness, tachycardia, tachypnea, skin rash, swelling of the lips/tongue or hands/feet, conjunctival injection, or lymphadenopathy. Severe abdominal pain, difficulty breathing, cyanosis of the lips or face, or new confusion/altered level of consciousness in a child with this picture needs immediate emergency assessment.

Risk factors for progression to severe disease — adults

Risk increases with age, comorbidity burden, and vaccination status.^[3]

• Age over 60 years (risk continues to climb with advancing age).
• Pregnancy.
• No prior COVID-19 vaccine dose, or vaccination not up to date on the current schedule.
• Chronic lung disease, including COPD, asthma, interstitial lung disease, and cystic fibrosis.
• Chronic kidney disease, including end-stage renal disease or dialysis.
• Cardiovascular disease, including prior stroke.
• Dementia or other significant neurologic disease requiring ventilatory support (e.g., Bi-PAP).
• Diabetes, particularly insulin-treated diabetes.
• Hypertension.
• Obesity (BMI ≥ 40 kg/m²).
• Active cancer or current cancer treatment.
• Immunocompromising conditions or medications — see the detailed eligibility list in Choose Initial Treatment.
• Severe intellectual or developmental disability.
• Rare blood or genetic disorders (e.g., sickle cell disease, thalassemia).

Risk factors for progression to severe disease — children

Evidence in children is more limited, but the following are reasonable indicators of elevated risk:

• Medically fragile children or those with complex medical needs.
• More than one comorbidity.
• Neurologic disorders.
• Chronic lung disease.
• Down syndrome.
• Immunocompromising conditions.

Differentiating COVID-19 from other common respiratory illnesses

Symptom overlap between COVID-19, influenza, RSV, and the common cold is substantial enough that testing — not symptom pattern — is the standard of care for confirming COVID-19 before prescribing antiviral therapy.^[1] A few features shift the differential without being diagnostic on their own:

None of these features reliably rules COVID-19 in or out — a positive test remains required before treatment decisions are made.

History

• Exact date of symptom onset — this single data point determines whether the patient still falls within the 5-day (nirmatrelvir/ritonavir) or 7-day (remdesivir) treatment window.
• Vaccination history, including date of most recent dose.
• Full current medication list — essential before considering nirmatrelvir/ritonavir given its extensive interaction profile (see Choose Initial Treatment and Drug Tables).
• Immunocompromising conditions or immunosuppressive medications.
• Comorbidities relevant to risk stratification (see Risk Factors).
• Exposure history and symptom trajectory (improving vs. worsening).

Examination

• Vital signs, including oxygen saturation — a home pulse oximeter is a reasonable objective adjunct for higher-risk patients monitoring symptoms at home.
• Respiratory examination for work of breathing, wheeze, or crackles.
• Hydration status, especially with prominent GI symptoms.

Investigations

A positive rapid antigen test (RAT) or PCR test is required before prescribing antiviral therapy; either is an acceptable microbiological diagnosis and a confirmatory PCR is not required after a positive RAT.^[4] If clinical suspicion remains high after an initial negative RAT, repeat testing 24-48 hours later increases sensitivity, since RATs are least sensitive in the first day of infection.

When testing is not needed: patients with mild, cold-like symptoms who are not being considered for antiviral therapy (e.g., because they are low-risk and clearly improving) do not need to be tested purely to confirm a COVID-19 diagnosis for symptom management purposes — supportive care is the same regardless. Baseline renal function does not need to be ordered before prescribing full-dose nirmatrelvir/ritonavir in a patient with no known renal disease; check renal function specifically in patients with known or suspected renal impairment, since it changes the dose.^[4]

• Prevent progression to severe disease, hospitalization, or death in eligible high-risk patients through antiviral therapy started within the treatment window.
• Relieve symptoms with supportive and symptomatic care in all patients.
• Limit transmission to others through respiratory hygiene and appropriate distancing from vulnerable contacts while symptomatic.
• Keep patients current on the annually updated vaccine schedule appropriate to their risk tier.
• Identify and escalate red-flag presentations promptly.
• Recognize and appropriately evaluate post-COVID-19 condition (long COVID) when symptoms persist beyond 12 weeks.

• Hand hygiene — wash hands often, or use hand sanitizer when soap and water are unavailable.
• Respiratory etiquette — cough or sneeze into a tissue (or a sleeve if no tissue is available) and dispose of used tissues immediately.
• Masking while symptomatic and unable to stay away from others, when caring for someone who is ill, when required in a specific setting, or as personal choice. Children under 2 years of age should not wear masks, visors, eye protection, or other face coverings, due to the risk of asphyxiation.
• Stay home from work or school while symptomatic, and avoid visiting hospitals, long-term care facilities, or other individuals at high risk of complications until symptoms are clearly improving — return to usual activity is guided by symptom resolution rather than a fixed isolation period.
• Rest and adequate fluid intake for symptomatic relief.
• Saline nasal irrigation (1-2 sprays each nostril, up to 4 times daily for the duration of illness) is a reasonable option for patients who want a product for symptom relief — low cost, low harm, and some evidence of symptom benefit.
• Home pulse oximetry as an objective adjunct for higher-risk patients tracking their trajectory, rather than as a routine measure for everyone.
• Vitamin, antioxidant, and probiotic supplements have no demonstrated benefit specifically for COVID-19 and should not be recommended as a treatment.

Pregnancy

Pregnancy is a risk factor for more severe COVID-19 compared with non-pregnant patients of the same age, and vaccination rates in this population tend to lag age-matched cohorts.^[4] Most infants born to patients with COVID-19 are born healthy at term, though there is an increased risk of preterm delivery, low birth weight, and NICU admission; vertical transmission is rare. Nirmatrelvir/ritonavir has no human pregnancy data — use only if potential benefit outweighs potential risk to the fetus, with individualized specialist input; low concentrations of nirmatrelvir cross into breast milk but ritonavir transfers more, so it is not recommended during breastfeeding unless benefit outweighs risk.^[6] Remdesivir has no formal pregnancy data either, but has been used in pregnant patients in independent series without a signal of harm; it has poor oral absorption and very low breast milk transfer, so breastfeeding does not need to be avoided during treatment, with careful infant monitoring.^[7] Patients on nirmatrelvir/ritonavir who use hormonal contraception should use a backup method during and for a short period after treatment, since ritonavir lowers plasma estrogen levels and can reduce contraceptive efficacy.^[4]

Breastfeeding

COVID-19 is unlikely to be transmitted via breast milk. Patients with suspected or confirmed infection should continue breastfeeding if able, taking precautions: wash hands before holding the baby or handling feeding equipment, wear a mask while holding or feeding the baby, and cough or sneeze away from the baby. If too unwell to breastfeed directly, expressing milk with a pump is a reasonable alternative.

Pediatrics

Nirmatrelvir/ritonavir is not authorized in Canada for patients under 18 years of age.^[6] Remdesivir is authorized for hospitalized pediatric patients (at least 4 weeks of age and weighing at least 3 kg) with pneumonia requiring supplemental oxygen, and for non-hospitalized high-risk patients (adult or pediatric) weighing at least 40 kg; it is not authorized for non-hospitalized pediatric patients under 40 kg.^[7] Immunocompromised children generally have a lower risk of severe COVID-19 than immunocompromised adults, and risk appears more related to underlying comorbidities than to immune suppression alone — treatment decisions in this group should involve pediatric infectious diseases or the relevant subspecialist.

Renal impairment

See the renal dose-adjustment table for nirmatrelvir/ritonavir in Choose Initial Treatment and Drug Tables. Remdesivir requires no dose adjustment at any degree of renal impairment, including dialysis, per the current product monograph, though safety data in severe renal impairment and end-stage renal disease remain more limited and closer monitoring is reasonable.^[7]

Hepatic impairment

Nirmatrelvir/ritonavir needs no dose adjustment in mild or moderate hepatic impairment (Child-Pugh A/B) but is not recommended in severe hepatic impairment (Child-Pugh C), where no safety data exist.^[6] Remdesivir needs no dose adjustment across mild, moderate, or severe hepatic impairment, though liver function monitoring during treatment is reasonable given more limited safety data at the severe end.^[7]

Immunocompromised patients

The single group most consistently shown to benefit from antiviral treatment across risk categories. Also a priority group for 2 vaccine doses per year rather than 1 — see Drug Tables.^[2]

For patients started on an antiviral, monitor for symptom improvement over the following days and reinforce safety-net advice: return for reassessment if not clearly improving, or sooner, by day 4-5 of illness, or immediately if any red flag develops. Home pulse oximetry is a reasonable adjunct for higher-risk patients. Check renal and hepatic function during treatment when clinically indicated by the patient's baseline status, rather than routinely in every patient. Routine "test of cure" after completing antiviral treatment is not recommended — patients can remain RAT-positive for weeks after clinical recovery, and a positive test alone does not indicate ongoing infectiousness or treatment failure. For vaccination, track and record each dose against the patient's current risk tier and the minimum 3-month interval since the last dose or confirmed infection, and flag patients due for their next dose per the current NACI schedule at the next pharmacy interaction.^[2] If symptoms — most commonly fatigue, shortness of breath, cognitive difficulties ("brain fog"), sleep disturbance, or persistent myalgia/arthralgia — persist beyond 12 weeks from the acute infection, evaluate for post-COVID-19 condition (long COVID) and refer for the appropriate work-up; a detailed discussion of long-COVID management is outside the scope of this chapter.^[9]

1. Symptoms not improving, or worsening, by day 4-5 of illness: reassess in person, check for red flags, and confirm the patient hasn't been missed for antiviral eligibility — if still within the 5-day (nirmatrelvir/ritonavir) or 7-day (remdesivir) window and otherwise eligible, treatment can still be started.
2. Suspected "rebound" (recurrence of symptoms or a new positive test after initial improvement): reassure the patient that rebound occurs whether or not antiviral treatment was given and is not caused by treatment. It is not an indication to repeat, extend, or restart antiviral therapy — manage supportively and reassess only if a genuine red flag develops.
3. A contraindicated or unmanageable drug interaction is found on the pre-prescribing interaction screen for a high-risk patient: switch to remdesivir if infusion access is available and the patient is within the 7-day window, or consult pharmacy/infectious diseases about temporarily holding or dose-adjusting the interacting medication rather than abandoning antiviral therapy altogether in a genuinely high-risk patient.
4. Clinical deterioration continues despite a completed antiviral course: escalate per Red Flags & Refer. Antivirals prevent progression in eligible patients — they do not treat established severe or hospitalized disease, and ongoing deterioration needs escalation of care, not a repeated or extended antiviral course.
5. Symptoms persisting beyond 12 weeks: evaluate for post-COVID-19 condition, rule out alternative diagnoses that could explain the symptom pattern, and refer per the appropriate post-COVID-19 condition care pathway.

• Prescribing or dispensing nirmatrelvir/ritonavir without a systematic drug-interaction screen — the single most consequential and most pharmacist-catchable error with this drug.
• Missing the treatment window — starting antiviral therapy beyond 5 days (nirmatrelvir/ritonavir) or 7 days (remdesivir) of symptom onset, when timing is the largest driver of benefit.
• Treating low-risk, mildly symptomatic patients who are already improving on their own — no demonstrated benefit, with real interaction and adverse-effect exposure for no gain.
• Withholding or stopping antiviral therapy because of a "rebound" positive test or symptom recurrence — rebound is not treatment-caused and is not an indication to repeat or extend a course.
• Prescribing antivirals with the goal of relieving symptoms faster — they do not shorten symptom duration or severity; the goal is preventing progression to severe disease in eligible patients.
• Using fluvoxamine, oral or inhaled corticosteroids, colchicine, or natural health products for outpatient COVID-19 — none has demonstrated benefit for this indication.
• Assuming nirmatrelvir/ritonavir is automatically off the table in renal or hepatic impairment — dose reduction (moderate renal impairment) or switching to remdesivir, not automatic exclusion, is usually the right response.
• Recommending hormonal contraception without a backup method during a nirmatrelvir/ritonavir course — ritonavir lowers estrogen levels and can reduce contraceptive efficacy.
• Skipping a due vaccine dose because the patient "already had COVID" or "was already vaccinated once" — both reinfection and waning immunity occur; follow the current interval-based NACI schedule regardless of prior infection.
• Diagnosing COVID-19 by symptoms alone without testing — overlap with influenza, RSV, and the common cold makes clinical diagnosis unreliable, and a positive test is required before prescribing antivirals.

Every drug/formulation this chapter covers, with full dosing, adverse effects, interactions, and comments. Pharmacologic Treatment Options above is the smaller decision-focused summary; this is the comprehensive reference.

Outpatient antivirals

Both require a positive COVID-19 test and treatment within the stated symptom-onset window; neither treats established severe/hospitalized disease.

Major nirmatrelvir/ritonavir drug interactions requiring action before prescribing

Ritonavir is a potent CYP3A4 inhibitor; nirmatrelvir and ritonavir are themselves CYP3A substrates. Always confirm against the current product monograph or a dedicated interaction checker (e.g., the University of Liverpool COVID-19 drug interaction resource) before prescribing — this table covers the major classes, not every possible interacting drug.

COVID-19 vaccines currently authorized in Canada

Two platforms are currently authorized: mRNA vaccines and an adjuvanted protein-subunit vaccine. Strain composition is updated by Health Canada/NACI to match circulating variants ahead of each fall season (recent seasons have used strains such as LP.8.1 and JN.1) — confirm the current season's specific product before administering rather than relying on a prior season's formulation.

Subsequent (previously known as "booster") dose frequency

Minimum interval between any 2 doses, or since a confirmed COVID-19 infection, is 3 months.

• 2 doses per year, strongly recommended: adults 80 years and older; residents of long-term care or other congregate living settings for seniors; individuals 6 months and older who are moderately to severely immunocompromised; adults 65-79 years with an underlying medical condition placing them at higher risk of severe disease.
• 2 doses per year may be offered: all other adults 65-79 years of age.
• 1 dose per year: all other individuals 6 months of age and older — programs are encouraged to offer this broadly, not only to those in a specifically listed higher-risk group.