INDICATIONS FOR TREATMENT — Antiviral treatment can be considered for otherwise healthy symptomatic outpatients with suspected or confirmed influenza who are not at increased risk for influenza complications if it can be started within 48 hours after illness onset.

Antiviral treatment is recommended as soon as possible for any patient with suspected or confirmed influenza who is hospitalized, has severe, complicated, or progressive illness, or is at higher risk for complications, even if it is started >48 hours after illness onset.1-3 False negative results can occur with some influenza tests; patients in the above groups should receive antiviral treatment even if they test negative, especially when influenza viruses are known to be circulating in the community.4

While influenza viruses and SARS-CoV-2 are co-circulating, hospitalized patients with respiratory illness, outpatients with severe, complicated, or progressive respiratory illness, and outpatients at higher risk for complications who present with symptoms of acute respiratory illness (with or without fever) should receive empiric antiviral treatment for influenza as soon as possible, without waiting for results of influenza and/or SARS-CoV-2 testing. Empiric antiviral treatment for influenza can be considered for outpatients with suspected influenza who are not at high risk if it is started within 48 hours after illness onset. None of the drugs that are FDA-approved for treatment of influenza have clinically relevant antiviral activity against SARS-CoV-2.

TREATMENT — A neuraminidase inhibitor (oral oseltamivir, IV peramivir, or inhaled zanamivir) or the oral cap-dependent endonuclease inhibitor baloxavir marboxil is recommended for treatment of influenza in nonpregnant outpatients this season. Both drug classes are active against influenza A and B viruses.

Oseltamivir is preferred for treatment of influenza in pregnant women, hospitalized patients, and outpatients with severe, complicated, or progressive illness.1

Effectiveness – Use of a neuraminidase inhibitor or baloxavir for treatment of acute uncomplicated influenza in adults shortens the duration of symptoms by about one day.5-8 In a meta-analysis of 26 randomized, placebo-controlled trials that included 11,897 healthy adults and children with influenza-like illness, zanamivir was associated with the shortest time to alleviation of influenza symptoms and baloxavir was associated with the lowest risk of influenza-related complications.9 Oseltamivir and peramivir appear to be similarly effective.10

A meta-analysis of randomized trials in children with influenza found that starting oseltamivir within 48 hours after illness onset reduced illness duration by about 18 hours (by about 30 hours when trials that enrolled children with asthma were excluded) and decreased the risk of otitis media.11 In children hospitalized with laboratory-confirmed influenza, antiviral treatment started within 48 hours after illness onset was associated with shorter lengths of hospital stay compared to no antiviral treatment.12

Although most controlled trials of antiviral drugs have not been powered to assess their efficacy in preventing serious influenza complications, experts have generally interpreted the combined results of controlled trials, observational studies, and meta-analyses as showing that early antiviral treatment of influenza in high-risk patients can reduce the risk of complications.6,13-15

Oseltamivir vs Baloxavir – In 173 children 1-11 years old, the median time to improvement in symptoms was similar with a single dose of baloxavir or 5 days' treatment with oseltamivir (138 vs 150 hours).16 Oseltamivir is FDA-approved for treatment of influenza in patients ≥2 weeks old; baloxavir is approved for use in patients ≥12 years old.

In a randomized, double-blind trial in 2184 adolescents and adults with uncomplicated influenza who were at high risk of developing complications, the median time to improvement of symptoms was similar with a single dose of baloxavir or 5 days' treatment with oseltamivir (both started within 48 hours after illness onset) in patients infected with influenza A(H3N2), but was significantly shorter with baloxavir than with oseltamivir in those infected with influenza B (median difference 27.1 hours). Use of either drug was associated with reduced rates of influenza-related complications and fewer antibiotic prescriptions compared to placebo.5

Timing – Neuraminidase inhibitors are most effective when started within 48 hours after illness onset, but the results of some observational studies in hospitalized and critically ill patients suggest that treatment started as late as 4-5 days after illness onset can shorten the length of hospitalization and reduce the risk of complications such as pneumonia, respiratory failure, and death.17-20 No data are available on the efficacy of baloxavir when it is started >48 hours after illness onset.

Adults (outpatient or hospitalized) with communityacquired pneumonia who test positive for influenza should receive antiviral treatment regardless of the duration of illness.21

CHEMOPROPHYLAXIS — Oseltamivir, zanamivir, and baloxavir are FDA-approved for post-exposure prophylaxis of influenza. Post-exposure prophylaxis should be considered for persons at increased risk of complications who have not received a 2021-2022 influenza vaccine, received one within the previous 2 weeks, or might not respond to vaccination, or when the match between the vaccine and circulating strains is poor. It is not recommended for healthy persons exposed to influenza or when >48 hours have elapsed since exposure. Antiviral chemoprophylaxis is also recommended to help control institutional influenza outbreaks.1

Effectiveness – Neuraminidase inhibitors have generally been about 70-90% effective in preventing influenza caused by susceptible strains of influenza A or B viruses.1 In a randomized, double-blind trial in 752 household contacts of patients with influenza, a single dose of baloxavir was significantly more effective (86%) than placebo in preventing clinical influenza in household contacts.22

Timing – When indicated, chemoprophylaxis with oseltamivir or zanamivir should be started no later than 48 hours after exposure and it should be continued for 7 days after the last known exposure.

For institutional outbreaks, the CDC recommends chemoprophylaxis for at least 2 weeks and continuing for up to 1 week after the end of the outbreak. A single dose of baloxavir is recommended for post-exposure prophylaxis if it can be given within 48 hours of exposure.

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PREGNANCY AND LACTATION — Pregnant women are at increased risk for severe complications of influenza. Oseltamivir and zanamivir appear to be safe for use during pregnancy.23,24 Prompt treatment with oseltamivir is recommended for women with suspected or confirmed influenza who are pregnant or ≤2 weeks postpartum.25-27 Oseltamivir is preferred for treatment of women who are breastfeeding. No data are available on use of baloxavir in pregnant or breastfeeding women.

Antiviral chemoprophylaxis can be considered for pregnant women who have had close contact with someone likely to have influenza. Zanamivir may be preferred because of its limited systemic absorption, but oseltamivir is a reasonable alternative, especially in women at increased risk for respiratory problems.

RESISTANCE — Over 99% of the recently circulating influenza virus strains tested by the World Health Organization (WHO) have been susceptible to neuraminidase inhibitors.28 Reduced susceptibility of some influenza virus strains, particularly influenza A(H1N1), to oseltamivir or peramivir can emerge during or after treatment, especially in immunocompromised patients with prolonged viral shedding and in young children.29-34 Resistant isolates have usually remained susceptible to zanamivir, but reduced susceptibility to zanamivir has been reported.35 In immunocompromised patients, a double dose of oseltamivir reduced the incidence of oseltamivir resistance compared to standard dosing, but it did not improve efficacy and can cause more adverse effects.36

Baloxavir is active against neuraminidase inhibitor-resistant strains of influenza A and B viruses, including A(H1N1), A(H5N1), A(H3N2), and A(H7N9). Amino acid substitutions associated with reduced susceptibility to baloxavir have occurred following treatment with a single dose of the drug.8,37 Reduced susceptibility to baloxavir appears to be more frequent in persons infected with influenza A(H3N2) and A(H1N1)pdm09 viruses, particularly children.38,39 Baloxavir monotherapy is not recommended for severely immunosuppressed patients because of concerns that prolonged replication of the influenza virus in these patients could lead to emergence of resistance. Oseltamivir and peramivir may be active against influenza virus strains with reduced susceptibility to baloxavir.40

The adamantanes amantadine and rimantadine are active against influenza A viruses, but not influenza B viruses. As in recent past seasons, resistance to these drugs is high (>99%) among circulating influenza A(H3N2) and A(H1N1)pdm09 viruses; neither amantadine nor rimantadine is recommended for antiviral treatment or chemoprophylaxis of currently circulating influenza A viruses.

ADVERSE EFFECTS — Nausea, vomiting, and headache are the most common adverse effects of oseltamivir; taking the drug with food may minimize GI adverse effects. Oseltamivir has been associated with bradycardia in critically ill patients.41 Diarrhea, nausea, sinusitis, fever, and arthralgia have been reported with zanamivir. Inhalation of zanamivir can cause bronchospasm; the drug should not be used in patients with underlying airway disease. Diarrhea and neutropenia have occurred with peramivir.42 Neuropsychiatric events, including self-injury and delirium, have been reported in patients taking neuraminidase inhibitors, but a causal relationship has not been established, and neuropsychiatric dysfunction can be a complication of influenza illness itself.43

Baloxavir was well tolerated in clinical trials. It appears to cause less nausea and vomiting than oseltamivir.44 Neuropsychiatric events have also been reported with use of baloxavir, but a causal relationship has not been established.

Hypersensitivity reactions, including anaphylaxis, have been reported with all of these drugs.

DRUG INTERACTIONS — Use of oseltamivir or zanamivir within 48 hours before, peramivir within 5 days before, or baloxavir within 17 days before or <2 weeks after administration of the intranasal live-attenuated influenza vaccine (FluMist Quadrivalent) could inhibit replication of the vaccine virus, reducing the vaccine's efficacy, and is not recommended.45 Revaccination with an inactivated or a recombinant influenza vaccine is recommended in persons who receive any one of these antiviral drugs within 2 weeks after receiving the intranasal influenza vaccine.46

Coadministration of antacids, laxatives, multivitamins, or other products containing polyvalent cations, such as calcium, aluminum, iron, magnesium, selenium, or zinc, can reduce serum concentrations of baloxavir and should be avoided.

Additional Content Available:
Comparison Table: Antiviral Drugs for Influenza for 2021-2022

REFERENCES