Seasonal influenza in children: Prevention with antiviral drugs

INTRODUCTION — Influenza is an acute respiratory illness caused by influenza A or B viruses, and rarely influenza C viruses. It occurs in outbreaks worldwide every year, mainly during the winter seasons in temperate climates.

Influenza usually is an acute, self-limited, and uncomplicated disease in healthy children, but it can be associated with severe morbidity and mortality. Certain groups of children are at increased risk of severe or complicated influenza infection (table 1).

Although immunization is the most important public health measure for the prevention of influenza infection, not all children can receive influenza immunization or have an adequate response to immunization. Chemoprophylaxis with antiviral drugs is an adjunct or alternative to immunization for children who cannot be vaccinated, children expected to have a poor response to immunization, or children who have not had time to respond to immunization.

The prevention of influenza in children with antiviral drugs will be presented here. Influenza vaccination in children; the clinical features, diagnosis, and management of influenza in children; and the prevention of influenza with antiviral drugs in adults are discussed separately.

●(See "Seasonal influenza in children: Prevention with vaccines".)

●(See "Seasonal influenza in children: Clinical features and diagnosis".)

●(See "Seasonal influenza in children: Management".)

●(See "Seasonal influenza in adults: Role of antiviral prophylaxis for prevention".)

INFLUENZA ACTIVITY — Information about influenza activity at the national, regional, and state level is available from the United States Centers for Disease Control and Prevention (FluView interactive) and the World Health Organization (FluNet). Surveillance information is updated regularly during influenza season.

Local hospital or clinic laboratories may also collect information to monitor influenza activity in a specific geographic area.

During the coronavirus 2019 (COVID-19) pandemic, influenza virus circulation has been low in the northern and southern hemispheres [1]. The impact of the anticipated cocirculation of influenza and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is uncertain.

CHEMOPROPHYLAXIS DECISION-MAKING

Potential benefits and harms

●Potential benefits

•Although chemoprophylaxis with neuraminidase inhibitors (oseltamivir, zanamivir, laninamivir) or baloxavir decreases the risk of symptomatic laboratory-confirmed influenza compared with placebo (risk ratios ranging from 0.14 to 0.36) [2,3], the absolute difference is small to modest (absolute difference of approximately 2 to 3 percent for individuals and 8 to 15 percent for households) [3-6]. (See 'Available antiviral agents' below.)

•It is uncertain whether chemoprophylaxis prevents serious complications of influenza [7]. (See 'Available antiviral agents' below.)

Trials of chemoprophylaxis with neuraminidase inhibitors were predominantly performed in healthy subjects; children with chronic conditions were generally excluded [2].

●Potential harms – Potential harms of influenza chemoprophylaxis with antiviral drugs include [8]:

•Promotion of resistance to antiviral medications if chemoprophylaxis is used indiscriminately

Neuraminidase inhibitor resistance is uncommon and occurs primarily in severely immunocompromised people who may have prolonged asymptomatic virus replication, although it has been reported among healthy children [9-14].

Baloxavir resistance was reported among healthy subjects and subjects at increased risk for severe or complicated influenza in prelicensure clinicals of baloxavir treatment [15,16]. Similarly, in a randomized trial comparing baloxavir with placebo for postexposure prophylaxis in immunocompetent household contacts, influenza viruses with mutations associated with reduced baloxavir susceptibility were detected in 4 percent of participants in the baloxavir group (some of whom were diagnosed at baseline before receiving baloxavir) [3].

•Reduced availability of antiviral medications for the treatment of individuals who are severely ill or at high risk of influenza complications

•Medication adverse effects (discussed below) (see 'Available antiviral agents' below)

Alternatives to chemoprophylaxis — Widespread or routine chemoprophylaxis for influenza should be avoided [17].

Adjuncts and alternatives to chemoprophylaxis for the prevention of influenza in children at very high risk of severe or complicated influenza (eg, those who are severely immunocompromised) include [17-19]:

●Immunization of children at increased risk for severe or complicated influenza, their household contacts, and their out-of-home caregivers is preferred to chemoprophylaxis for the prevention of influenza. (See "Seasonal influenza in children: Prevention with vaccines", section on 'Target groups' and "Seasonal influenza vaccination in adults", section on 'Whom to vaccinate'.)

Immunization is the primary means of prevention of influenza infection in individuals ≥6 months of age, particularly those who at high risk for severe or complicated influenza (table 1); chemoprophylaxis should not be substituted for immunization unless influenza immunization is specifically contraindicated [20,21].

●Early treatment of exposed individuals is an alternative to chemoprophylaxis, although the two strategies have not been directly compared [7,17]. For most children at very high risk of severe or complicated influenza (eg, those receiving chemotherapy), we suggest early treatment over chemoprophylaxis. Early treatment may reduce the use and/or duration of antiviral agents, lessening the risk of emergent resistance. (See "Seasonal influenza in children: Management", section on 'Antiviral therapy'.)

●Nonpharmaceutical and behavioral interventions similar to those recommended during the coronavirus disease 2019 (COVID-19) pandemic (eg, avoidance of ill contacts, physical distancing, use of face masks, limited travel), which have been associated with decreased prevalence of influenza [22,23].

Other factors — Other factors in the decision whether to provide chemoprophylaxis for influenza include [7,8]:

●The risk of developing severe or complicated influenza for the patient or close contact of the patient (the risk is greatest in children who are severely immunocompromised).

●The nature of the exposure (eg, duration, closeness of contact, other mitigation measures).

●The timing of the exposure.

Did it occur during the infectious period? The infectious period is typically one day before the onset of symptoms until 24 hours after resolution of fever, but it may be prolonged in immunocompromised individuals. (See "Seasonal influenza in children: Clinical features and diagnosis", section on 'Transmission'.)

●Ability to administer prophylaxis within 48 hours of exposure (for postexposure prophylaxis).

●Ability to complete the course of therapy (failure to complete therapy may contribute to the development of resistance).

●Advice from public health authorities and/or hospital epidemiologists.

OUTPATIENT SETTINGS

●Possible indications — Neither postexposure nor pre-exposure prophylaxis is routinely recommended [17]. Chemoprophylaxis of select children ≥3 months of age who are at very high risk of severe or complicated influenza (eg, those who are severely immunocompromised) or their close contacts is an alternative to early initiation of antiviral therapy.

•Children at increased risk for severe or complicated influenza – We make decisions about postexposure or pre-exposure chemoprophylaxis on a case-by-case basis in shared decision-making with the patient and/or caregivers for [7,17]:

-Children at very high risk of severe or complicated influenza infection (eg, those who are severely immunocompromised) who have a contraindication to the influenza vaccine, are otherwise unable to receive the influenza vaccine, or may not respond to influenza vaccination.

-Children at high risk of severe or complicated influenza (table 1) who have not been vaccinated for influenza or were vaccinated less than two weeks before exposure (for postexposure prophylaxis) or onset of influenza activity in the community (for pre-exposure prophylaxis).

Unvaccinated children who are eligible for the influenza vaccine should receive the influenza vaccine simultaneously [7]. They should receive the inactivated influenza vaccine (IIV) or recombinant influenza vaccine (RIV; if they are ≥18 years of age). (See "Seasonal influenza in children: Prevention with vaccines".)

•Contacts of children at increased risk for severe or complicated influenza

-Postexposure prophylaxis – Postexposure chemoprophylaxis may be warranted for household contacts of children who are severely immunocompromised (eg, hematopoietic cell transplant recipients) if the household contact is unvaccinated against influenza and ≥3 months of age [7].

-Pre-exposure prophylaxis – Pre-exposure prophylaxis may be warranted for unvaccinated family members who are ≥3 months of age or unvaccinated health care providers who are likely to have ongoing close contact with children at very high risk of severe or complicated influenza who are unable to be vaccinated and unable to take chemoprophylaxis [7].

Unvaccinated contacts who are eligible for the influenza vaccine should receive the influenza vaccine simultaneously [7]. They should receive IIV or RIV (if they are ≥18 years of age). (See "Seasonal influenza in children: Prevention with vaccines" and "Seasonal influenza vaccination in adults".)

●Postexposure prophylaxis regimens — Postexposure prophylaxis should not be initiated if exposure occurred ≥48 hours previously [7,17].

•Choice of agent – For postexposure prophylaxis in children, we prefer oseltamivir or zanamivir (for those ≥5 years of age) based on viral surveillance and resistance data from recent influenza seasons (table 2) [24-26]. Baloxavir is also available for children ≥5 years of age [27]. Doses are provided below and summarized in the tables (table 3A-B). (See 'Available antiviral agents' below.)

Laninamivir (not available in the United States) is available for postexposure prophylaxis in Japan.

Peramivir has not been studied for the prevention of influenza in children and is not indicted for chemoprophylaxis. Adamantane drugs should not be used for pre-exposure prophylaxis given high levels of resistance [8,17,28].

•Initiation – Postexposure prophylaxis should be initiated as soon as possible after and within 48 hours of exposure [7,17].

•Duration – The duration of postexposure prophylaxis varies with the agent.

-Oseltamivir or zanamivir – Oseltamivir or zanamivir should be administered until seven days after the last known exposure [17].

-Baloxavir – Postexposure prophylaxis with baloxavir (for patients ≥5 years) consists of a single dose [27].

●Pre-exposure prophylaxis regimens

•Choice of agent – For pre-exposure prophylaxis, we prefer oseltamivir or zanamivir (for those ≥5 years of age) based on viral surveillance and resistance data from recent influenza seasons (table 2) [24-26]. Doses are provided below and summarized in the tables (table 3A-B). (See 'Available antiviral agents' below.)

Baloxavir is not approved for pre-exposure prophylaxis [27]. Given high levels of resistance, adamantane drugs should not be used for pre-exposure prophylaxis [8,17,28].

•Initiation – Pre-exposure prophylaxis should be initiated as soon as influenza activity is detected in the community [7]. (See 'Influenza activity' above.)

•Duration – The duration for pre-exposure prophylaxis depends upon the indication.

-Awaiting vaccine response – For individuals receiving chemoprophylaxis with oseltamivir or zanamivir while awaiting the development of an immune response to the influenza vaccine, chemoprophylaxis is administered until 14 days after receipt of the influenza vaccine [17].

-Other indications – For other indications, pre-exposure chemoprophylaxis with oseltamivir or zanamivir should be administered every day for the duration of influenza activity in the community. Oseltamivir and zanamivir regimens as long as 16 weeks have been well tolerated [29,30].

INSTITUTIONAL OUTBREAK — Chemoprophylaxis is one of multiple interventions to control outbreaks of influenza in a closed institutional setting with residents or patients at high risk of severe or complicated influenza (eg, long-term care facility, hospital rehabilitation unit, transplant unit) [7,17].

We use the definition for outbreak provided in the Infectious Diseases Society of America guidelines: ≥2 cases of health care-associated, laboratory-confirmed influenza among patients or residents of the same ward or unit within 72 hours [7].

●Potential indications for chemoprophylaxis – In addition to other outbreak control measures (described below), potential target groups for chemoprophylaxis include [7]:

•Asymptomatic exposed residents/patients – For asymptomatic exposed residents/patients and residents/patients of outbreak-affected units (with or without known exposure), we suggest chemoprophylaxis, even if they have been vaccinated against influenza. Influenza transmission can occur rapidly in closed settings. Children with disabilities residing in long-term care facilities are at increased risk for influenza complications and mortality [31,32].

Chemoprophylaxis for institutional outbreaks of influenza is supported by randomized and observational studies in older patients) [33-36] and observational studies in hospitalized children and neonates [37-39]. In a cluster randomized trial, the influenza attack rate among nursing home residents was lower with influenza prophylaxis and treatment than with treatment alone (23 versus 35 percent) [34]. Prophylaxis also reduced the mean duration of outbreaks (11 versus 24 days).

•Symptomatic exposed residents/patients – Exposed residents/patients who develop symptoms compatible with influenza or who are suspected to have influenza should undergo testing for influenza and receive antiviral therapy for influenza without waiting for test results. (See 'Breakthrough infection' below.)

In the setting of an institutional outbreak, there should be a low threshold for suspected breakthrough influenza infection. People with chronic illness who develop nosocomial infection may not have typical signs and symptoms; they may be nonverbal or unable to describe their symptoms [7,40,41]. They may lack respiratory symptoms, have hypothermia rather than fever, or demonstrate behavioral changes [40,41].

Chemoprophylaxis should be resumed after treatment is completed if there is ongoing influenza exposure on the ward or unit [7].

•Staff – Decisions regarding chemoprophylaxis for unvaccinated staff or staff who received influenza immunization with the inactivated influenza vaccine or recombinant influenza vaccine in the two weeks before the outbreak should be made in consultation with hospital epidemiologists and/or public health authorities. Chemoprophylaxis for staff who received the live attenuated influenza vaccine is discussed separately. (See 'Receipt of LAIV' below.)

●Choice of agent – For outbreak control in the institutional setting, we generally prefer oseltamivir or zanamivir to other antiviral agents [7].

Baloxavir is an alternative for staff who receive chemoprophylaxis while awaiting a response to the influenza vaccine [27].

●Duration of chemoprophylaxis – The duration of prophylaxis in the institutional outbreak setting depends upon the indication.

•Exposed patients and residents – For exposed patients and residents, oseltamivir or zanamivir should be administered for a minimum of two weeks and up to one week after the last known case was identified [7].

•Awaiting vaccine response – For staff receiving chemoprophylaxis while awaiting the development of an immune response to the influenza vaccine, chemoprophylaxis with oseltamivir or zanamivir is administered until 14 days after receipt of the influenza vaccine [17]. Baloxavir prophylaxis is provided as a single dose.

●Other measures – Other measures for outbreak control include [7]:

•Surveillance for new cases.

•Cohorting ill residents and staff members.

•Use of facemasks by ill residents or patients and ill staff members.

•Minimizing nonessential visits.

(See "Infection control measures for prevention of seasonal influenza".)

SPECIAL CIRCUMSTANCES

Pregnancy — Influenza chemoprophylaxis for pregnant people is discussed separately. (See "Seasonal influenza and pregnancy", section on 'Role of antiviral prophylaxis'.)

Receipt of LAIV — Individuals immunized with the intranasal live-attenuated influenza vaccine (LAIV) should not receive chemoprophylaxis for 14 days after receipt of the LAIV because the vaccine strains are susceptible to antiviral drugs [20]. Those who require chemoprophylaxis before the 14-day interval should be revaccinated with the inactivated influenza vaccine or recombinant influenza vaccine (if ≥18 years of age).

Breakthrough infection — Children who develop symptoms of influenza despite chemoprophylaxis should undergo testing for influenza and start antiviral therapy while awaiting results [7]. Given the possibility of emergent resistance, particularly with baloxavir, children who develop symptoms of influenza during or after prophylaxis ideally should receive treatment with a different antiviral agent. (See "Seasonal influenza in children: Management", section on 'Antiviral therapy'.)

CHEMOPROPHYLAXIS COUNSELING — Education of patients and/or caregivers of children who receive chemoprophylaxis should include the following information [17,19,21]:

●For eligible children, immunization is the primary means of influenza prevention. (See "Seasonal influenza in children: Prevention with vaccines".)

●Chemoprophylaxis lowers, but does not eliminate, the risk of influenza. (See 'Potential benefits and harms' above.)

●Susceptibility to influenza returns with discontinuation of the antiviral medication.

●Individuals at increased risk of severe or complicated influenza (table 1) should seek medical evaluation if they develop a febrile respiratory illness or moderate to severe illness despite chemoprophylaxis so that they can receive antiviral therapy (See 'Breakthrough infection' above.)

AVAILABLE ANTIVIRAL AGENTS — The pharmacology of antiviral agents is discussed separately. (See "Antiviral drugs for influenza: Pharmacology and resistance".)

Neuraminidase inhibitors — Neuraminidase inhibitors prevent the release of virions from the host cell [42]. Neuraminidase inhibitors are active against influenza A viruses and influenza B viruses [17].

Oseltamivir — In the United States, oseltamivir is available for the prophylaxis of influenza in individuals ≥1 year of age, but it may be used in younger children when indicated [17,43].

●Route and dose – Oseltamivir is generally administered orally. Limited data suggest that it may be administered by orogastric or nasogastric tube [44,45], but the pharmacokinetics with enteric tube administration may be different than with oral administration, resulting in decreased concentrations of the active metabolite [46,47].

The dose of oseltamivir varies according to age and weight.

•Full-term infants <1 year of age (table 3A):

-Age <3 months – Not recommended, except in critical situations (consultation with an specialist in pediatric infectious diseases is recommended)

-Age 3 through 8 months – 3 mg/kg per dose once daily

-Age 9 through 11 months – 3.5 mg/kg per dose once daily

If weight is not known, dosing should be determined by age as follows [48]:

-Age 4 through 5 months – 17 mg once daily

-Age 6 through 11 months – 24 mg once daily

•Age 1 through 12 years (table 3B)

-Weight ≤15 kg – 30 mg orally once daily

-Weight >15 to 23 kg – 45 mg orally once daily

-Weight >23 to 40 kg – 60 mg orally once daily

-Weight >40 kg – 75 mg orally once daily

•Age ≥13 years – 75 mg orally once daily

●Efficacy – In meta-analyses of randomized trials, oseltamivir prophylaxis reduced the risk of symptomatic laboratory-confirmed influenza in exposed individuals (2.6 versus 5.6 percent; risk difference [RD] 3 percent, 95% CI 1.8-3.9; three trials, 2479 participants) and households (3.4 versus 17.0 percent; RD 13.6 percent, 95% CI 0.5-15.5; one trial 405 participants) [4].

●Adverse effects – Nausea and vomiting are the most common adverse effects of oseltamivir (occurring in 12 percent of recipients of once-daily dosing) [49].

Zanamivir — In the United States, dry powder zanamivir is available for the prophylaxis of influenza in individuals ≥5 years of age [17].

●Route and dose – Dry powder zanamivir is administered by oral inhalation. Each inhalation is 5 mg. The recommended dose is two inhalations (10 mg total) once per day. Use of a dry powder inhaler requires high enough inspiratory flow to mobilize and aerosolize the medication. It is prudent to ask the child to demonstrate proper technique before prescribing zanamivir.

An intravenous (IV) form of zanamivir has been evaluated for prophylaxis in a clinical trial [50]. The IV form is available in the United Kingdom and Europe but is not available in the United States [51,52].

●Efficacy – In meta-analyses of randomized trials, zanamivir reduced the risk of symptomatic influenza in exposed individuals (1.2 versus 3.1 percent; RD 1.9 percent, 95% CI 1.0-2.5; four trials, 5275 participants) and households (4.1 versus 19.0 percent; RD 14.9 percent, 95% CI 12.2-16.6; two trials, 824 participants) [4].

●Adverse effects – In addition to gastrointestinal side effects, zanamivir is associated with increased risk of bronchospasm in patients with a history of wheezing or chronic respiratory disease [53].

Laninamivir — Laninamivir is a long-acting inhaled neuraminidase inhibitor that is available for prevention of influenza in Japan [5,6].

●Route and dose – Clinical trials of laninamivir used the following doses [5,6]:

•Age 2 through 9 years – Single dose of 20 mg

•Age ≥10 years – A single dose of 40 mg or 20 mg daily for two days

●Efficacy – In a randomized trial, laninamivir (a single dose of 40 mg or 20 mg daily for two days) reduced laboratory-confirmed influenza among family members (≥10 years of age) of influenza patients who were within 48 hours of symptom onset (4.5 versus 12.1 percent in the placebo group) [5]. Most of the patients with influenza had influenza A H3N2 infection. In a separate randomized trial, laninamivir (a single 20 mg dose) also reduced laboratory-confirmed influenza among family members between 2 and 10 years of age (11 versus 19 percent in the placebo group; relative risk reduction 46 percent, 95% CI 8-68) [6].

Baloxavir — Baloxavir blocks influenza proliferation by inhibiting initiation of mRNA synthesis [54]. It has activity against influenza A and influenza B viruses.

In Japan, baloxavir is licensed for postexposure prophylaxis of influenza in people ≥12 years and children <12 years who weigh at least 20 kg. In the United States, it is licensed for postexposure prophylaxis in people ≥5 years of age [54-57].

●Route and dose – Baloxavir is administered orally. The dose varies according to weight:

•Weight <20 kg (oral suspension) – 2 mg/kg orally as a single dose

•Weight 20 to <80 kg (tablet or oral suspension) – 40 mg orally as single dose

•Weight ≥80 kg (tablet or oral suspension) – 80 mg orally as a single dose

●Efficacy – The efficacy of baloxavir for postexposure prophylaxis was demonstrated in a randomized trial that included 749 household contacts of 545 index patients with influenza who received antiviral therapy (53 percent with baloxavir) per standard practice in Japan [3]. Baloxavir decreased the risk of laboratory-documented influenza in household contacts compared with placebo (1.9 versus 13.6 percent; adjusted risk ratio 0.14, 95% CI 0.06-0.30), without an increase in adverse effects. In subgroup analysis, baloxavir was effective in individuals at high risk for severe influenza and individuals who were unvaccinated.

●Adverse effects – Emergent baloxavir resistance is the most important adverse effect of baloxavir. (See 'Potential benefits and harms' above.)

Other adverse effects of baloxavir include diarrhea and vomiting, which occur in 2 to 6 percent of recipients [15,58].

Adamantane drugs — Amantadine and rimantadine are adamantanes (also called M2 inhibitors). Adamantanes target the M2 protein of influenza A, which forms a protein channel in the viral membrane that is essential for efficient viral replication. Adamantanes are not active against influenza B viruses.

Circulating strains of influenza A virus have been widely resistant to adamantane drugs since 2009 and they are not recommended for prophylaxis of influenza [8,28,59]. (See "Antiviral drugs for influenza: Pharmacology and resistance".)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Treatment and prevention of seasonal influenza with antivirals".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or email these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient education" and the keyword[s] of interest.)

●Basics topic (see "Patient education: Flu (The Basics)")

●Beyond the Basics topics (see "Patient education: Influenza prevention (Beyond the Basics)" and "Patient education: Influenza symptoms and treatment (Beyond the Basics)")

SUMMARY AND RECOMMENDATIONS

●Chemoprophylaxis decision-making – Influenza chemoprophylaxis decision-making for children at very high risk of severe or complicated influenza is individualized, considering:

•The small to modest absolute benefit and potential harms, particularly development of antiviral resistance (see 'Potential benefits and harms' above)

•Alternatives to chemoprophylaxis, including (see 'Alternatives to chemoprophylaxis' above):

-Availability, suitability, and timing of influenza immunization, which is the primary means of preventing influenza infection

-Early initiation of antiviral therapy (as soon as possible after onset of symptoms)

For most children at very high risk of severe or complicated influenza, we suggest early initiation of antiviral therapy (Grade 2C); early treatment may reduce the use and/or duration of antiviral agents, lessening the risk of emergent resistance (see "Seasonal influenza in children: Management", section on 'Antiviral therapy')

•The potential for severe or complicated influenza in the child or child's contacts (table 1) and the nature and timing of exposure (see 'Other factors' above)

●Chemoprophylaxis in outpatient settings – Neither postexposure nor pre-exposure prophylaxis for influenza is routinely recommended for children in the outpatient setting. Possible indications for chemoprophylaxis of children who are at very high risk of severe or complicated influenza (eg, this receiving chemotherapy) or their close contacts as an alternative to early initiation of antiviral therapy are discussed above. (See 'Outpatient settings' above.)

●Institutional outbreak – Chemoprophylaxis is one of multiple interventions to control outbreaks of influenza in a closed institutional setting with patients at high risk of severe or complicated influenza (eg, long-term care facility, hospital rehabilitation unit, transplant unit). In this setting, we consider ≥2 cases of health care-associated, laboratory-confirmed influenza among patients or residents of the same ward or unit within 72 hours an outbreak. (See 'Institutional outbreak' above.)

For asymptomatic exposed residents/patients and residents/patients of outbreak-affected units (with or without known exposure), we suggest chemoprophylaxis, even if they have been vaccinated against influenza (Grade 2C). Influenza can spread rapidly in closed settings.

Symptomatic patients should be tested for influenza infection and treated with antiviral therapy without waiting for the results. (See 'Breakthrough infection' above.)

●Counseling – Patients and/or caregivers of patients who receive chemoprophylaxis should understand that (see 'Chemoprophylaxis counseling' above):

•Immunization is the primary means of influenza prevention. (See "Seasonal influenza in children: Prevention with vaccines" and "Seasonal influenza vaccination in adults".)

•Chemoprophylaxis does not eliminate the risk of influenza. (See 'Potential benefits and harms' above.)

•Susceptibility to influenza returns with discontinuation of prophylaxis.

•Individuals at high risk of severe or complicated influenza (table 1) should seek medical evaluation if they develop symptoms of influenza so they can receive antiviral therapy. (See 'Breakthrough infection' above.)