How contagious is influenza and what are the prevention and treatment options?

Influenza Contagiousness and Management

How Contagious is Influenza?

Influenza is highly contagious, spreading from person to person primarily through respiratory droplets from coughing and sneezing, with infected individuals capable of transmitting the virus from 24 hours before symptoms appear until 5-6 days after symptom onset in adults and up to 10 days in children. 1

Transmission Dynamics

Primary transmission routes:

• Respiratory droplets expelled during coughing or sneezing are the main mode of spread 1
• Direct contact with influenza virus-contaminated surfaces also facilitates transmission 1
• Environmental persistence: The virus survives on hard surfaces for 24-48 hours, on porous materials for 8-12 hours, and on human hands for up to 3 hours 2

Infectious Period by Population

Adults:

• Contagious from 1 day before symptom onset through 5-6 days after symptoms begin 1, 2
• Viral shedding peaks shortly after symptom onset and decreases rapidly by days 3-5 2
• Most adults complete viral shedding by 5-7 days after illness onset 2

Children:

• Can be infectious before symptoms begin and for up to 10 days after symptom onset 1, 2
• Young children can shed virus for 6 days before illness onset 1, 2
• Attack rates in children are estimated at 10-40% annually during community outbreaks 1

Immunocompromised patients:

• May shed virus for weeks to months, requiring extended isolation precautions 1, 2
• This includes solid organ transplant recipients and those with advanced HIV disease 1

Community Impact

During outbreaks:

• Community outbreaks last 4-8 weeks or longer once influenza activity begins 1
• Highest attack rates occur among school-aged children, with secondary spread to adults and other family members being common 1
• The incubation period is 1-4 days, with an average of 2 days 1, 2

Prevention Strategies

Vaccination

Annual influenza vaccination is the cornerstone of prevention and should be administered to all persons 6 months and older without contraindications. 3

Vaccine effectiveness:

• Both trivalent inactivated vaccine (TIV) and live attenuated influenza vaccine (LAIV) are effective 1
• LAIV provides 52-55% increased protection compared to TIV in young children aged 6-71 months 1
• Vaccination reduces morbidity and mortality, though it may not prevent all infections 1

Non-Pharmaceutical Interventions

Key infection control measures:

• Cough etiquette and use of face masks during illness 4
• Hand hygiene to prevent transmission from contaminated surfaces 4
• Droplet precautions in healthcare settings, including surgical masks, gloves, and gowns 2
• Isolation of infected individuals: 5-6 days for adults, up to 10 days for children, and potentially longer for immunocompromised patients 2

Chemoprophylaxis

For high-risk exposed individuals:

• Oseltamivir 75 mg once daily or baloxavir are conditionally recommended for asymptomatic persons exposed to seasonal influenza who are at very high risk of hospitalization 5
• Prophylaxis should be considered for unvaccinated healthcare workers and household contacts during outbreaks 1
• In institutional outbreaks, chemoprophylaxis should be administered to all residents for at least 2 weeks or until 1 week after the outbreak ends 1

Important caveat: Children under 9 years receiving influenza vaccine for the first time may require 6 weeks of prophylaxis (4 weeks after first dose plus 2 weeks after second dose) 1

Treatment Options

Antiviral Therapy

Treatment should be initiated within 48 hours of symptom onset, with greatest benefit when started within 24 hours. 3

For non-severe influenza:

• Baloxavir is conditionally recommended if the risk of severe illness is high 5
• Antivirals are not recommended if the risk is low 5
• Treatment reduces illness duration by approximately 24 hours in otherwise healthy patients 6, 3

For severe influenza:

• Oseltamivir 75 mg twice daily for 5 days is conditionally recommended 5
• Peramivir and zanamivir are not recommended 5

Pediatric dosing:

• Children 1-12 years: 2 mg/kg twice daily for 5 days (started within 48 hours of symptom onset reduces illness by 1.5 days) 6
• Infants 2 weeks to <1 year: 3 mg/kg twice daily for 5 days 6

What NOT to Use

Strong recommendations against:

• Antibiotics if bacterial co-infection is unlikely 5
• Macrolide antibiotics in the absence of co-infection 5
• Corticosteroids, mTOR inhibitors, and plasma therapy 5

Common pitfall: Empiric antibiotic coverage for Staphylococcus aureus (including MRSA) should be included during influenza season when treating pneumonia, as secondary bacterial pneumonia carries high mortality 7

Diagnostic Testing

For suspected severe influenza:

• Nucleic acid amplification test (NAAT) or RT-PCR is strongly recommended 5
• Rapid molecular assays are preferred for point-of-care testing due to high accuracy 3

For suspected non-severe influenza:

• NAAT or digital immunoassay (DIA) is recommended 5
• Clinical diagnosis alone has limited accuracy (positive predictive value 30-88% depending on population) 1

Critical consideration: Rapid diagnostic tests have high specificity (>90%) but low to moderate sensitivity (20-70%), so negative results should not exclude influenza when community activity is high 8

Isolation Precautions in Healthcare Settings

Standard and Droplet Precautions should be implemented immediately upon suspicion of influenza: 2

• Continue until hospital discharge OR until symptoms resolve and patient is afebrile for at least 24 hours 2
• For immunocompromised patients, maintain precautions until hospital discharge or negative testing 2
• Healthcare personnel with influenza-like illness should be removed from direct patient contact 2

Drug resistance consideration: To reduce transmission of drug-resistant virus, avoid contact between high-risk individuals and patients taking antivirals during therapy and for 2 days after discontinuation 2