Duration of influenza b infection
Pro Research Analysisby 
Searched over 200M research papers
Duration of Influenza B Infection: Key Findings
Illness Duration in the General Population
Studies show that the duration of illness caused by influenza B is similar to that of influenza A. In a large study from France and Turkey, the average duration of influenza B illness was about 7.4 days for children (0–14 years), 8.7 days for adults (15–64 years), and 10.5 days for older adults (65+ years) in France. In Turkey, the durations were 6.3, 8.2, and 9.2 days for the same age groups, respectively. The duration increased with age but did not differ significantly between influenza A and B infections. Use of antibiotics was linked to longer illness duration, but the virus type itself was not a factor in prolonging illness .
Duration of Fever in Children
In children, the duration of fever from influenza B can vary depending on the antiviral treatment used. A recent study in Japan found that, after treatment, the median duration of fever for influenza B was about 20.3 hours with baloxavir, and around 34–35 hours with neuraminidase inhibitors (oseltamivir, zanamivir, laninamivir). This suggests that baloxavir may shorten fever duration in pediatric influenza B cases compared to other antivirals .
Viral Shedding and Infectious Period
The period during which a person with influenza B can spread the virus (infectious period) is generally around 4.8 days, based on studies of viral shedding in healthy adults. Most people stop being infectious after about 5 days, but some may shed the virus for up to 10 days, especially children, elderly, or those with weakened immune systems Carrat2008Moghadami2016. Only about 5% of cases remain infectious for more than 2.9 days, and isolation for four days after symptom onset is usually enough to prevent most transmissions .
Special Populations: Immunocompromised and Hospitalized Patients
In immunocompromised individuals, such as pediatric solid organ transplant recipients, influenza B infection can be more severe and may require longer hospital stays. In one study, the median hospital stay was 3 days, but some cases lasted much longer (up to 79 days), especially if complications occurred . Among ICU-admitted patients with complications, those infected with the B/Yamagata lineage had longer hospital and ICU stays compared to those with B/Victoria (median 15 vs. 7.5 days in hospital; 8.5 vs. 5.5 days in ICU) .
Outbreaks in High-Risk Settings
During outbreaks in high-risk settings, such as cystic fibrosis centers, influenza B can cause significant illness and prolonged symptoms. In one outbreak, patients experienced a persistent reduction in lung function for at least three months after infection, highlighting the potential for longer-term impacts in vulnerable groups .
Conclusion
The typical duration of influenza B illness in healthy individuals ranges from about 6 to 10 days, increasing with age. Fever usually resolves within 1–2 days with effective antiviral treatment in children. Most people are infectious for about 5 days, but this period can be longer in children, the elderly, or immunocompromised patients. In high-risk or hospitalized populations, illness and recovery may be prolonged, especially if complications arise.
Sources and full results
Most relevant research papers on this topic
Time lines of infection and disease in human influenza: a review of volunteer challenge studies.
Viral shedding in influenza virus infection peaked on day 2, with symptomatic infection occurring in 66.9% of healthy volunteers, and viral shedding peaks on day 4.
Influenza B virus infection in pediatric solid organ transplant recipients.
Influenza B infection is potentially life-threatening in pediatric solid organ transplant recipients, requiring annual immunization for these individuals and their household contacts.
Comparative Epidemiology of Influenza B by Lineage in Intensive Care Unit-Admitted Patients with Complications: A Nationwide Study in Taiwan, 2013-2017.
B/Yamagata influenza B virus causes more severe illness in ICU-admitted patients with complications than B/Victoria in Taiwan, with older patients and longer hospitalization durations.
DOI