Duration of fever in influenza
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Duration of Fever in Influenza: Key Findings
Fever Duration by Influenza Type and Subtype
Research consistently shows that the duration of fever in influenza varies by virus type and subtype. Influenza B infections generally cause a longer duration of fever compared to influenza A subtypes (A(H1N1)pdm09 and A(H3N2)) in both children and adults, regardless of the antiviral treatment used Sun2024Tani2020Tani2022+1 MORE. For example, in children treated with laninamivir, the median duration of fever for influenza B was significantly longer than for influenza A Tani2020Tani2022. This pattern has been observed across multiple influenza seasons and with different antiviral drugs.
Impact of Antiviral Treatments on Fever Duration
Several antiviral drugs are used to treat influenza, including baloxavir marboxil, oseltamivir, zanamivir, and laninamivir. Studies comparing these treatments found that:
- For influenza A (both H1N1pdm09 and H3N2), the median duration of fever after treatment is typically around 20–27 hours, with no significant differences between baloxavir and neuraminidase inhibitors (NAIs) such as oseltamivir, zanamivir, or laninamivir Sun2024Saito2020Ikematsu2019+1 MORE.
- For influenza B, baloxavir may shorten the duration of fever by about 15 hours compared to NAIs, with median durations of approximately 20 hours for baloxavir and 34–35 hours for NAIs .
- The effectiveness of laninamivir remains consistent across all influenza types, but fever duration is still longer for influenza B than for influenza A Tani2020Tani2022.
Factors Influencing Fever Duration
Other factors that influence the duration of fever in influenza include:
- Age: Younger children tend to have longer fevers than older children and adolescents Suzuki2007Mawatari2019.
- Initial Fever Severity: Higher body temperature at the time of clinic visit is associated with a longer duration of fever .
- Virus Resistance: In rare cases, resistance to oseltamivir (such as the H275Y mutation) can significantly prolong fever duration, with resistant cases showing mean fever durations of 89 hours compared to 40 hours in sensitive cases .
- Emergence of Drug-Resistant Variants: The appearance of polymerase acidic (PA) variants after baloxavir treatment does not significantly prolong fever duration Saito2020Ikematsu2019Wagatsuma2022.
Natural Course of Fever Without Antivirals
In untreated or placebo-treated influenza, fever typically peaks within the first two days of illness and can last several days. In volunteer challenge studies, fever was observed in about 37–41% of influenza A cases and 7.5% of influenza B cases, with systemic symptoms peaking on day 2 and total symptom scores peaking on day 3 . In children, fever duration is negatively associated with age and positively associated with the highest recorded temperature .
Fever Duration and Severe Influenza
Fever lasting more than three days is a risk factor for severe influenza, both for type A and B, and is used in clinical models to help differentiate and predict severe cases in children .
Conclusion
The duration of fever in influenza varies by virus type, patient age, and treatment. Influenza B generally causes longer fevers than influenza A. Antiviral treatments can reduce fever duration, with baloxavir showing particular benefit for influenza B. Age, initial fever severity, and viral resistance also play important roles in how long fever lasts during influenza infection.
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