Severe symptoms decline sharply after 7 daysThe findings suggest that, during the first week, symptom burden was substantial.
Nearly 40% of participants reported severe fatigue on the first day of symptoms, and roughly 37% noted severe cough.
Approximately 80% reported moderate-to-severe cough and moderate-to-severe fatigue on the first day.
Two weeks after the onset of symptoms, less than 6% reported moderate-to-severe symptoms.
Mild symptoms persist for up to a monthWhile severe and moderate-to-severe symptoms declined rapidly, milder symptoms persisted longer.
Influenza is often thought of as a miserable, short-lived illness. But a new study suggests that while the most severe symptoms may fade within days, many people experience lingering effects for up to a month after infection.
The study, published in BMC Infectious Diseases, tracked 724 US adults with confirmed influenza during the severe 2024-25 flu season. Researchers from Pfizer and CVS Health followed up with participants for four weeks after diagnosis, asking them to report the presence and severity of 12 common flu symptoms, including fatigue, fever, chills, body aches, sore throat, and gastrointestinal (gut) symptoms.
Participants completed symptom surveys daily during the first week after the onset of symptoms, then again on days 10 and 14, and at the four-week mark. The average participant was 42 years old, nearly three-quarters were female, and about half had received a flu vaccine. Roughly 28% had at least one underlying medical condition.
Severe symptoms decline sharply after 7 days
The findings suggest that, during the first week, symptom burden was substantial. On the first day of illness, participants reported an average of nearly nine symptoms, including roughly six that were moderate-to-severe and two to three that were severe. The most common symptoms were runny or stuffy nose, cough, fatigue, headache, body aches, fever, chills, and sore throat.
Nearly 40% of participants reported severe fatigue on the first day of symptoms, and roughly 37% noted severe cough. Approximately 80% reported moderate-to-severe cough and moderate-to-severe fatigue on the first day.
Four weeks after infection, 32% of participants still reported a runny or stuffy nose, roughly 27% continued to have a cough, and around 19% reported lingering fatigue.
Prevalence of both severe and moderate-to-severe symptoms dropped sharply over time, with roughly 17% reporting moderate-to-severe cough and just under 8% reporting moderate-to-severe fatigue on the seventh day. Two weeks after the onset of symptoms, less than 6% reported moderate-to-severe symptoms. Participants reporting severe symptoms dropped to under 1.5% by the seventh day and approached zero by the two-week mark.
Mild symptoms persist for up to a month
While severe and moderate-to-severe symptoms declined rapidly, milder symptoms persisted longer.
Four weeks after infection, 32% of participants still reported a runny or stuffy nose, roughly 27% continued to have a cough, and around 19% reported lingering fatigue. Smaller numbers of participants reported headaches (15.0%), body aches (13.6%), and sore throat (8.6%) at the four-week mark.
Adults at higher risk for flu complications, including adults over 65 years and those with chronic health conditions, experienced a greater overall symptom burden than other participants, but their trajectory of recovery closely mirrored that of the general study population, with severe symptoms tapering off within days and milder symptoms lingering for weeks.
Findings reflect flu’s extended symptom burden
Using clinical indicators, like hospitalization rates, “which primarily reflect complications rather than day-to-day symptom experience,” to assess symptom severity may underestimate the true symptom burden of influenza, write the researchers. The new findings help capture the condition’s full impact on daily life.
“The persistence of milder symptoms being present [at] 4 weeks’ follow-up highlights the extended burden of influenza illness among adults,” they write.
The results support continued investment strategies to mitigate the burden of influenza.
The results highlight the value of preventive measures and closely tracking recovery trajectories. “The results support continued investment strategies to mitigate the burden of influenza, including vaccination, and reinforce the public-health relevance of monitoring outpatient symptom trajectories to inform clinical care, workplace policies, and seasonal preparedness,” write the researchers.
