A few days after contracting the flu, muscle aches fade, exhaustion lifts, and the couch starts to lose its appeal. A return to ordinary life feels overdue.
But according to a study published last week in the American Journal of Epidemiology, some people diagnosed as having flu might shed the most virus RNA just as their symptoms start to improve. It’s also when roughly four-fifths of study participants stopped isolating and started interacting with the broader community.
Viral shedding typically peaks by day 4
In 2024, the Centers for Disease Control and Prevention (CDC) issued guidance for how long people with the flu should stay apart from others to help keep them safe from infection, stating that isolation should continue until the person is fever-free for 24 hours and other symptoms have started to improve, then additional precautions, such as masking and social distancing, should be taken for another five days.
The current study, led by CDC researchers, looked at how well this guidance aligns with influenza virus RNA shedding—which they used as a proxy for infectiousness—and self-reported isolation.
The team analyzed data from 1,294 people with symptomatic influenza during the 2023–24 and 2024–25 respiratory virus seasons. Participants in Missouri, New York, and Tennessee completed symptom diaries and collected daily nasal swabs. Fifty-six percent of participants were children, and 70% had not received an annual flu vaccine.
Peak viral shedding occurred a median of four days after symptoms began, according to the data. Depending on how viral shedding was measured, 69% to 80% of participants reached their peak by the end of the recommended five-day isolation period. Ninety-seven percent to 100% had reached their peak by day six.
The authors cautioned that while viral loads may correlate with virus shedding, they do not directly measure infectiousness. “Further studies that include virus culture data would provide the opportunity to compare infectiousness with symptom duration,” they wrote.
Only 21% isolated at any point during their illness
According to the findings, most participants isolated for less than the recommended period, if they did so at all. Only 21% of participants with the flu reported isolating from household members at any point during their illness. Of those who did, the mean number of isolation days was two. More people (80%) reported isolating from the broader community during their illness, but only for a median of four days.
Among all participants, less than 1% isolated from household members for the entire recommended period, and just 5.5% isolated from the broader community for the full period.
These findings highlight the challenge of balancing practicality and impact on transmission in public health isolation recommendations.
The researchers acknowledge that isolating at home can be difficult when people live in close proximity and caregivers may need to care for children regardless of their or their children’s illness status. “These findings highlight the challenge of balancing practicality and impact on transmission in public health isolation recommendations,” write the researchers.
Still, the findings suggest that the CDC’s guidance generally aligns with the period of peak viral shedding—if people follow it.
