RESPIRATORY viruses may temporarily alter susceptibility to other infections, with effects varying by pathogen and age. A prospective US cohort study involving 3,560 adults and children suggests that experiencing one respiratory illness may reduce the likelihood of developing another within the following two weeks. However, certain infections, including COVID-19, were associated with increased risks of subsequent illness.
Researchers followed participants aged 6 months to 49 years in Washington and Oregon between 2022 and 2024, collecting weekly nasal swabs and additional samples during symptomatic illness. Testing covered nine respiratory virus groups, including SARS-CoV-2, influenza, respiratory syncytial virus (RSV), rhinovirus, and adenovirus. Overall, 83% of participants experienced at least one respiratory illness during 4,299 person-years of follow-up. Adjusted analyses indicated that recent respiratory illness was frequently associated with a reduced likelihood of developing another symptomatic viral illness within 14 days. The findings support the possibility of viral interference, whereby immune responses triggered by one infection temporarily suppress another virus.
The strongest associations involved rhinovirus, a leading cause of the common cold. Recent rhinovirus illness was associated with substantially lower short-term risks of subsequent RSV, parainfluenza, and endemic coronavirus illnesses, with adjusted hazard ratios ranging from 0.07–0.27. These associations were observed across age groups, suggesting that common respiratory infections may influence susceptibility to unrelated viruses. However, the apparent protective effect was neither universal nor sustained across longer observation periods.
Unlike rhinovirus, recent COVID-19 illness was associated with increased short-term risks of adenovirus and parainfluenza illnesses, with adjusted hazard ratios of 3.20–3.33. Among children aged 6 months–4 years, recent influenza illness was also associated with increased susceptibility to subsequent symptomatic SARS-CoV-2 infection. Young children experienced particularly frequent respiratory illnesses, averaging 4.7 episodes per person-year compared with 1.4 among adults. Importantly, associations generally weakened when asymptomatic infections were included, highlighting differences between infection and symptomatic disease.
Although the findings suggest that respiratory viruses may influence subsequent illness risk, they do not establish that one infection directly protects against another. Changes in social contact, testing frequency, and underlying immune responses may contribute to the observed associations. The study also excluded adults aged 50 years and older, limiting its generalizability. For clinicians, these findings highlight the complexity of sequential respiratory infections and the importance of distinguishing symptomatic illness from viral infection when interpreting respiratory surveillance data.
Reference
Hoffman KL et al. Interactions between respiratory viruses in US adults and children: a prospective cohort study. medRxiv. 2026. doi:10.64898/2026.10.15.26365672.
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