Incidence of influenza and RSV
Influenza A(H3N2) was the predominant circulating influenza virus in both years. The median time from symptom onset to collection of nasal swabs was 1 day (IQR 1–2) and nasopharyngeal swabs was 2 days (IQR 1-3), which were not different among participants with or without cardiopulmonary conditions (p-values 0.589 and 0.746, respectively). Overall, 105 (3.3%) cohort participants had an influenza virus infection (influenza A(H1N1)=17; A(H3N2)=74; B=14) and 81 (2.5%) had an RSV infection. Laboratory-confirmed influenza infections were detected most in the first year in September (18.4%), followed by October in the second year (12.2%); most RSV infections were detected during August to October in the second year, with the peak in September at 22.0% (Figure 1).
Participants with cardiopulmonary conditions had a higher incidence of ARI (645/1000 person-years vs 350/1000 person-years, p<0.001) and SARI (87.8/1000 vs 3.6/1000, p<0.001) than those without. Adjusted IRR for laboratory-confirmed RSV, all influenza, influenza A, and influenza B viruses were 2.02 (95% confidence interval [CI] 1.10-3.72), 1.84 (95% CI 1.05-3.23), 1.83 (95% CI 1.00-3.34), and 1.96 (95% CI 0.42-9.21), respectively (Table 3).