This community-based prospective cohort study measured the incidence of PCR-confirmed respiratory syncytial virus (RSV) acute respiratory infection (RSV-ARI) among adults aged 50 years and older following the initial COVID-19 pandemic period. The primary aim was to estimate incidence and attack rates during two consecutive surveillance years: October 2021–September 2022 (year 3) and October 2022–September 2023 (year 4). Secondary aims included describing healthcare utilisation and ARI-related complications within four weeks of illness.
The analysis covers years 3 and 4 of a 4-year prospective cohort of community-dwelling adults residing in southeast Minnesota. The study enrolled participants who were aged 50 years or older, received primary care within the Mayo Clinic system, and agreed to self-collect nasal swabs at home for laboratory testing when meeting ARI criteria. Participants were excluded if they did not meet age, residency, or provider criteria, could not comply with swab procedures, or were away from the study region beyond predefined limits during surveillance periods.
ARI was defined using previously described criteria requiring multiple upper, lower, or systemic signs or symptoms for at least 24 hours. Lower respiratory tract disease (LRTD) definitions and ARI severity categories (mild, moderate, severe) were applied based on symptom location and the need for outpatient, emergency, or inpatient care.
When participants met ARI criteria they were instructed to self-collect a nasal swab for PCR testing unless they were known to have active COVID-19; in that situation RSV testing was not performed and the episode was considered missing for RSV. Self-collected specimens were tested using FDA-approved RT-PCR assays that detect RSV and influenza A/B.
The primary outcome was the first PCR-confirmed RSV-ARI during each surveillance year. Secondary outcomes included healthcare utilisation (outpatient, ED visits, hospitalisation, ICU stay) and ARI complications (pneumonia, hospitalisation, death) within four weeks of ARI. Incidence was expressed as first RSV-positive cases per 1000 person-years (PY) with exact Poisson 95% CIs; attack rate was the percentage of participants with at least one RSV-ARI during the period with exact 95% CIs. Analyses included age- and gender-standardised incidence rates based on the 2020 US Census white population.
At the start of year 3 there were 2,500 participants (mean age 68.2 years, SD 9.3), 60.1% female and 96.4% non-Hispanic white. Prevalent conditions included diabetes (12.9%), asthma (10.7%), chronic obstructive pulmonary disease (6.7%) and congestive heart failure (4.8%). COVID-19 vaccination coverage in the cohort was 64.6% and influenza vaccination in the prior year was 44.6%. RSV vaccine became available in May 2023; only 66 participants received RSV vaccination prior to the end of follow-up, accounting for 2.37 PY after vaccination.
In 2021–2022 (year 3) there were 15 RSV-ARI cases among 2,500 participants. The crude incidence was 6.12/1000 PY (95% CI 3.42–10.09) with an attack rate of 0.60% (95% CI 0.34–0.99). Age-gender-standardised incidence was 6.17/1000 PY.
In 2022–2023 (year 4) there were 39 RSV-ARI cases among 2,446 participants. The crude incidence was 16.40/1000 PY (95% CI 11.66–22.42) with an attack rate of 1.59% (95% CI 1.14–2.17). Age-gender-standardised incidence was 16.99/1000 PY.
When stratified by age, incidence estimates for adults 50–59 years and those 60 years or older showed overlapping confidence intervals in both years. In year 4, incidence was higher among women than men (22.44/1000 PY vs 7.35/1000 PY), with corresponding differences in attack rates reported in the source data.
RSV subtype distribution shifted between seasons: RSV B predominated in 2021–2022 (92% of typed cases) and RSV A predominated in 2022–2023 (74% of typed cases). Seasonal peaks occurred in December 2021 and November 2022. No coinfections were reported in year 3; four RSV cases in year 4 had coinfections with other viruses including SARS-CoV-2, rhinovirus, parainfluenza and OC43 coronavirus.
Most RSV-ARI episodes were classified as severe due to lower respiratory tract symptoms in year 3. Standardised incidence of RSV-LRTD was 2.79/1000 PY (95% CI 0.05–5.53) in year 3 and 8.90/1000 PY (95% CI 4.26–13.53) in year 4. The study reported no hospitalisations, clinical pneumonia diagnoses, or deaths within 30 days of the identified RSV-ARI episodes across the surveillance years covered in the source report.
Strengths of the study include a community-based prospective design, PCR confirmation of infections over two respiratory seasons, and prospective follow-up for healthcare utilisation and severe outcomes. Limitations noted by the authors include single geographic location, reliance on a single collection method for PCR testing, and the policy of not testing participants for RSV when they had active COVID-19 infection. The latter led to 574 ARI episodes (25% of ARIs during years 3–4) with missing RSV status; roughly 70% of those were COVID-19 positive and thus not swabbed for RSV, which may have resulted in underestimation of RSV incidence.
Compared with the pre-pandemic period reported previously by the cohort, RSV-ARI incidence remained lower during October 2021–September 2023 but increased substantially in 2022–2023 relative to 2021–2022 (a 2.7-fold rise). The seasonal patterns, subtype shifts, and rise in incidence underscore the continued community-level relevance of RSV among adults aged ≥50 years. The authors note these findings support the rationale for ongoing RSV vaccination efforts to mitigate RSV burden in older adults. All data relevant to the analysis are reported in the article or supplementary files.