P-1951. Incidence of SARS-CoV-2 Infection among those 6 Months to 49 Years of Age in the Cascadia Prospective Cohort Study, Oregon and Washington, United States, July 1, 2022 – March 31, 2024

Abstract Background With the ending of the public health emergency and changes in the testing landscape, less is known about SARS-CoV-2 incidence in the community than earlier in the pandemic. Our goal was to describe SARS-CoV-2 incidence among individuals 6 months to 49 years of age enrolled in the Cascadia prospective community cohort study in Oregon and Washington, USA, by age, sex, and COVID-19 vaccination status. Covid-like illness (CLI) and vaccination status definitions Methods We included Cascadia participants enrolled July 1, 2022 – March 31, 2024. We collected weekly nasal swabs and surveys from participants; more frequently among those with SARS-CoV-2 infection. We defined SARS-CoV-2 infection as a positive or inconclusive swab result and assessed covid-like illness (CLI) symptoms ±7 days from collection date (Fig.1). We followed individuals from receipt of their first nasal swab until the earliest of SARS-CoV-2 infection, loss to follow-up, or the end of the study period. We considered age and COVID-19 vaccination status (Fig. 1) as time-varying variables. We estimated incidence as the number of SARS-CoV-2 infections per 1,000 person-weeks (p-w) of observation, along with robust-error 95% confidence intervals (CI); we considered incidence rates to vary between strata if confidence intervals did not overlap. Table Incidence of SARS-CoV-2 Infection among Cascadia study participants, by sex, age, and vaccination status, July 1, 2022-March 31, 2024 Results We observed 1,495 SARS-CoV-2 infections among 3,464 included individuals with 164,290 p-w of follow-up; overall incidence rate was 9.10 (CI 9.09, 9.11). Incidence was similar by sex, lowest among those 6m-4y, and highest among those 12-17y (Table). Overall SARS-CoV-2 incidence was higher among those previously up-to-date (10.17 [CI 9.24, 11.18]) than among those currently up-to-date with vaccines (8.42 [CI 7.72, 9.19]; Table). A quarter (178/716) of SARS-CoV-2 infections among those < 18y were asymptomatic, compared with 10% (76/785) among those ≥18. The proportions reporting ≥2 CLI symptoms were 54% (49/91) among those 6m–4y, 62% (219/354) among those 5–11y, 69% (184/265) among those 12–17y, and 85% (666/785) among those 18–49y (Fig. 2). Frequency of covid-like illness (CLI) symptoms1 reported among Cascadia study participants, by age, July 1, 2022-March 31, 2024 Conclusion Our study fills a knowledge gap about the incidence of SARS-CoV-2 in the community later in the pandemic. Incidence was lowest among those recently vaccinated. These findings will inform continued discussions about COVID-19 vaccination strategies. Disclosures Mark A. Schmidt, PhD, MPH, HilleVax: Grant/Research Support|Janssen: Grant/Research Support|Moderna: Grant/Research Support|Pfizer: Grant/Research Support|Vir Biotechnology: Grant/Research Support Neil D. Yetz, M.P.H., N/A: Expert Testimony Teresa Kimes, MS, Moderna: Grant/Research Support|Vir Biotechnology: Grant/Research Support Stephen P. Fortmann, MD, Pfizer: Grant/Research Support Holly C. Groom, MPH, Hillevax: Grant/Research Support|Moderna: Grant/Research Support Jennifer L. Kuntz, MS, PhD, Hillevax, Inc: Grant/Research Support Richard A. Mularski, MD, MSHS, MCR, Pfizer, Inc: Grant/Research Support Janet A. Englund, MD, Abbvie: Advisor/Consultant|AstraZeneca: Advisor/Consultant|AstraZeneca: Grant/Research Support|GlaxoSmithKline: Advisor/Consultant|GlaxoSmithKline: Grant/Research Support|Meissa Vaccines: Advisor/Consultant|Merck: Advisor/Consultant|Pfizer: Board Member|Pfizer: Grant/Research Support|Pfizer: Speaker at meeting|SanofiPasteur: Advisor/Consultant|Shinogi: Advisor/Consultant Helen Y. Chu, MD, MPH, Abbvie: Advisor/Consultant|Merck: Advisor/Consultant|Vir: Advisor/Consultant