IMPORTANCE Characterizing rates of SARS-CoV-2 infection among vaccinated and unvaccinated persons with the same exposure is critical to understanding the association of vaccination with the risk of infection with the Delta variant. Additionally, evidence of Delta variant transmission by children to vaccinated adults has important public health implications. OBJECTIVE To characterize transmission and infection of SARS-CoV-2 among vaccinated and unvaccinated attendees of an indoor wedding reception. DESIGN, SETTING, AND PARTICIPANTS This cohort study included attendees at an indoor wedding reception in Minnesota in July 2021. Data were collected from REDCap surveys and routine surveillance interviews. The full list of attendees and a partial list of emails were obtained. Fiftyseven attendees completed the emailed survey. Eighteen additional attendees were identified from the state health department COVID-19 surveillance database. EXPOSURES Attendance at an indoor event. MAIN OUTCOMES AND MEASURES Risk of SARS-CoV-2 infection among vaccinated and unvaccinated attendees, identification of an index case, whole genome sequencing (WGS) to identify the COVID-19 variant, understanding of transmission patterns, and assessment of secondary transmission. The primary case definition was an individual with a positive SARS-CoV-2 test who attended the wedding in the 14 days prior to their illness. RESULTS Data were gathered for 75 attendees (mean [SE] age, 37.5 [13.7] years; 57 [ 76%] female individuals), of whom 56 (75%) were fully vaccinated, 4 (5%) were partially vaccinated, and 15 (20%) were unvaccinated. Of 62 attendees who were tested, 29 (47%) tested positive, including 16 of 46 fully vaccinated attendees (35%), 2 of 4 partially vaccinated attendees (50%), and 11 of 12 unvaccinated attendees (92%). Being unvaccinated was associated with a higher risk of infection compared with being vaccinated (risk ratio, 2.64; 95% CI, 1.71-4.06; P =.001). One unvaccinated adult required hospitalization. An unvaccinated child who was symptomatic on the event date was identified as the index case. Eleven specimens were available forWGS. All sequenced specimens were closely related andwere identified as the Delta variant. WGS supported secondary transmission from a vaccinated individual with SARS-CoV-2. CONCLUSIONS AND RELEVANCE This cohort study identified a COVID-19 Delta variant outbreak at an indoor event despite a high proportion of vaccinated attendees. It found that vaccination was associated with a reduced risk of infection.

The Minnesota Department of Health investigated a coronavirus disease 2019 (COVID-19) outbreak at a fitness center in Olmsted County, Minnesota. Twenty-three severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections (5 employees and 18 members) were identified. An epidemiological investigation supported by whole genome sequencing demonstrated that transmission of SARS-CoV-2 occurred at the fitness center despite following recommended prevention strategies.