Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Outbreak at a College With High Coronavirus Disease 2019 (COVID-19) Vaccination Coverage-Connecticut, August 2021-September 2021

Background. During August 2021-September 2021, a Connecticut college experienced a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Delta variant outbreak despite high (99%) vaccination coverage, indoor masking policies, and twice-weekly testing. The Connecticut Department of Public Health investigated characteristics associated with infection and phylogenetic relationships among cases. Methods. A case was a SARS-CoV-2 infection diagnosed by a viral test during August 2021-September 2021 in a student. College staff provided enrollment and case information. An anonymous online student survey collected demographics, SARS-CoV-2 case and vaccination history, and activities preceding the outbreak. Multivariate logistic regression identified characteristics associated with infection. Phylogenetic analyses compared 115 student viral genome sequences with contemporaneous community genomes. Results. Overall, 199 of 1788 students (11%) had laboratory-confirmed SARS-CoV-2 infection; most were fully vaccinated (194 of 199, 97%). Attack rates were highest among sophomores (72 of 414, 17%) and unvaccinated students (5 of 18, 28%). Attending in-person classes with an infectious student was not associated with infection (adjusted odds ratio [aOR], 1.0; 95% confidence interval [CI], .5-2.2). Compared with uninfected students, infected students were more likely to be sophomores (aOR, 3.3; 95% CI, 1.1-10.7), attend social gatherings before the outbreak (aOR, 2.8; 95% CI, 1.3-6.4), and complete a vaccine series >= 180 days prior (aOR, 5.5; 95% CI, 1.8-16.2). Phylogenetic analyses suggested a common viral source for most cases. Conclusions. SARS-CoV-2 infection in this highly vaccinated college population was associated with unmasked off-campus social gatherings, not in-person classes. Students should stay up to date on vaccination to reduce infection.