The July 30, 2021 CDC report on an outbreak of COVID-19 (delta variant) in Barnstable County, Massachusetts (July 6-25, 2021) used to justify guidance for vaccinated persons to wear masks indoors appears to indicate the COVID vaccines failed to reduce either the rate of infection with COVID in the vaccinated or the hospitalization rate if infected, contrary to widespread statements in the mass media by public health authorities and others. Although there are some possible if improbable explanations, no explanation for this seeming contradiction appears to be present in the report. Outbreak of SARS-CoV-2 Infections, Including COVID-19 Vaccine Breakthrough Infections, Associated with Large Public Gatherings — Barnstable County, Massachusetts, July 2021 Brown et al Morbidity and Mortality Weekly Report Early Release / Vol. 70 July 30, 2021 U.S. Department of Health and Human Services Centers for Disease Control and Prevention https://www.cdc.gov/mmwr/volumes/70/wr/pdfs/mm7031e2-H.pdf Relevant Number from Report: cases of COVID-19 associated with multiple summer events and large public gatherings in a town in Barnstable County, Massachusetts (July 6-25, 2021) 469 vaccination coverage among eligible Massachusetts residents 69.00% cases in fully vaccinated 346 73.77% cases in unvaccinated, which may include partially vaccinated and recently vaccinated (less than 14 days since final shot) 123 26.23% number of vaccinated hospitalized 4 number of unvaccinated hospitalized 1 hospitalization rate of vaccinated infected with COVID-19 1.16% hospitalization rate of unvaccinated infected with COVID-19 0.81% Number of cases sequenced 133 Number of cases with B.1.617.2 (Delta) variant 120 90.23% Number of symptomatic vaccinated cases 274 Number of symptomatic unvaccinated cases ? About Us: Main Web Site: https://mathematical-software.com/