Effectiveness of Vaccination Against Reported SARS-CoV-2 Infection in United States Coast Guard Personnel Between May and August 2021: A Time-Series Analysis

Background: The United States Coast Guard (CG) began voluntary use of SARS-CoV-2 vaccines under an Emergency Use Authorization on December 16, 2020. Vaccination status is monitored through a service-wide immunization registry. Active Duty and Reserve (military) CG members are required to report any new positive test for COVID-19 to a centralized database. Methods: Between May and August 2021, vaccination effectiveness (VE) against any new report of COVID-19 was calculated according to standard formulas, using registry immunization status of cases and monthly mid-point vaccine coverage data. CG members recorded as fully vaccinated with a two-dose vaccine were compared with those with any other vaccine status. Sub-analyses were also conducted according to geographic area (Atlantic vs Pacific), age, and type of vaccine received. Results: Effectiveness of full vaccination reached a peak of 89.0% in June, then declined over the rest of the study period to 62.7% in August. In July and August, steeper declines in VE were seen in the Atlantic region. The rate of breakthrough infections remained under 1% in two-dose vaccine recipients, and did not differ between those who received the Moderna or Pfizer vaccines. No hospitalizations or deaths due to COVID-19 disease were recorded in fully vaccinated Coast Guard members. Conclusions: Coincident with the national spread of the Delta variant of SARS-CoV-2, overall vaccine effectiveness among CG personnel decreased during the summer months of 2021, but continued to provide substantial protection, as well as full protection against the most serious outcomes. Policy initiatives and outreach intended to increase vaccine coverage within this and other military populations could extend the disease prevention benefits seen in this study.