Outbreak of COVID-19 Among a Highly Vaccinated Population Aboard a U.S. Navy Ship After a Port Visit — Reykjavik, Iceland, July 2021

On July 27, 2021, a fully vaccinated* crew member on a U.S. Navy ship who had been symptomatic with cough and congestion for 4 days was evaluated in the ship’s onboard medical department and received a positive test result^† for SARS-CoV-2, the virus that causes COVID-19. The ship had approximately 350 personnel on board^§; COVID-19 vaccination rate was >98%.^¶ The ship had been on an 8-week deployment with port visits in Norway (July 13–14) and in Reykjavik, Iceland (July 18–21). Masking and physical distancing mandates on the ship were relaxed while at sea but were immediately reimplemented upon identification of the crew member’s positive test result. During the deployment, personnel had permission to go ashore only during the Iceland port visit and only if they were fully vaccinated. Before July 27, no one had been evaluated at the onboard medical department for respiratory symptoms. Although reported COVID-19 incidence was low in Iceland just before the port visit (17.5 per 100,000 population on July 18), incidence increased approximately elevenfold, to 219.5 per 100,000 on July 27 with emergence of the B.1.617.2 (Delta) variant.** At the onset of the COVID-19 pandemic, outbreaks on some U.S. Navy ships led to attack rates greater than 25% (1) of the crew in the confined environment. In this outbreak during Delta variant predominance, the combination of a high vaccination rate with prevention strategies resulted in a lower (6.3%)