Vaccine-preventable outbreak of acute respiratory illness and pneumonia associated with adenovirus at a U.S. Marine Corps training center.

Adenovirus outbreaks have long been a cause of acute respiratory disease, hospitalization, and death in otherwise young, healthy military recruits. The administration of oral, live attenuated adenovirus (AdV) vaccine against AdV types 4 and 7 has been critical in preventing outbreaks in this population.1-4 In early July 2024, a spike in recruit hospitalizations for AdV pneumonia was recognized at the Marine Corps Recruit Depot (MCRD) San Diego, and an outbreak investigation commenced. From July 1 through September 23, 2024, a total of 212 AdV cases, including 28 hospitalizations, were identified among trainees and staff. Non-pharmaceutical interventions, including aggressive environmental cleaning, separation of sick and well recruits, and masking, were implemented. The outbreak was not appreciably slowed, however, until AdV vaccine administration was advanced from day 11 to day 1 post-arrival of recruits to MCRD San Diego. This outbreak report demonstrates that early AdV vaccination for newly arriving recruits is an effective and essential step in preventing AdV morbidity and mortality in a recruit training setting. Despite the availability and widespread use of effective vaccines during recruit training, adenovirus outbreaks remain a significant threat to military recruits if the vaccine is not administered expediently, upon arrival to the recruit training center.