Introduction Military Establishments are at increased risk of rapid spread of respiratory infections. Little was known in March 2020 about SARS-CoV-2 prevalence, serological conversion, asymptomatic transmission or risk factors for transmission, particularly in military populations. Early identification, swift implementation of control measures, and investigation of a potential outbreak with Public Health England in an Army barracks following one confirmed case and 11 symptomatic personnel, allowed exploration of these questions.

Methods All adult personnel, including civilians working or living at the Barracks were invited to participate at initial visit and day-36 follow-up. Participants completed a symptom and transmission risk factor questionnaire; gave nasal and throat swabs for SARS-CoV-2 RT-PCR, infectious virus isolation, whole genome sequencing (WGS); and blood samples to detect SARS-CoV-2 and neutralising antibodies. Risk factors were statistically analysed using STATA v15.0, described in univariate analysis by relative risks and assessed using Fisher’s Exact test.

Results At first visit, 24/304 (8%) participants were RT-PCR positive and infectious virus recovered from 7/24 (29%). Seropositivity was 7% (19/285). 64% of all positive participants were asymptomatic. WGS identified more than three separate introductions, and evidence of asymptomatic transmission through genetically indistinguishable samples. Significant transmission risk factors included contact with a confirmed case, female gender, and two-person shared bathrooms. After 36 days, there were no new cases, all previously RT-PCR positive participants seroconverted, but not all developed neutralising antibodies; seropositivity was 13% (25/193).

Conclusions Most positive military personnel were asymptomatic, but those with symptoms reported ageusia or anosmia. Some RT-PCR positive participants, but none who were simultaneously positive for neutralising antibodies, had infectious virus. Initial infection rates were five times general London estimates, but effective implementation of control measures including enhanced cleaning, social distancing, and prompt isolation mitigated on-site spread. 36 days later seropositivity was below London’s rate. Ongoing risks include new COVID-19 introduction into the barracks from off-site personnel, asymptomatic transmission between cases and contacts, and use of two-person bathrooms.