Outbreak of SARS-CoV-2 in a prison: Low effectiveness of a single dose of the adenovirus vector ChAdOx1 vaccine in recently vaccinated inmates

Introduction To analyse the effectiveness of a dose of adenovirus vector ChAdOx1 vaccine (AVChOx1) in an outbreak of SARS-CoV-2 detected in a prison. Methods Observational study carried out at Brians-1 Prison, Barcelona. After detecting a case of infection, rt-PCR was administered to all prisoners (some of whom had been vaccinated 21-23 days previously with a dose of AVChOx1) and to staff. Infection rates in vaccinated and unvaccinated populations were calculated, as was vaccine effectiveness. Results One hundred and eighty-four asymptomatic prisoners (50.3% vaccinated) and 33 staff were screened. Forty-eight (25.9%) infections by the SCV-B.1.17 variant were recorded in prisoners and none in staff. Infection rates were higher in younger prisoners, immigrants, and those admitted ≥7 days previously. In all, 22.6% of vaccinated subjects were infected vs. 29.3% of unvaccinated. Vaccine effectiveness was 23%. Only 6.2 cases would have been prevented by vaccinating the unvaccinated individuals. At seven days, the rt-PCR was negative in 46.2% of vaccinated subjects vs. 13.6% of unvaccinated (p = 0.02). Discussion In a prison outbreak, a dose of AVChAdOx1 administered 21-23 days earlier did not significantly prevent the occurrence of infections, but did reduce the duration of rt-PCR positivity. Maintaining post-vaccination preventive measures is essential.