Covid-19 vaccine: care home residents gain 62% protection from one dose

A single dose of either the Oxford-AstraZeneca or Pfizer vaccines gives residents of care homes substantial protection against SARS-CoV-2 infection after four weeks, according to new pre-print findings from the Vivaldi study led by UCL researchers. The study, funded by the Department of Health and Social Care, looked at data from more than 10,000 care home residents in England, with an average age of 86, between December and mid-March, comparing the number of infections occurring in vaccinated and unvaccinated groups (as identified by the gold-standard PCR test. It found that a single vaccine dose was effective at preventing 56% of infections after four weeks, rising to 62% of infections after five weeks. Looking at the Pfizer and Oxford-AstraZeneca vaccines separately, they found that the timing and size of the protective effect was similar for both vaccines. Dr Maddie Shrotri (UCL Institute of Health Informatics) said: "Our study suggests that both vaccines currently in use in the UK are effective at reducing infections in frail, older adults. This new evidence is important because there is currently limited data about vaccine efficacy for this vulnerable population." Researchers also found evidence suggesting the infections occurring post-vaccine may be less infectious. Analysing lab samples from positive PCR tests, they found that samples taken at least 28 days after the first vaccine dose contained less of the virus, meaning they were "weaker" positives.