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 research.

The Vivaldi study, led by UCL researchers, 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.

Funded by the Department of Health and Social Care, the study found that a single vaccine dose was effective at preventing 56% of infections after four weeks, rising to 62% of infections after five weeks.

It also discovered that the timing and size of the protective effect was similar for both the Pfizer and Oxford-AstraZeneca vaccines.

Dr Maddie Shrotri of the UCL Institute of Health Informatics commented: ‘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.

Dr Laura Shallcross of the UCL Institute of Health Informatics said: ‘Our findings show that a single dose has a protective effect that persists from four weeks to at least seven weeks after vaccination.

‘Vaccination reduces the total number of people who get infected, and analysis of lab samples suggests that care home residents who are infected after having the vaccine may also be less likely to transmit the virus.

‘We can also infer that the vaccines protect against the highly transmissible UK variant, as this was prevalent during the study period.’

Commenting on the findings, chief medical officer for England, Professor Chris Whitty said: ‘These data add to the growing evidence that vaccines are reducing COVID-19 infections and doing so in vulnerable and older populations, where it is most important that we provide as much protection from COVID as possible.’

Minister for Care, Helen Whately said: ‘This virus sadly has the most serious and profound effect on older people living in care homes and making sure they are protected has been our priority for the last year.

‘It is brilliant to see this is having the positive effect the science suggested, not only by preventing death, but also reducing the chance of infection. This is particularly important in keeping those most at risk from the virus safe.’