The Vivaldi 2 study, led by University College London, was set up in June 2020 to investigate SARS-CoV-2 transmission, infection outcomes, and immunity in residents and staff in care homes in England. Between 8 December 2020 and 15 March 2021 the study compared vaccinated and unvaccinated care home residents in England by using routine asymptomatic polymerase chain reaction (PCR) testing. Of 10,412 residents, 9,160 were vaccinated with either the Oxford/AstraZeneca or Pfizer vaccines, and PCR test results were used to compare the number of infections occurring in vaccinated and unvaccinated groups in order to estimate the effects of a first vaccine dose.
One of the most significant findings of the study is 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.
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.
Among care home residents who had previously been infected by SARS-CoV-2, a single vaccine dose appeared to have little impact, suggesting people who have a prior infection are already well protected. However, only 11% of people in the study had been infected with SARS-CoV-2 before, so these early findings require further research.
The researchers also looked at the Ct (Cycle threshold) values of the PCR-positive tests. The Ct value is the number of cycles it takes to detect the viral DNA – a higher Ct value means there is less viral material to detect and suggests a lower level of infectiousness (transmissibility). The researchers found a higher average Ct value for positive tests 28 days after vaccination compared to positive tests before vaccination (an average Ct value of 31.3 compared to 26.6).
The first vaccine dose was associated with substantially reduced SARS-CoV-2 infection risk in care home residents from 4 weeks to at least 7 weeks.
It also infers that the vaccines protect against the highly transmissible UK variant, as this was prevalent during the study period 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.