Coronavirus infections in care home residents fell by 62% from five weeks after they received their first vaccine dose, Government-funded research has found.
Residents in England who were infected after having the vaccine may also be less likely to transmit the virus, according to initial findings from the Vivaldi study.
The study, led by researchers from University College London, examined data on 10,412 elderly care home residents from 310 care homes between December and mid-March.
They looked at the number of infections confirmed with PCR tests within specific time periods after vaccination and compared this to the number of infections that occurred before vaccination.
They calculated the risk of infection was 56% lower from four weeks after a single dose of either the Pfizer or Oxford-AstraZeneca vaccine, and 62% lower after five weeks.
The “substantial protection” lasted until at least seven weeks after vaccination, which the researchers say “provides some evidence” to support the UK’s decision to extend the dose interval.
They also found samples from positive coronavirus tests taken at least 28 days after a first dose of the vaccine contained less viral material, which suggests a lower level of transmissibility.
The results also suggest both vaccines are effective against the highly transmissible UK variant, which was prevalent in care homes and wider society during the study period.
Laura Shallcross, from UCL’s 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.”
Among care home residents who had previously contracted coronavirus, a single vaccine dose appeared to have little impact, suggesting they were already well protected.
The authors say this early finding requires further study, as just 11% of the participants had previously contracted the virus.
Further research is also needed to examine how effective a first dose is after eight to 12 weeks, and the impact of a second dose.
This will feed into policy decisions regarding the ongoing need for disease control measures in care homes, such as visitor restrictions, which the authors note continue to have a detrimental impact on families.
A small number of residents – 439 – were living in care homes where the vaccine had been rolled out but they had not been vaccinated.
These residents had lower rates of infection confirmed by PCR tests than the wider never vaccinated group.
This suggests there may be a lower risk of exposure or it could be the result of a herd immunity effect in the care home.
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.”
The study is a collaboration between researchers from UCL and the University of Birmingham and healthcare providers including Four Seasons Healthcare, HC-One, The Orders of St John Care Trust, and Friends Of The Elderly.
It is published on the preprint server medRxiv.