A government-funded study of care home residents in England has found that their risk of infection with Covid-19 – either symptomatic or asymptomatic – fell by 62% five weeks after they received their first Oxford/AstraZeneca or Pfizer/BioNTech vaccine dose.
Those who were infected after having the vaccine may also be less likely to transmit Covid-19, initial findings showed. The study, funded by the Department of Health and Social Care, is key, given that most clinical trials and observational studies evaluated the impact of the vaccines on symptomatic infections, but whether the vaccines can reduce asymptomatic infections – which play a crucial role in the spread of the virus – is still unclear.
“It’s helpful to look at people who don’t have symptoms because what you want to do is reduce the total number of people who’ve been infected,” noted UCL’s Dr Laura Shallcross, an author of the analysis.
Researchers tracked more than 10,400 care home residents in England (with an average age of 86) between December and March, comparing the number of infections occurring in vaccinated and unvaccinated groups – using data retrieved from routine monthly PCR testing.
Both vaccines reduced the risk of infection by about 56% at 28-34 days after the first dose, and 62% at 35-48 days. The effect is maintained for at least seven weeks, the authors concluded in their analysis, which has not yet been peer-reviewed.
This data is notable, given older adults with underlying illnesses have largely been excluded from vaccine trials. It also supports the UK’s decision to extend dose intervals beyond what was studied in clinical trials.
The beneficial impact on transmission was further bolstered by the findings of a lower viral load in positive tests post-vaccination – if there is less viral material detected, that suggests a lower level of infectiousness, which is linked to transmissibility.
In clinical trials, both vaccines were found to be very effective in reducing the risk of severe illness and death. But understanding whether these interventions can thwart the spread of the disease is imperative to public health policy, given vaccinating the world will take a long time, the risk of vaccine-resistant virus variants emerging and percolating, and that vaccines have not yet been proved safe and effective in children.
Other evidence that both vaccines have a beneficial impact on transmission has been previously reported.
Preliminary data on the Oxford vaccine from the UK arm of the trial, in which the researchers obtained weekly respiratory samples from volunteers, indicated that after a single dose of vaccine, PCR-positive cases of Covid fell between 49% and 78%, suggesting it may help to reduce the spread of the disease. However, this study was not designed to assess the impact of the vaccine on transmission so participants weren’t randomised and other factors could explain the findings.
Separately, a single dose of the Pfizer vaccine could reduce asymptomatic infections by 75%, doctors in Cambridge suggested, after recording the sharp fall in infections after 12 days of the first shot in an analysis of Covid tests performed on healthcare workers in January. The findings build on similarly positive results from Public Health England’s Siren study, which found infections in healthcare workers, some with symptoms and some without, fell by 70% after one dose of the Pfizer vaccine, and by 85% after two.
The main limitation of the latest analysis – which is derived from the larger Vivaldi study that was launched last June to investigate Covid-19 infections in care homes – was that it only tracked the impact of one vaccine dose, said Shallcross.
“We need to keep doing this kind of work to see what happens after two doses.”