First hit of Covid reduces risk of infection by 62% among residents of care centers in England | Vaccines and immunization

A government-funded study of nursing home residents in England found their risk of Covid-19 infection – symptomatic or asymptomatic – decreased by 62% five weeks after receiving their first dose of Oxford / AstraZeneca vaccine or Pfizer / BioNTech.

Those who have been infected after receiving the vaccine may also be less likely to transmit Covid-19, initial results have shown. The study, funded by the Department of Health and Social Affairs, is critical, given that most clinical trials and observational studies have assessed the impact of vaccines on symptomatic infections, but whether vaccines can reducing 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 have been infected,” noted UCL’s Dr Laura Shallcross, author of the ‘analysis.

Researchers followed more than 10,400 nursing home residents in England (with an average age of 86) between December and March, comparing the number of infections occurring in the vaccinated and unvaccinated groups – using data extracted from the routine monthly PCR tests.

Both vaccines reduced the risk of infection by approximately 56% at 28 to 34 days after the first dose and by 62% at 35 to 48 days. The effect is maintained for at least seven weeks, the authors concluded in their review, which has yet to be peer reviewed.

These data are remarkable, given that older people with underlying diseases have been largely excluded from vaccine trials. It also supports the UK’s decision to extend dose intervals beyond what has been studied in clinical trials.

The beneficial impact on transmission was reinforced by the results of a lower viral load in positive tests after vaccination – if there is less viral material detected, this suggests a lower level of infectivity, which is related to transmissibility.

In clinical trials, both vaccines have been shown to be very effective in reducing the risk of serious illness and death. But understanding whether these interventions can thwart the spread of the disease is imperative for public health policies, given that vaccination worldwide will take a long time, the risk of the emergence and percolation of vaccine-resistant viral variants and that the vaccines have not yet been proven safe. and effective in children.

Other evidence that both vaccines have a beneficial impact on transmission has already been reported.

Preliminary data on the Oxford vaccine from the UK arm of the trial, in which researchers obtained weekly respiratory samples from volunteers, indicated that after a single dose of vaccine, PCR-positive Covid cases ranged from 49% and 78%, which suggests that it may help reduce the spread of the disease. However, this study was not designed to assess the impact of the vaccine on transmission, so participants were not randomized and other factors may explain the results.

Separately, a single dose of the Pfizer vaccine could reduce asymptomatic infections by 75%, Cambridge doctors suggested, after recording the sharp drop in infections after 12 days of the first shot in an analysis of Covid tests performed on workers at health in January. The findings build on equally positive results from Public Health England’s Siren study, which found infections among healthcare workers, some with symptoms and others without, fell by 70% after one dose of Pfizer vaccine and 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 nursing homes – was that it only tracked the impact of ‘a dose of vaccine, Shallcross said.

“We have to keep doing this kind of work to see what happens after two doses.”