The effectiveness of the vaccines of Pfizer and of Moderna against COVID-19 decreased from 91% to 66% since the Delta variant became predominant in the US, according to figures released today by the US health authorities.
These data come from a study involving thousands of health workers and hospital workers in six states, which was conducted to test the performance of vaccines in real-life settings. Participants were tested weekly for symptomatic and asymptomatic infections.
Almost all of the vaccinated health workers had been vaccinated with Pfizer or Moderna vaccines.
Between December 2020 and April 2021, the effectiveness of vaccines in preventing infection was 91%, according to data published by the US Centers for Disease Control and Prevention (CDC), the country's main federal public health agency.
However, during the weeks when the Delta variant became dominant, i.e. when it was responsible for more than 50% of cases according to the sequence, efficacy dropped to 66%.
The authors of the study note, however, that this decline may not only be caused by the Delta variant, but also by an efficiency that declines over time.
«While these interim data suggest a modest decline in the effectiveness of Covid-19 vaccines in preventing infection, the fact that the reduction in infections remains at 66% underscores the continued importance and benefit of vaccination,» the authors of this study highlight.
The data released today showed no difference between the efficacy of Moderna's vaccine and Pfizer's vaccine.
This reduction in efficiency compared to the Delta variant has been noted by several studies, even though the exact size differs from one to another.
That's one of the reasons health authorities proposed last week to announce a booster vaccination campaign starting in mid-September, which will apply to all American adults who received their second dose eight months earlier.
Vaccine protection against severe cases of the disease, hospitalisation and deaths, however, remains high, authorities insist.
Another study released today by the CDC, conducted on patients in Los Angeles from early May to late July, shows that the unvaccinated were 29 times more likely to be hospitalized than those who were vaccinated.
Delta became dominant in the United States in early July and is now responsible for over 98% of infections.
