The effectiveness of Pfizer and Moderna vaccines against Covid-19 has fallen from 91% to 66% since the Delta variant became dominant in the US, according to data released today by the US health authorities. These data come from a study of thousands of health care workers and hospital workers in six states that looked at the performance of vaccines in real life.
Participants were screened weekly for symptomatic and asymptomatic infections. Almost all 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 released by the US Centers for Disease Control and Prevention (CDC), the country's leading federal public health agency.
However, during the weeks that the Delta variant became dominant, that is, when it was responsible for more than 50% of the cases according to the sequence, the effectiveness dropped to 66%.
The study authors note, however, that this reduction may have been caused not only by the Delta variant, but also by an efficiency that declines over time. "While these intermediate data suggest a moderate reduction in the effectiveness of vaccines for Covid-19 in the prevention of infection, the fact that the reduction of infections remains at 66% underscores the continuing importance and benefit of vaccination ", emphasize the authors of this study.
The data released did not differ between the efficacy of the Moderna vaccine and that of Pfizer. This reduction in efficiency against the Delta variant has been noted by various studies, even if the exact size varies from one to another.
That's one of the reasons health officials last week announced a mid-September booster vaccination campaign for all American adults who received their second dose eight months earlier.
Vaccine protection against serious cases of illness, hospitalization and death, however, remains high, authorities insist.
Another study published today by the CDC, conducted on patients in Los Angeles from early May to late July, found that 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.