The effectiveness of Pfizer and Moderna's vaccines against Covid-19 infection has fallen from 91% to 66% since the Delta variant became dominant in the United States, according to data released Tuesday by US health authorities.
These data come from a study of thousands of healthcare center and hospital workers in six states to examine the performance of vaccines in real conditions.
Participants are tested weekly for both symptomatic and asymptomatic infections.
Almost all of the vaccinated health workers received the sera from Pfizer or Moderna.
Between December 2020 and April 2021, the effectiveness of vaccines in preventing infection was 91%, according to data released by the Centers for Disease Control and Prevention (CDC), the country's main federal public health agency.
But over the weeks when the Delta variant became dominant, that is, when it was responsible for more than 50% of the cases according to the sequencing, the efficiency fell to 66%.
The authors of the study warn, however, that this decline may not be caused only by the Delta variant, but also by an efficiency eroding over time.
"Although these interim data suggest a moderate reduction in the effectiveness of Covid-19 vaccines in preventing infection, the fact that the reduction in infections remains at two-thirds underscores the continued importance and benefit of vaccination." , write the authors of this work.
Data released Tuesday did not differentiate between the effectiveness of Moderna's vaccine and that of Pfizer's serum.
This drop in efficacy against the Delta variant has been pointed out by several studies, even if the precise figure differs from one to another.
This is one of the reasons put forward last week by health authorities to announce a recall campaign from mid-September, which will affect all American adults who received their second dose eight months previously.
The protection of vaccines against severe cases of the disease, hospitalizations and deaths, however, remains high, insist the authorities.
Another study released Tuesday by the CDC, conducted on patients in Los Angeles between early May and late July, shows that unvaccinated people were 29 times more likely to be hospitalized than those vaccinated.
Delta became dominant in the United States in early July, and is currently responsible for over 98% of infections.
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