Pfizer and Moderna vaccines are powerfully effective against Covid-19, a C.D.C. analysis confirmed.

By Emily Anthes

The Pfizer-BioNTech and Moderna coronavirus vaccines are 94 percent effective at preventing symptomatic Covid-19 illness, according to a new study of more than 1,800 health care workers in the United States.

The research, which the Centers for Disease Control and Prevention released on Friday, provides yet more evidence that the vaccines are working well even outside controlled clinical trials.

“This report provided the most compelling information to date that Covid-19 vaccines were performing as expected in the real world,” Dr. Rochelle Walensky, the C.D.C. director, said in a statement on Friday.

“This study, added to the many studies that preceded it, was pivotal to C.D.C. changing its recommendations for those who are fully vaccinated against Covid-19.”

The findings are based on an ongoing study of health care workers in 25 states. This interim analysis included data on 1,843 health care workers who were routinely tested for infection with the coronavirus. More than 80 percent of participants were female.

Some 623 workers tested positive between January and mid-March. Those who were fully vaccinated were 94 percent less likely to develop symptomatic coronavirus infections than their unvaccinated peers, the researchers found. The figures are consistent with the efficacy estimates from the clinical trials.

The scientists also found that a single dose of the two-shot regimen was 82 percent effective at preventing symptomatic infection. That figure is higher than has been reported in other studies and may be a result of the relative youth of the study participants, who had a median age of 37 to 38. Fewer than 2 percent were 65 or older.

C.D.C. scientists had previously found that fully vaccinated health care, frontline and essential workers were 90 percent less likely to contract the coronavirus. Those findings helped allay fears that vaccinated people might still be likely to carry the virus, even asymptomatically, and spread it to others.

The concern was one of the main rationales for asking vaccinated Americans to continue to wear masks, a recommendation that the C.D.C. lifted on Thursday.

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