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The Moderna and Pfizer-BioNTech vaccines are very effective in real-world conditions at preventing infections, the C.D.C. reported.

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The coronavirus vaccines made by Moderna and Pfzer-BioNTech are proving highly effective at preventing symptomatic and asymptomatic infections under real-world conditions, federal health researchers reported on Monday.

Consistent with clinical trial data, a two-dose regimen prevented 90 percent of infections by two weeks after the second shot. One dose prevented 80 percent of infections by two weeks after vaccination.

There has been debate over whether vaccinated people can still get asymptomatic infections and transmit the virus to others. The study, by researchers at the Centers for Disease Control and Prevention, suggested that transmission is extremely unlikely.

There also has been concern that variants may render the vaccines less effective. The study’s results do not confirm that fear. Troubling variants were circulating during the time of the study — from December 14, 2020 to March 13, 2021 — yet the vaccines still provided powerful protection.

The C.D.C. enrolled 3,950 people at high risk of being exposed to the virus because they were health care workers, first responders, or others on the front lines. None had previously been infected with the coronavirus.

Most — 62.8 percent — received both shots of the vaccine during the study period, and 12.1 percent had one shot. The participants had no previous infections with the coronavirus.

Participants collected their own nasal swabs each week, which were sent to a central location for P.C.R. testing, the most accurate type of test. The weekly swabs allowed the researchers to detect asymptomatic infections as well as symptomatic ones.

The investigators also asked participants about symptoms associated with infection, including fever, chills, cough, shortness of breath, sore throat, diarrhea, muscle aches, or loss of smell or taste. The researchers also analyzed patients’ medical records to detect illnesses.

Fifty-eight percent of the infections were detected before people had symptoms. Just 10.2 percent of infected people never developed symptoms.

Among those who were fully vaccinated, there were .04 infections per 1,000 person-days, meaning that among 1,000 persons there would be .04 infections in a day.

There were 0.19 infections per 1,000 person-days among those who had had one dose of the vaccine. In contrast, there were 1.38 infections per 1,000 person-days in unvaccinated people.

“This study shows that our national vaccination efforts are working.,” said Dr. Rochelle P. Walensky, the C.D.C. director in a news release.

The C.D.C. said this is the first of many vaccine effectiveness studies it will be conducting.

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