The U.S. government's first look at the real-world use of COVID-19 vaccines found their effectiveness was nearly as robust as it was in controlled studies.

The two vaccines available since December—Pfizer and Moderna—were 90% effective after two doses, the Centers for Disease Control and Prevention reported Monday. In testing, the vaccines were about 95% effective in preventing COVID-19.

"This is very reassuring news," said the CDC's Mark Thompson, the study's lead author. "We have a vaccine that's working very well."

The study is the government's first assessment of how the shots have been working beyond the drugmakers' initial experiments. Results can sometimes change when vaccines are used in larger, more diverse populations outside studies.

With nearly 4,000 participants from six states, the study focused on health care workers, first responders and other front-line workers who had first priority for the shots. They were given nasal swab test kits to use every week to check for signs of infection.

"The evidence base for (currently available) COVID-19 vaccines is already strong, and continues to mount ever higher with studies like this one," said David Holtgrave, dean of the University at Albany's School of Public Health, in an email.

The study included roughly 2,500 volunteers who got two vaccine doses, about 500 who got one dose and about 1,000 who did not get vaccinated.

The researchers counted 205 infections, with 161 of them in the unvaccinated group. Of the remaining 44, the CDC said 33 of them were in people apparently infected with two weeks of their last shot, the point at which they are considered fully vaccinated.

No one died, and only two were hospitalized. Thompson did not say whether the people hospitalized were vaccinated or not.

"These findings should offer hope to the millions of Americans receiving COVID-19 vaccines each day and to those who will have the opportunity to roll up their sleeves and get vaccinated in the weeks ahead," CDC Director Dr. Rochelle Walensky, in a statement. "The authorized vaccines are the key tool that will help bring an end to this devastating pandemic.

Different researchers have tried to look at how the vaccines have performed including work done in Israel and the United Kingdom, and a U.S. study of Mayo Clinic patients.

Unlike the Mayo study, which focused on hospitalization and death, the CDC study looked for any infection—including infections that never resulted in symptoms, or were identified before people started feeling sick.

About two-thirds of the participants who were vaccinated got Pfizer shots, one-third got Moderna and five got the newest shot from Johnson & Johnson. The study was done in Miami; Duluth, Minnesota; Portland, Oregon; Temple, Texas; Salt Lake City; and Phoenix and other areas in Arizona. .

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—Mike Stobbe