The agency updated its guidance because of several newly released studies documenting the strong real-world effectiveness of coronavirus vaccines in protecting against infection and spread of the virus, and the rapid pace of vaccinations, now close to 3 million people a day.

For international travel, fully vaccinated people do not need to be tested before their trips unless it is required by the destination, the guidance says. For their return to the United States, fully vaccinated people should get tested and have a negative result before they board an international flight back to the United States.

Nearly 40 percent of all adults have received at least one dose of a vaccine and more than 1 in 5 adults are now fully vaccinated, including more than half of seniors, according to the CDC. On Friday, the United States is expected to pass the milestone of 100 million people getting at least one dose.

Friday’s guidance builds on the agency’s initial recommendations last month that said fully vaccinated Americans can gather with other vaccinated people indoors without wearing a mask or social distancing. Those recommendations also say that vaccinated people can come together in the same way with unvaccinated members in one other household considered at low risk for severe diseases, such as vaccinated grandparents visiting healthy children and grandchildren. Fully vaccinated people, the agency said, should keep following health and safety precautions in public, including wearing a mask.

Since the initial guidance was released on March 8, the number of Americans who are fully vaccinated has almost doubled, to over 56 million people, or about 17 percent of the total population, according to the CDC.

The agency updated its recommendations because of newly released studies showing the real-world effectiveness of the vaccines.

In a study released Monday of about 4,000 healthcare personnel and essential workers, the CDC found the Pfizer-BioNTech and Moderna vaccines reduced the risk of infection by 80 percent two weeks after one shot. Protection increased to 90 percent two weeks after the second dose. The study is significant because it is one of the first to estimate vaccine effectiveness against infection — rather than just monitoring for symptomatic cases — including infections that resulted in no symptoms, according to the CDC.

The updated guidance is another piece of good news for the air and travel industries, which have been pushing the CDC to update its recommendations. The shift is likely to give another boost to airlines, which last month recorded their best month for passenger traffic since the pandemic began more than a year ago.

Officials with the Transportation Security Administration said there were 26 days in March when more than a million people moved through security checkpoints. That number is still far below 2019 when TSA was routinely screening more than 2 million passengers a day, but significantly higher than this point last year. For example on April 1 of last year, only 124,021 people moved through those checkpoints.

In addition to urging the CDC to update its travel guidance for those who have been fully vaccinated, the industry is also pushing the administration to lift restrictions on international visitors.

Last month, a coalition of travel groups, including Airlines for America and the U.S. Travel Association, sent a four-page letter to Jeffrey Zients, the White House’s covid-19 response coordinator, saying they were eager to partner with the administration on a plan to reopen the U.S. to international visitors.

The coalition said in the letter it hoped to have a plan by May 1 that could be implemented this summer, as long as “vaccine distribution and epidemiological trends continue in a positive direction.”

But doing away with the ban may prove trickier given infection trends in Europe, where several countries are reimposing restrictions because of a new surge.