“Every day you get more data, and you change your guidance based on the existing data,” said Dr. Ali Khan, dean of the University of Nebraska’s College of Public Health.

Khan said the update reinforces the safety and effectiveness of the vaccines, and is another incentive for people to get vaccinated.

According to the CDC, nearly 100 million people in the U.S. — or about 30% of the population — have received at least one dose of a Covid-19 vaccine. A person is considered fully vaccinated two weeks after receiving the last required dose of vaccine.

Unvaccinated people are still advised to avoid unnecessary travel.

The new guidance says:

• Fully vaccinated people can travel within the U.S., without getting tested for the coronavirus or quarantining. People should still wear a mask, socially distance and avoid crowds, the agency says.
• For international travel, the agency says vaccinated people do not need to get a Covid-19 test before leaving, though some destinations may require it.
• Vaccinated people should still get a negative Covid-19 test before boarding a flight to the U.S., and be tested 3 to 5 days after returning. They do not need to quarantine. The agency noted the potential introduction of virus variants and differences in vaccine coverage around the world for the cautious guidance on overseas travel.

The CDC cited recent research on the real-world effects of the vaccines for its updated guidance. Already, the agency had said fully vaccinated people could visit with each other indoors without wearing masks or social distancing. It also said vaccinated people could visit with unvaccinated people from a single household under similar conditions, as long as the unvaccinated individuals were at low-risk for severe illness if infected

The U.S. began its vaccine rollout in mid-December. The first vaccines — from Pfizer and Moderna — require two doses taken a few weeks apart. A one-shot vaccine by Johnson & Johnson was given the green light by regulators at the end of February.