• The NIH said it started a trial looking at what happens when an adult who is fully vaccinated with one type of COVID vaccine, like Pfizer's, is boosted with a different shot about three to four months later.
• The trial will include about 150 adults who have been fully vaccinated with the Johnson & Johnson, Moderna or Pfizer Covid vaccines, according to the agency.
• Federal health officials said people who have not yet received an authorized vaccine are also eligible to enroll in the trial in a separate group.

The National Institutes of Health announced Tuesday it has started an early stage clinical trial looking at what happens when an adult who is fully vaccinated with one type of COVID-19 vaccine, like Pfizer's, is boosted with a different shot about three to four months later.

The trial will include about 150 adults who have received one of the three COVID vaccine regimens currently available under the Food and Drug Administration's emergency use authorization: Johnson & Johnson's, Moderna's or Pfizer's.

Federal health officials said people who have not yet received an authorized vaccine are also eligible to enroll in the trial in a separate group. Those volunteers will receive two doses of Moderna's vaccine and will be assigned to receive a booster dose of one of the three vaccines about 12 to 20 weeks later, officials said.

"Although the vaccines currently authorized by the U.S. Food and Drug Administration offer strong protection against COVID-19, we need to prepare for the possibility of needing booster shots to counter waning immunity and to keep pace with an evolving virus," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, which is part of the NIH.

"The results of this trial are intended to inform public health policy decisions on the potential use of mixed vaccine schedules should booster doses be indicated," he added.

The trial comes as drugmakers and some scientists now say people will likely need a booster dose of the COVID-19 vaccines and possibly additional shots each year, just like for the seasonal flu.

Pfizer's and Moderna's COVID-19 vaccines currently require two doses given three to four weeks apart, while Johnson & Johnson's shot requires just one jab. All three vaccines have been shown to be highly effective against COVID, though company executives now say they expect that strong protection to wane over time.

Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, said last month that COVID-19 booster shots could be needed for fully vaccinated people within a year.

"So, hopefully, you know, it would be nice if it'll turn out that it'll be a year before anyone might need a booster," Marks said May 18 during a virtual news conference on the COVID-19 vaccines with high school and middle school journalists.

"But we still don't know," he added. "It could be more, it could be a little less but ... this is just something we're going to have to figure out as we go." 

Each vaccine group in the NIH trial will enroll about 25 people ages 18 through 55 years old and approximately 25 people age 56 years and older.

Twelve to 20 weeks following their initial vaccination regimen, participants will receive a single booster dose of the Moderna vaccine as part of the trial.