U.S. regulators are considering revising the rules for COVID-19 vaccines to allow people to opt for a different type of shot for their booster than what they originally received, a move that would enable people who got the Johnson & Johnson vaccine to receive the Moderna or Pfizer dose as their next shot.
Likewise, a person who got the Moderna or Pfizer vaccine might be able to boost with J&J or the other mRNA shot.
No decision has been made, and it's not clear how soon mixed doses could happen.
The U.S. Food and Drug Administration would need to amend its authorizations of the three vaccines available to Americans, and the Centers of Disease Control and Prevention would have to endorse the idea.
But in a meeting Friday with independent advisers, senior government officials suggested they were open to the idea.
"It does seem like there's some consensus that this is an important option for people to have," said Dr. Peter Marks, a senior FDA official who oversees vaccine regulation.
Amanda Cohn, a senior adviser for vaccines at CDC's National Center for Immunization and Respiratory Diseases, said having "allowable language" from the FDA would be helpful from a public health perspective. One concern, she said, are the 15 million people who have received the J&J shot but either might not have access to a second dose or are concerned about the risk of rare but serious blood clots that the vaccine poses to women of childbearing age.
"If there's not any allowable language in the FDA factsheets or EUA authorization, then those individuals are left behind," she said.
While Marks said providing regulator flexibility was possible, he asked the advisory panel to weigh in on what data might be needed to make such a decision. He didn't offer a timetable and suggested he would be interested in collecting more real-world data first.
"We don't know from the short studies what the longer-term effects of mix and match will be. And we just don't have those data," he told the advisory panel.
Early results from a recent study by the National Institutes of Health found that boosting with a different shot than what was received the first time around appears to be safe and effective. What's more is that the study found J&J recipients wound up with higher antibody levels if they were boosted with Moderna or Pfizer.
The ability to mix vaccine brands also could be of interest to male teens and young adults, who are more likely to experience heart inflammation following a shot of Moderna or Pfizer. While treatable and typically mild, there have been reports of hospitalization among that population.
Cohn said there do not appear to be any safety concerns with mixing booster doses of any type.
"I think the safety data that has been presented today is very supportive, especially in light of the culmination of the millions of doses of these products that we've seen given and the safety evidence from all of those vaccines," she said.
Dr. Ofer Levy of Boston Children's Hospital, a panel member, said the government should be ready to move forward quickly to allow for mixed boosters.
"In the real world, all these kinds of combinations or extra boosters are already happening," he said.
ABC's Sony Salzman contributed to this report.