Dr. Anthony Fauci said the Johnson & Johnson COVID-19 vaccine likely "should have been a two-dose vaccine to begin with."
An FDA panel of outside experts last week recommended that the agency authorize J&J's request to offer a booster dose.
On ABC News' "This Week," Fauci said people who received the J&J shot not be worried about the recommendation.
Dr. Anthony Fauci, the longtime director of the National Institute of Allergy and Infectious Diseases, said Sunday that a recent recommendation by a US Food and Drug Administration advisory panel indicated that the Johnson & Johnson COVID-19 vaccine should've required two doses from the get-go.
An FDA panel consisting of outside experts on Friday voted to support Johnson & Johnson's application to offer a booster shot of its vaccine, as Insider previously reported. The panel recommended that all individuals who were vaccinated with the single-shot J&J vaccine get a second dose, regardless of their age or any underlying conditions.
When it was authorized for emergency use by the FDA in February, the J&J shot was offered as a single-shot option for vaccination, compared to offerings from Pfizer and Moderna, which required two doses for full vaccination.
"We know this vaccine was not as effective as others," ABC News' Martha Raddatz said Sunday during on "This Week." "So should those 15 million people who got the vaccine be concerned, given these recommendations?"
"No, not at all, Martha," Fauci replied. I think that they should feel good about it because what the advisors to the FDA felt is that given the data that they saw, very likely this should have been a two-dose vaccine to begin with," Fauci said.
The FDA committee voted unanimously in favor of recommending the additional shot, recommending that anyone who received the shot get an additional dose two months after their first one.
"I think that's a very good thing and I think it's very favorable for those who have received the J&J vaccine," Fauci added of the FDA panel's recommendation.
"I don't see that as a problem at all," he told Raddatz.
The FDA last week released its own summary of J&J's application that noted several limitations in the evidence that J&J submitted for a booster shot, as Insider previously reported. The FDA does not have to accept the panel's recommendation to authorize the booster, but it typically accepts such recommendations, per the previous Insider report.
One recent study suggested that mixing a J&J shot with a dose of Moderna or Pfizer's vaccine would provide a better initial immune response.
"But the data of boosting the J&J first dose with the J&J second dose is based on clinical data," Fauci said Sunday. "So what's going to happen is that the FDA is going to look at all those data, look at the comparison and make a determination of what they will authorize."
The agency last month authorized boosters of the Pfizer vaccine for individuals over the age of 65 or other people at high risk from COVID-19. A panel of outside experts at the FDA also last week recommended authorizing the third dose of Moderna's vaccine for high-risk groups.