In Review, F.D.A. Stays Neutral on the Need for a Moderna Booster

The advisory committee is also likely to consider whether Moderna’s protection against severe disease and hospitalization may eventually weaken. Booster proponents argue that Moderna’s potency is declining much like Pfizer’s, which is based on the same technology, only more slowly because its initial dosage is significantly stronger than Pfizer’s and the interval between the first two shots is a week longer.

“The fundamental issue is, what are we trying to do?” said Dr. El Sahly. “Both Moderna and Pfizer seem to have waned when it comes to mild to moderate disease, especially with the Delta variant. So, if that’s the goal, a booster is needed.”

“If we want to prevent only severe disease that lands people in the hospital,” she said, “that’s a different mathematics.”

The F.D.A. has yet to release its review of Johnson & Johnson’s booster data. But in its application for a booster authorization, the company supplied details from two studies.

In a study it announced in August, the company gave a second shot of the vaccine to 17 volunteers six months after the first. In these subjects, Johnson & Johnson reported, the levels of antibodies against the coronavirus rose twelve times higher than after the initial dose. Researchers did not determine exactly how much protection that boost provided against Covid-19.

The second study, which Johnson & Johnson reported last month, indicated that a booster can indeed improve protection. In November 2020, the company launched a clinical trial with 32,000 volunteers, this time giving a second dose of the vaccine two months after the first.

The company announced that in the portion of the trial that took place in the United States, the efficacy rose to 94 percent. Worldwide, the increase was more modest, at 75 percent — around the same efficacy reported for a single dose in the U.S. over the winter. Against severe to critical Covid-19, two shots had an efficacy of 100 percent.

Noah Weiland contributed reporting from Washington, D.C.; Amy Schoenfeld Walker from Trumbull, Conn.; and Carl Zimmer from Guilford, Conn.