Moderna officials reported in an earnings call last week that six months after a person received their second dose, the strength of their antibodies against variants was found to have waned substantially. But many scientists say that waning antibodies are expected and should not be viewed as a sign that a vaccine is working less well.

In general, U.S. scientists are frustrated with how bits and pieces of often conflicting data are dribbling out in company news releases and studies that have not been peer-reviewed or published in scientific journals.

The C.D.C. has said it is following breakthrough infections in specific population segments, including health care workers, emergency medical workers and nursing home residents. But the reports posted on its website rely on data from months ago, before the Delta variant became dominant.

“Everyone is confused because there is not a lot of published data,” Dr. Peter J. Hotez, a vaccine expert with the Baylor College of Medicine in Houston, said. “The question is how vulnerable are we, and the numbers are a bit all over the map.”

While some federal officials have argued that any booster should be tailored to the Delta variant, the administration is expected to use the same vaccines that have already been manufactured and that studies indicate work well against the Delta variant.

Mr. Bancel, Moderna’s chief executive, said his company would not have a vaccine targeted specifically to the Delta variant until the end of the year. Pfizer is on a similar track.

Both companies are expected to apply soon to the F.D.A. for authorization of a third shot of their existing two-dose vaccines. Moderna is studying the effectiveness of both a full dose and half dose as a booster.

N.I.H. researchers are conducting trials to determine if giving people a booster shot of a different vaccine from the one they originally received provides better protection. While a mix-and-match effort could be hard to organize, officials want to see that data.

“It’s a little compressed from what you would normally want,” said Dr. John Beigel, who is leading the study. “But we want to make sure we have data to inform decisions this fall.”

Benjamin Mueller contributed reporting from London, and Noah Weiland from Washington. Kitty Bennett contributed research.

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