An advisory committee for the Food and Drug Administration will meet for two days beginning Thursday to evaluate requests from Johnson & Johnson and Moderna to authorize booster doses.

The meeting will begin on Thursday at 8:30 a.m. EDT and last for several hours. The panel is expected to decide Thursday afternoon whether to recommend the Moderna booster for recipients over 18. Friday's meeting will evaluate the extra Johnson & Johnson dose, which is produced by subsidiary Janssen.

Last month, the FDA approved a third shot of the vaccine made by Pfizer for people over 65, as well as recipients between 18 and 64 who are at high risk of severe illness and who face frequent institutional or occupational exposure to COVID-19.

The agency recommended the Pfizer booster dose six months after the second shot.

Almost 9 million people in the United States have received a booster dose of the Pfizer vaccine over the past few weeks, according to the Centers for Disease Control and Prevention.

FDA scientists announced findings for both the Johnson & Johnson and Moderna boosters this week.

Tuesday, they said that data show that Moderna's two-dose regimen provides sufficient immunity that precludes the need for a third dose. They said, generally, a Moderna booster seems unnecessary to protect against the more contagious Delta variant.

However, the committee may still recommend a Moderna booster for more vulnerable Americans who are at greater risk of hospitalization and death from the virus.

A study released by Moderna in August showed that its vaccine remained 93% effective in recipients six months after the second dose, and that a third shot created a "robust antibody response" against the Delta variant.

In their remarks about the Johnson & Johnson shot on Wednesday, FDA scientists said its booster appears to offer added protection, but they're unsure when is the best time to receive the extra dose. Johnson & Johnson recommends the booster between two and six months after the first shot.

The Johnson & Johnson vaccine, unlike those from Moderna and Pfizer, requires only one dose.

In their evaluation for a booster, the scientists said some of the data provided by Johnson & Johnson was incomplete. For example, they said, the company offered only a small sample size of recipients who received a booster after six months.

Johnson & Johnson in September reported that a booster after two months was 100% effective in protecting recipients from severe COVID-19, and 75% effective in preventing symptomatic COVID-19 in recipients worldwide.

The company said the booster had greater efficacy in the United States, 94%, in preventing symptomatic COVID-19.

Johnson & Johnson also noted that one dose of the vaccine showed to be 79% effective against infection in the United States between March and late July, and 81% effective in preventing hospitalization.

Another development that may impact the distribution of booster doses came on Wednesday, when a National Institutes of Health-led study concluded that "mixing and matching" booster doses appears to be effective, and will also be reviewed by the FDA advisory committee.

In fact, the study said that people who received the Johnson & Johnson vaccine are better off getting a booster dose of the Moderna or Pfizer formulation.

That means that the FDA and CDC, for example, may ultimately approve a Moderna booster for people who received the Pfizer vaccine and vice versa. It's not yet known when the FDA panel will make its recommendation on that possibility.

The NIH study is expected to be a factor in the FDA panel's recommendations for Moderna and Johnson & Johnson boosters.

After the panel makes its recommendation, it would have to be approved by the FDA and CDC before Americans could begin to receive the Moderna and Janssen boosters. The soonest the agencies could grant that approval is Oct. 22.

At Thursday's meeting, Israel's public health director will give a presentation describing the effectiveness of Pfizer vaccine booster shots there.

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