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(Bloomberg) -- Moderna and Johnson & Johnson said that data they’ve gathered supports the need for booster shots for their Covid-19 vaccines, ahead of a key regulatory meeting later this week.

A panel of scientific experts who advise the US Food and Drug Administration on vaccines is scheduled to meet Thursday and Friday to weigh the evidence for booster doses for each of the two vaccines. The regulator has already authorized a booster for the Covid shot made by Pfizer Inc. and BioNTech SE for those 65 and older and other at-risk adults.

The Biden administration has been pushing to make booster shots available to as many people as possible after a wave of infections over the summer tied to the delta variant. In addition, some studies have indicated that protection afforded by the US-authorized shots wanes over time. In documents prepared for the meeting, both companies said the supplemental doses would be advisable after six months.

Moderna was little changed in trading as of 11:30 a.m. in New York, while J&J shares fell as much as 1.2%.

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In a report, FDA officials didn’t make a specific recommendation for what the advisers should do regarding the Moderna booster. Findings from real-world studies are mixed on whether the vaccine’s efficacy has waned over time against the delta variant, the report said. And if the vaccine is still effective in preventing harm from Covid, the benefit from getting a boost is likely to be more limited than if efficacy has waned substantially, the staff noted.

Moderna’s vaccine uses a higher dose than the Pfizer vaccine; even the reduced booster dose Moderna is proposing contains more active ingredient than Pfizer’s shot. While data are still emerging, some real-world studies have suggested that Moderna’s shot produces more antibodies or may hold up slightly better over time compared to the Pfizer vaccine.

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Pfizer’s booster shot was cleared last month, and regulators have allowed some immunocompromised people to receive an additional dose of the Moderna shot. According to the Centers for Disease Control and Prevention, some 7.8 million people have received a Covid vaccine booster so far.

While Pfizer’s booster is the same dose as the first two shots, Moderna is asking the FDA to authorize a 50-microgram booster, half the amount in each of the initial two. The company noted that breakthrough cases increased among participants in its final-stage trial during July and August, as the highly infectious delta variant took over. Such cases were higher among people who got the vaccine early in the trial compared with those who got it later, another possible indication of waning immunity.

In a trial of the booster, the half-dose raised levels of protective antibodies at least 15-fold within one month, compared to levels before the boost, Moderna said in the documents. Antibody levels were also significantly higher after the booster than after the second shot, the company said.

In its staff report, the FDA pointed out that the isn’t known whether the risk of types of heart inflammation linked to the two messenger RNA vaccines, called myocarditis and pericarditis, will rise after boosters of Moderna’s shot. The vaccine maker pointed to the safety of other booster shots in the documents it submitted.

“Lower doses for boosters have been shown to be safe and immunogenic for other vaccines, such as diphtheria,” the company said. In addition, lowering the booster dose would help Moderna increase its vaccine supply for the rest of the world.

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Use of J&J’s vaccine has lagged those from Pfizer and Moderna, and as of Oct. 12, only about 15 million people in the US have received the single shot, according the the CDC. J&J said that data support a favorable benefit-risk profile for a booster in adults 18 and older. The booster will likely result in increased protection against symptomatic Covid, as well as the current variants, the company said.

The New Brunswick, New Jersey-based drug giant recommended a booster at six months or more after the initial shot, but said it may also be administered as early as two months, based on the strength of the immune response. The need for a booster dose -- and the timing in which it should be given -- must be determined by local Covid conditions and the needs of individuals or specific populations, J&J said.

“In the early stages of a pandemic, a single dose vaccine is an efficient tool to rapidly increase vaccine uptake and reduce the burden on health care systems by preventing severe disease outcomes, especially where supply limitations were present,” J&J said. “In the current stage of the pandemic, and given emergence of different variants under certain circumstances, focus may shift to protecting individuals by maximizing and prolonging vaccine-induced protection.”

In parts of the US where vaccination rates remain low, J&J said, the focus isn’t just on preventing severe disease. A booster can also help slow transmission as the delta variant continues to spread in areas with high vaccine-hesistancy rates, the company said.

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The most intriguing part of the two-day meeting could come at the close, where the FDA advisers are scheduled to hear a presentation from University of Maryland scientists leading a US government-sponsored study of whether it’s possible to mix and match Covid boosters with different vaccines than were used for the primary inoculation. The FDA has not released details of what the Maryland scientists will say.

©2021 Bloomberg L.P.