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Moderna is urging the US Food and Drug Administration to authorize a half-size booster of its Covid-19 vaccine, according to a briefing document posted Tuesday, ahead of a key meeting of the agency’s vaccine advisers.

The company is asking for authorization for a 50-microgram booster dose – half the size of the 100-microgram doses used in the primary series of the two-dose vaccine. The company says halving the dose increases protection against the coronavirus while helping the worldwide Covid-19 vaccine supply.

The FDA’s Vaccines and Related Biological Products Advisory Committee is meeting this week to consider booster doses of the Moderna and Johnson & Johnson Covid-19 vaccines. The FDA previously authorized boosters of Pfizer’s Covid-19 vaccine for some adults.

Moderna is requesting authorization for a 50-microgram booster dose at least six months after the second dose for certain groups: people age 65 and older; people ages 18 to 64 who are at high risk of severe Covid-19; and people ages 18 to 64 whose exposure to the coronavirus in their settings or jobs put them at risk for Covid-19 complications or severe illness.

Antibodies have been shown to wane several months after people complete their Moderna or Pfizer vaccine series, but protection against severe disease and death remain strong after two doses. However, Moderna says, trial data and real-world evidence “support interventions to restore or enhance” antibodies that have waned over time.

“Clinical trial data breakthrough infections as well as real world evidence of reduced effectiveness against the Delta variant indicate that a booster dose of mRNA-1273 for those vaccinated more than 6 months previously could be beneficial to restore antibody titers to higher than post-dose 2 levels and reduce the number of breakthrough cases particularly against (variants of concern),” Moderna said in its briefing document.

Moderna’s authorization request includes results from a study of 171 people who received the authorized 100-microgram doses of Moderna’s vaccine and a 50-microgram booster dose at least six months later. Moderna also included safety data from another group of Moderna trial participants.

Efficacy against symptomatic Covid-19 wasn’t evaluated among booster dose recipients, the FDA notes in its analysis released Tuesday.

Just as with Pfizer’s booster dose submission, the FDA did not take a position on EUA for a booster dose of Moderna’s vaccine in its VRBPAC briefing document.

“The expected benefit of booster vaccination will depend on the impact that booster vaccination has in reducing disease relative to the primary series,” the FDA’s analysis said. “If the primary series of Moderna COVID19 Vaccine is still effective in preventing important COVID-19-related outcomes, then the benefit of booster vaccination is likely to be more limited than if effectiveness following the primary series has waned substantially.”

It said factors to consider include effectiveness of the primary series over time and against variants in circulation, the effectiveness of a booster against outcomes such as death, hospitalization and long Covid, the dynamics of the pandemic in the United States and the risks of a booster dose.

“Some real world effectiveness studies have suggested declining efficacy of Moderna COVID-19 Vaccine over time against symptomatic infection or against the Delta variant, while others have not,” the agency’s analysis said. “However, overall, data indicate that currently US-licensed or authorized COVID-19 vaccines still afford protection against severe COVID-19 disease and death in the United States. There are many potentially relevant studies, but FDA has not independently reviewed or verified the underlying data or their conclusions.”

Moderna booster safety

In addition to effectiveness, the briefing documents included some details about safety of booster doses of Moderna’s vaccine.

The two-dose mRNA Covid-19 vaccines made by Moderna and Pfizer have been linked to rare cases of heart inflammation, particularly among younger men. Studies have found such myocarditis and pericarditis cases are generally mild.

The FDA’s analysis notes that trials conducted before authorization might not be large enough to fully understand uncommon, potentially serious adverse reactions to a booster dose.

“It is currently not known if there will be an increased risk of myocarditis/pericarditis or other adverse reactions after a booster dose of the Moderna COVID-19 Vaccine,” the FDA’s analysis says. “These risks and associated uncertainties have to be considered when assessing benefit and risk.”

There was a higher rate of swollen lymph nodes after the booster dose when compared with the second dose in the primary Moderna series, but safety data didn’t otherwise show increased frequency or severity of reactions after a booster dose.

What happens next?

On Thursday, the FDA’s independent vaccine advisers are expected to discuss and vote on whether to recommend authorization of boosters for the Moderna Covid-19 vaccine.

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On Friday, the advisers are scheduled to discuss and vote on whether to recommend authorization of boosters for Johnson & Johnson’s Covid-19 vaccine.

Both vaccines are already authorized for use in people age 18 and older.

VRBPAC members will also hear a presentation on Friday on “mix and match” booster doses.

The committee of independent advisers typically discusses and makes recommendations to the FDA about vaccine authorizations and approvals, and the agency then makes the final decision about whether to OK vaccines. If they’re authorized, the CDC’s vaccines advisers are scheduled to meet October 20-21 to discuss whether to recommend them for use.