An independent advisory panel to the Food and Drug Administration on Thursday unanimously recommended a booster dose of the Moderna coronavirus vaccine for people 65 and older and adults who are at high risk of severe illness or are exposed at work. 

The recommendation mirrors the eligibility criteria for the Pfizer-BioNTech booster, which was authorized in September. Nearly 70 million Americans have received both doses of the Moderna vaccine.

The vote comes after a full-day examination of data on the safety and effectiveness of a booster, and the recommendation will now be considered by FDA officials, who are expected to reach a decision on the Moderna booster within days. An advisory committee to the Centers for Disease Control and Prevention that makes recommendations on how vaccines should be used is scheduled to meet Wednesday.

At the start of the meeting, Peter Marks, director of the Center for Biologics Evaluation and Research, outlined the potential rationale for boosters. He showed that all three of the vaccines available in the United States remain highly protective against severe outcomes but said even milder cases may warrant prevention.

"Vaccine effectiveness against mild and moderate disease does appear to wane over time for the different vaccines, and we do need to account for the fact that mild-to-moderate covid-19 can be associated with adverse outcomes, such as blood clots and long covid-19, even in those who have breakthrough infections after vaccination," Marks said.

Moderna's two-shot regimen remains robustly protective at more than five months after vaccination: 93% effective in preventing all virus-related illness and 98% protective against severe cases, according to data presented by Jacqueline Miller, head of Moderna's infectious-disease therapeutic area.

To make the case for boosters, she presented data showing that six to eight months after vaccination, antibody levels dropped in vaccine recipients. A half-dose booster at least six months after initial vaccination restored those antibodies in a study of nearly 300 people.

Miller also presented data on breakthrough cases among people in the clinical trial of the company's vaccine, which began in summer 2020. Among those vaccine recipients, there was a clear uptick in breakthrough infections this July and August.

A Moderna analysis showed that protection against infections appears to diminish over time. People vaccinated earlier in the large clinical trial were more likely to have breakthrough cases this July and August compared with the group that received the placebo in the trial and thus were vaccinated more recently.

"Prior to July, the maximum number of cases reported in [vaccinated individuals] in a single month was 23," Miller said. "This increased to 81 cases in July and 169 cases in August, with 97% of these cases due to the delta variant."

During the meeting, questions were raised about why Moderna has not made a bigger commitment to supply doses to low- and middle-income countries. The Biden administration has become frustrated that Moderna has not made a greater commitment to supplying doses beyond wealthy countries.

Miller referred to an open letter from Moderna chief executive Stephane Bancel. She said the company was not enforcing its patents during the pandemic and added that a half-dose instead of a full-dose booster will "make more vaccine available for the world, so that frees up a billion extra doses."

Representatives from Israel presented data on that country's experience with Pfizer-BioNTech booster doses, showing that as the country faced an uptick in infections this summer, the implementation of a booster campaign - initially targeted at adults 60 and older - helped thwart the rise in infections and severe cases in vaccinated people.

They also presented data showing that across age groups, including younger people, the rate of infection dropped after a booster dose. The rate of severe disease also dropped for people older than 40. There were not enough severe cases among younger people to estimate a benefit.

Fully vaccinated people "were part of the wave, some of them getting severely ill and dying," said Sharon Alroy-Preis, director of public health services for Israel's Ministry of Health. "There is no question in my mind that the break of the curve was due to the booster dose."