This Wednesday, Dec. 23, 2020, file photo shows a vial of the Moderna COVID-19 vaccine in the first round of staff vaccinations at a hospital in Denver. Credit: David Zalubowski / AP

U.S. public health advisers voted unanimously to recommend COVID-19 booster shots from Moderna Inc. and Johnson & Johnson, clearing the way for a widespread campaign hoped to stave off a resurgence of the virus.

In two votes, the 15-member Centers for Disease Control and Prevention’s vaccine advisory committee Thursday supported the additional shots in certain populations, following the outlines of U.S. regulators who cleared the shots Wednesday.

Boosters are becoming more widely available at a crucial time when the U.S. is still fighting the summer surge of infections driven by the highly contagious delta variant of COVID. Health officials are keen to protect those most vulnerable to the disease as the winter months approach, bringing the added threat of a potentially harsh flu season.

While the unvaccinated account for the vast majority of the most seriously ill in the current outbreak, breakthrough infections among the vaccinated have fueled concerns that the shots’ efficacy wanes over time. Moderna and J&J both presented evidence at Thursday’s meeting that their boosters restore infection-fighting power.

The recommendation from CDC’s Advisory Committee on Immunization Practices echoes the clearance granted to the boosters by the Food and Drug Administration. The FDA authorized Moderna’s additional shot for people 65 and older along with other adults at high risk of severe COVID because of health conditions or work at least six months after their initial inoculation. The agency also cleared J&J boosters for people 18 and older who got the single-shot vaccine at least two months ago.

Several committee members made it clear they were voting for the recommendation in the interests of making a clear message to the public that wouldn’t conflict with the earlier recommendation for the vaccine from Pfizer Inc. and BioNTech SE.

Many committee members also voiced support for mixing and matching of the three cleared boosters. Those who received J&J’s single dose vaccine must wait two months before taking any other booster, while those whose primary vaccination was an mRNA vaccine should wait at least six months before getting any additional shot, including J&J’s.

CDC Director Rochelle Walensky must sign off on the committee’s recommendations before doctors, drugstores and other sites can begin giving shots. The recommendation for the Pfizer-BioNTech shot, which was cleared by the FDA last month, is for older people and adults at high risk, including teachers, nurses and others who risk infection from contact with the public through their jobs.

Last time the CDC’s advisers met to discuss boosters, they determined these front-line workers may be at greater risk of infection, but not for developing severe COVID. While the advisers didn’t recommend boosters for this group, Walensky overruled them, broadening out the booster ruling to include those at risk because of their jobs.

Moderna’s booster dose is half the amount of its regular shot, and during the discussion, Michael Hogue, a committee liaison from the American Pharmacists Association, raised concerns that twice the number of needles stuck in the product vial might raise sterility problems.

Moderna is looking at repackaging the product for their children’s program, said Jacqueline Miller, Moderna’s therapeutic area head for infectious diseases. However, data support the safety of 20 entries into the vial, she said.

Using the standard vial for boosters “is a measure that we’re putting in place now given the current context of the delta surge and the need to make these booster doses available,” she said.

The FDA also said people who have been fully immunized and meet the criteria to get a booster can receive a supplemental dose made by a different manufacturer than the maker of their original shot.

Americans have been able to get an additional vaccine dose since Aug. 13, when regulators authorized an extra dose of messenger RNA vaccines for people with impaired immune systems. Since then, more than 11 million booster doses have been administered.

Clearance of J&J’s booster was highly anticipated by the 15 million people in the U.S. who received it, including the immunocompromised, who have been more vulnerable to COVID symptoms.

The booster program has drawn criticism from the head of the World Health Organization because of the difficulty of getting primary immunizations in many lower- and middle-income countries. Critics say distributing extra doses in rich countries exacerbates the inequity.

Fiona Rutherford, Robert Langreth and Riley Griffin, Bloomberg News