U.S. health advisors discussed Friday who should receive boosters of Johnson & Johnson’s single-shot COVID-19 vaccination and when. They also explored whether a different brand might offer better protection.

After the Food and Drug Administration approved third doses for the Pfizer vaccine, the push for boosters began last month. This was because seniors and younger adults were at greater risk of contracting the coronavirus from their jobs and living conditions. A FDA advisory panel recommended Thursday a half-dose booster for the Moderna vaccine.

Friday’s panel also discussed boosters of J&J vaccine. But the decision was more complicated. Moderna and Pfizer requested that the FDA approve boosters within six months of immunization. However, J&J suggested a sliding schedule with an additional dose up to two months later.

The experts will also discuss preliminary data from a “mix-and match” government study, which suggested that J&J recipients might have a stronger immune response if they receive a Moderna/Pfizer booster instead of a second J&J dosage.

The FDA will consider the recommendations of its advisors to decide whether boosters are authorized for Moderna and J&J next week. After that, another government agency will determine who should take up their sleeves.

All three vaccines offered strong protection against severe illness or death from COVID-19 in the United States. The priority for the nation is to get first doses of the vaccines to the 66 million eligible, but unvaccinated Americans most at risk.

Experts continue to debate the value of boosters for milder infections among most adults who have been vaccinated, an approach that was proposed by the Biden administration.

FDA scientists have recently highlighted flaws in J&J’s data in their application for a booster dose. This included little data about the strength of protection for people who received a six-month boost. An earlier review revealed that a two-month study suggests there may be a benefit.

J&J’s single shot vaccine has shown consistently lower effectiveness than the two-shot MRNA vaccines created by Moderna and Pfizer, which FDA reviewers pointed out in their review. This gap in protection will likely play a major role as panelists evaluate the need for boosters and their scope.

J&J’s vaccine was highly anticipated because of its unique formulation and different technology. Its rollout was hampered by manufacturing issues and side effects that were rare and serious, including a blood disorder and neurological reaction called Guillain Barre syndrome. Both cases were decided by regulators that the benefits outweighed the risks.

Most of the nearly 188 million Americans who have been fully vaccinated against COVID-19 are receiving the Moderna or Pfizer options. J&J recipients make up about 15 million.