J&J told the Food and Drug Administration that an extra dose adds important protection as early as two months after initial vaccination — but that it might work better if people wait until six months later.

The FDA isn’t bound by the recommendation as it makes a final decision — and adding to the debate’s complexity, new research also suggests that J&J recipients might have a stronger immune response if their booster dose is from a competing brand.

And the most dramatic jump came from giving a Pfizer or Moderna shot after the single-dose J&J vaccination.

But J&J’s vaccine has consistently shown lower effectiveness levels across a series of studies — and the FDA panel ultimately settled on another shot for any recipient 18 or older at least two months after their first vaccination.

The vast majority of the 188 million Americans who are fully vaccinated against COVID-19 have received the Pfizer or Moderna options, while J&J recipients account for only about 15 million.

J&J’s vaccine is made with a different technology and on Friday, the company pitched its booster as a way to strengthen a robust vaccine that they said has retained its protective power over eight months.

As for its booster, J&J presented results from a large study that found giving a second dose just two months after the first bumped protection against symptomatic COVID-19 to 94% from 70% in U.S.

And while FDA’s reviewers found no new safety concerns about a second J&J dose, they noted other shortcomings.