U.S. health advisers endorsed a booster of Johnson & Johnson’s COVID-19 vaccine Friday, citing concern that Americans who got the single-dose shot aren’t as protected as those given two-dose brands. J&J told the FDA 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. Unable to settle the best timing, the FDA’s advisory panel voted unanimously that the booster should be offered at least two months after people got their earlier shot.