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U.S. health advisers endorsed a booster of Johnson & Johnson’s COVID-19 vaccine Friday, citing growing worry that Americans who got the single-dose shot aren’t as protected as those given two-dose brands.

Advisers to the Food and Drug Administration weighed J&J’s proposal for a flexible booster schedule. The company said the 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.