WASHINGTON (AP) — A panel of United States health advisers endorsed booster doses of Johnson & Johnson’s single-shot COVID-19 vaccine Friday, saying they should be offered at least two months after immunisation.

J&J has asked the Food and Drug Administration for flexibility with its booster, arguing 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.

The FDA’s advisory panel voted unanimously that a booster should be offered without setting a firm time.

The advisers cited growing worried that recipients of J&J’s vaccination seem to be less protected than people who got two-dose Pfizer or Moderna options — and that most got that single-dose many months ago.

The FDA isn’t bound by the vote but its ultimate decision could help expand the nation’s booster campaign.

The government says all three US vaccines continue to offer strong protection against hospitalisation and death from COVID-19, and that the priority is getting first shots to the unvaccinated.

But there’s a growing push to shore up protection against “breakthrough” infections and the extra-contagious delta variant of the coronavirus.

Booster doses of Pfizer’s vaccine began last month for people at high risk of COVID-19, and the FDA advisory panel has recommended the same approach for Moderna recipients.

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