(NewsNation Now) — U.S. health officials Friday endorsed a booster dose of the Johnson & Johnson COVID-19 vaccine, saying they should be offered at least two months after immunization.

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 worry 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.

This is just the first step in a review process that also includes sign-off from the leadership of both the FDA and the Centers for Disease Control and Prevention. If both agencies give the go-ahead, Americans could begin getting J&J boosters later this month.

However, earlier this week, FDA scientists said they did not receive enough data in time to do their own analysis of J&J’s application for a booster, but the agency’s review of company studies raised some red flags.

In its review, FDA scientists repeatedly noted the limitations of small sample sizes of many of the company’s studies.

They noted that data J&J used to support administering a booster six months after the initial shot relied on a test to measure immune response that has not been validated and was possibly not sensitive enough.

That mismatch in assessment tools would make it challenging to make meaningful comparisons with data from the company’s larger two-dose booster trial.

The FDA on Thursday endorsed a half-dose booster of Moderna’s COVID-19 vaccine as a booster for seniors and other high-risk groups.

The FDA meetings come as U.S. vaccinations have climbed back above 1 million per day on average, an increase of more than 50% over the past two weeks. The rise has been driven mainly by Pfizer boosters and employer vaccine mandates.

56.6% of all Americans are fully inoculated and 4.7% of the total population has received a booster, according to data compiled by the CDC.