A US Food and Drug Administration (FDA) advisory panel has endorsed a booster shot of the Pfizer-BioNTech Covid-19 vaccine six months after full inoculation in people above 65 years and for individuals at high risk for severe disease.

The votes Friday represented a challenge for the Biden administration’s sweeping effort to shore up nearly all Americans’ protection amid the spread of the highly contagious Delta variant. The decision was made by a committee of outside experts who advise the Food and Drug Administration.

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The decision late Friday came after members of the FDA’s Vaccines and Related Biological Products Advisory Committee voted against advising the agency to approve boosters for anyone 16 and older after hours of debate, reports Xinhua news agency.

The initial vote on the widespread use of boosters was 16 to 2, with members expressing frustration that Pfizer had provided little data on the safety of extra doses. Many also raised doubts about the value of mass boosters, rather than ones targeted to specific groups.

Members of the committee expressed doubts about the safety of a booster dose in younger adults and teens and said they wanted to see more data about the safety and long-term efficacy of a booster dose.

A decision about boosters from the FDA is expected in the next few days.

Meanwhile, another advisory panel of the Centers for Disease Control and Prevention (CDC) is also scheduled to meet next week on how to apply the FDA’s eventual decision and can fine-tune the recommendation.

President Joe Biden’s administration has recently announced a plan to begin administering booster doses to the general population during the week of September 20, pending sign-off from the FDA and CDC.

The CDC published data on Friday showing that all three Covid-19 vaccines available in the US — Pfizer-BioNTech, Moderna, and Johnson & Johnson — still provide strong protection against hospitalization.

Moderna’s vaccine provided 93 percent protection against hospitalization, Pfizer’s was 88 percent and Johnson & Johnson’s Janssen vaccine was 71 percent, said the CDC study.