A US Food and Drug Administration (FDA) advisory panel on Friday voted against recommending a Pfizer Covid-19 booster for the general population but approved it for people over the age of 65 and for those at high risk of severe illness.
The FDA’s Vaccines and Related Biological Products Advisory Committee voted 16 to 2 against measure after a day-long meeting discussing the safety and effectiveness of Pfizer's Covid-19 booster vaccine.
“There are very clear populations where a booster may be appropriate, such as elderly or immunocompromised,” said National Institutes of Health scientist Michael Kurilla.
“It’s not clear to me that the data we’re seeing right now is applicable or necessary for the general population.”
A second vote resulted in unanimous approval to recommend the Pfizer Covid-19 booster shot for people over the age of 65 and anyone who is at high risk.
The FDA does not have to follow the committee's recommendations, but it often does.
The decision may delay President Joe Biden's planned September 20 roll-out for booster shots in the US, which has drawn criticism for promising booster vaccines before they were approved by the FDA and CDC.
Eight top health officials in the Biden administration - including the heads of the FDA and Centres for Disease Control and Prevention - advocated booster shots last month.
Pfizer pointed to data collected from a study in Israel that showed boosters were necessary, determining from the study that protection from its second dose weakens after six months and arguing the need for a third dose to restore protection to “95 per cent effectiveness".
But this is in contrast to the neutral stance the FDA took this week in a 23-page report.
The agency noted observational studies such as the one conducted in Israel could include “known and unknown biases that can affect their reliability” and added that US vaccines such as Pfizer can “still afford protection against severe Covid-19 disease and death in the United States".
“If the advisory committee votes not to support boosters, this will put the administration in an awkward position,” said Norman Baylor, former director of the FDA’s Office of Vaccines Research and Review and now chief executive at Biologics Consulting.
A CDC advisory committee will meet separately next week to discuss booster shots. The CDC urged health officials to wait to administer boosters until they are granted approval by the FDA and CDC, CNN reported.
A group of international experts, which included two outgoing FDA vaccine regulators, argued against the need for boosters in a paper released on Monday in a striking rebuke of Mr Biden's booster plan.
The report stated that the messaging for boosters could undermine the need for primary vaccination, noting that unvaccinated people remain at the highest risk of severe disease and are the main drivers of transmitting the illness.
US Surgeon General Dr Vivek Murphy on Friday said August's roll-out announcement was to provide transparency and planning.
White House Press Secretary Jen Psaki on Thursday defended the administration's plan, pointing to health officials determining in August that boosters would be needed.
“We really rely on the people who are leading our health and medical agencies. And there is broad agreement among those individuals about where things stand,” Ms Psaki said.
FDA Commissioner Janet Woodcock and CDC Director Rochelle Walensky both publicly supported the Biden administration's roll-out last month, but a negative review could put them at odds with their own agencies.
The argument for booster vaccines gained momentum as the highly transmissible Delta variant tore through the US this summer, particularly in states with low vaccination rates.
The US has already begun offering third doses to those with severely compromised immune systems.
Moderna has also asked for approval of its own booster shot, but it remains unclear when the FDA will debate that or the need for a Johnson & Johnson booster.
Mr Biden's proposed roll-out is part of a new six-pronged plan to combat the Covid-19 pandemic that includes sweeping mask and vaccine mandates.
To date, more than 670,000 Americans have died from the disease.