(Bloomberg) -- It was up to Centers for Disease Control and Prevention director Rochelle Walensky to sort out a crucial question: which Americans should be the first to get Covid-19 booster shots.

And there was little consensus.

The CDC’s Advisory Committee on Immunization Practice on Thursday voted to narrow eligibility for boosters of the Pfizer Inc.-BioNTech SE shot authorized by the Food and Drug Administration. Walensky had a choice: side with the advisers, citing the paucity of data on whether younger, vaccinated adults are truly at risk of severe breakthrough cases of Covid-19. Or overrule them, and err on the side of boosters for front-line health workers and others.

Shortly after the panel’s vote, Walensky called Health and Human Services Secretary Xavier Becerra to lay out the options in front of them, and then met privately for hours with CDC officials to come to a decision, according to people familiar with the process.

And late Thursday, she issued a statement. The CDC advisers were overruled, with Walensky expanding eligibility for boosters broadly to include those 18 to 64 at risk of workplace exposure, including healthcare workers.

She was particularly motivated by one factor: she believed that nurses and other health officials working in Covid-19 units, many of whom are now 8 months past their second shot, should be eligible for third shots, according to one of the people familiar with the matter. Under the CDC advisory committee’s guidance, they wouldn’t have been booster-eligible unless they had an underlying health condition.

Walensky and other CDC officials discussed the issue deep into the evening on Thursday before settling on a decision. She called Becerra and told him, and HHS informed the White House shortly after 11 p.m., the people said. 

She announced her recommendations publicly just before midnight.

Though she is a political appointee, there’s no indication the White House directly influenced her decision. President Joe Biden didn’t speak to her as she considered her statement, nor did two of his top aides, Chief of Staff Ron Klain and Covid-19 Coordinator Jeff Zients, one of the people said.

In the end, Walensky staked out something of a middle ground. She expanded eligibility for booster shots, but set up a two-tier recommendation. People 65 and up and people age 50-64 with underlying health conditions “should” get a booster, she said. Americans age 18-49 with health conditions, and those 16-64 with workplace exposure, “may” get one, she said.

Kathy Poehling, a member of the CDC advisory committee -- known as ACIP -- and a professor of pediatrics and epidemiology and prevention at the Wake Forest School of Medicine, said she supported Walensky’s move. The word “may” in Walensky’s recommendation for younger people was a crucial addition, she said. 

“‘May’ is a wonderful way to allow people who would like to get the booster dose and highlight that the primary series is highly effective,” Poehling said. “As you could see from the meeting, we struggled and reviewed a lot of data. And this decision is really difficult. And ultimately you have to decide what is the best decision for the greatest good.”

Ashish Jha, dean of the Brown School of Public health, applauded Walensky for the reversal, saying that ACIP would have blocked health workers from receiving boosters. 

“Dr. Walensky fixed it,” he tweeted. “This is why its good to have a strong CDC director.”

Still, there is sparse data showing a need for broad booster shots. It’s dependent on predictions that vaccine efficacy will eventually wane even for younger adults. At the ACIP meeting, the CDC’s own presenter noted that much of the data supporting boosters was short-term and of “very low” certainty.

While there is a hope among experts that a third shot given months later will lead to much longer-lasting immunity than the first two doses along, that remains to be proven. 

In the meantime, despite all the angst, breakthrough hospitalizations and deaths among vaccinated people under 65 remain are rare, compared to the more than 2,000 daily deaths in the U.S. from Covid-19. Almost all of those are unvaccinated people -- and the ACIP panel expressed fear that a round of boosters would reinforce doubts among the unvaccinated about whether the shots work at all.

Overall, there have been less than 4,000 U.S. hospitalizations for fully vaccinated people who contract Covid-19 and fewer than 400 deaths, according to CDC data through September 13.

Walensky acknowledged that she made a call off of incomplete data.

“We are tasked with analyzing complex, often imperfect data to make concrete recommendations that optimize health,” she said in her statement. “In a pandemic, even with uncertainty, we must take actions that we anticipate will do the greatest good.”

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