The U.S. launched a campaign to offer boosters of Pfizer’s COVID-19 vaccine to millions of Americans on Friday even as federal health officials stressed the real problem remains getting first shots to the unvaccinated.

“We will not boost our way out of this pandemic,” warned Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention — even though she took the rare step of overruling the advice of her own expert panel to make more people eligible for the booster.

The vast majority of COVID-19 cases and hospitalizations are among the unvaccinated, Walensky noted. And all three COVID-19 vaccines in the U.S. offer strong protection against severe illness, hospitalization and death despite the extra-contagious delta variant that caused cases to soar. But immunity against milder infection appears to wane months after initial vaccination.

People anxious for another Pfizer dose lost no time rolling up their sleeves after Walensky ruled late Thursday on who’s eligible: Americans 65 and older and others vulnerable because of underlying health problems or where they work and live — once they’re six months past their last dose.

Also Read: US President Joe Biden urges eligible citizens to take COVID booster shots

Jen Peck, 52, of Eau Claire, Wisconsin, qualified because of her job as an education math and science consultant. She was vaccinated back in March but worries about unknowingly picking up and spreading an infection. She travels between rural schools where many students and teachers don’t wear masks and the younger children can’t yet be vaccinated.

“I don’t want to be COVID Mary carrying it around to buildings full of unvaccinated kiddos. I could not live with myself if I carried it from one building to another. That haunts me, the thought of that,” said Peck, who got the extra shot first thing Friday morning.

Health officials must clear up confusion over who should get a booster, and why. For now, the booster campaign is what Walensky called “a first step.” It only applies to people originally vaccinated with shots made by Pfizer and its partner BioNTech. Decisions on boosters for Americans who received Moderna or Johnson & Johnson vaccines are still to come.

President Joe Biden said if you’re vaccinated, “You’re in good shape and we’re doing everything we can to keep it that way, which is where the booster comes in.” He urged those now eligible for an extra shot to “go get the booster,” saying he’d get his own soon — and that everyone should be patient and wait their turn.

Exactly who should get a booster was a contentious decision as CDC advisers spent two days poring over the evidence. Walensky endorsed most of their choices: People 65 and older, nursing home residents and those ages 50 to 64 who have chronic health problems such as diabetes should be offered one once they’re six months past their last Pfizer dose. Those 18 and older with health problems can decide for themselves if they want a booster.

Also Read: Ushering in new phase of vaccinations, US CDC endorses COVID booster

But in an extremely unusual move, Walensky overruled her advisers’ objections and decided an additional broad swath of the population also qualifies: People at increased risk of infection — not serious illness — because of their jobs or their living conditions. That includes health care workers, teachers and people in jails or homeless shelters.

“This was scientific close call,” Walensky said Friday. “In that situation it was my call to make.”

Experts say it was only the second time since 2000 that a CDC director overruled its advisory panel.

Health care workers can’t come to work if they have even a mild infection and hospitals worried about staffing shortages welcomed that decision.

But some of the CDC’s advisers worry that offering boosters so broadly could backfire without better evidence that it really will make a difference beyond the most medically vulnerable.

“My hope is that all of this confusion – or what may feel like confusion – doesn’t send a message to the public that there is any problem with the vaccine,” said Dr. Beth Bell, a University of Washington expert. “I want to make sure people understand these are fantastic vaccines and they work extremely well.”

Dr. Anthony Fauci, the U.S. government’s top infectious disease specialist, cautioned against seeking a Pfizer booster before the recommended six-month mark.

“You get much more of a bang out of the shot” by letting the immune system mature that long so it’s prepared to rev up production of virus-fighting antibodies, he explained.

Also Read: US CDC panel recommends Pfizer booster shots for certain adult Americans

The U.S. had already authorized third doses of the Pfizer and Moderna vaccines for certain people with weakened immune systems, such as cancer patients and transplant recipients. Other Americans, healthy or not, have managed to get boosters, in some cases simply by asking.

About 182 million Americans are fully vaccinated, or just 55% of the total population. Three-quarters of those 12 and older — the ages eligible for vaccination — have had a first dose.