ROCHESTER, Minn. — The Centers for Disease Control and Prevention on Friday, Sept. 24, formally recommended that a broad swath of the vaccinated population seek out a third shot.

Officially, the new recommendation applies to persons who had the Pfizer vaccine more than six months ago and are over 65, or those who are over 50 with underlying conditions — the latter a category believed to encompass tens of millions of Americans.

A separate determination — that front line workers over 18 "may" receive a booster — was issued late in the evening Friday by CDC Director Dr. Rochelle Walensky, who made the decision by overruling a CDC panel chosen to evaluate the science.

In addition, the CDC as of Friday now advises that persons over 18 with high risk conditions "may" get boosters. Like the language applied to its ruling on front-line workers, the agency has not formally recommended that group do so.

The CDC on Friday did not rule on Moderna or J&J vaccine boosters.

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That a third shot is recommended in the face of only mixed, limited evidence of their effectiveness caused Mayo Clinic vaccinologist Dr. Greg Poland to resurrect a metaphor from the early days of the pandemic.

"We are flying the airplane while we are building it," Poland said during a call with the media Friday. He added that "nobody has more than a 19-month experience with this. We have to incorporate new information and at the same time the virus is changing while we study it."

The momentum toward boosters began, as Poland explained, in the arrival last summer of Israeli effectiveness data showing increased hospitalization of the vaccinated with time and at younger ages.

These observations however were not observed in Scotland, England, Canada or the U.S., Poland said.

The Israeli studies had a host of limitations, he cautioned, leading the FDA to originally reject the Biden administration's call for boosters for all Americans. The new recommendations reflect a narrowing of that original proposal.

Poland says he agrees that not everyone should be advised to get a booster at this point, citing limited evidence. But he said that he supports Walensky's decision to defy her scientific advisors.

"Medicine has always had to appeal to the 'art of medicine,'" he said, "not just the science. And that occurs when we don't have the science, and we have to extrapolate."

Poland's comments sound like a reversal for an expert who has spent weeks decrying amateur advice-givers who would make a claim to scientific truth without a scientific background. But Friday he said that the booster policy reflects the practical limitations of studying a third shot when the vaccines have not been on the market long enough to do so.

"Would it be safer to wait two to three months and have the data from that study of 10,000?" he asked. "In the meantime there will be people who will get infected and die.

"In the next few months we are promised some data from a trial involving 10,000 people. Now I'm happy as a scientist and a physician, because those are data I can work with."

And while the booster shots are "born of less data than we would like," Poland said, he expects there will be "high quality data" in the next few months.

Poland said scientists do know from the less rigorous research that giving a third dose "brings immunity right back up and protects people."

He also agreed to the risks faced by the U.S. as it launches into third doses as large portions of the the rest of the world go without vaccines.

"To the extent that this virus continues to propagate throughout the globe in the population, we will — not might — see the continual evolution of viral mutation."

Although vaccine manufacturers in the U.S. have refused to share their technologies with poor countries as advocates have asked, Poland said that the U.S. had helped other countries and is currently in its own predicament.

"It's hard to go feed other people when your people are starving," he said.