By Dylan Scott

The Biden administration has settled on its initial plan for Covid-19 vaccine booster shots.

In the last week, the FDA and CDC have both endorsed providing booster shots to some — but not all — Americans who received the Pfizer/BioNTech vaccine. Their approval finally gives the Biden White House the authority it needs to start rolling out and promoting booster shots, a month after President Joe Biden announced a plan to start administering additional doses.

The president had originally said all Americans would receive a third shot — and that may still prove to be true, experts say. But the booster plan for now is more narrow, focused on the populations most likely to develop severe symptoms from Covid-19 and those who face more exposure because of their work.

The FDA’s approval and the CDC’s recommendations differ slightly in scope. But generally speaking, federal officials have signed off on booster shots for the following groups, with the third dose to be given at least six months after the person’s second shot:

-All people over age 65 and all nursing home residents who received the Pfizer vaccine

-Younger people who received the Pfizer vaccine and have medical conditions that raise the risk of severe Covid-19 symptoms (which could include cancer, diabetes, heart conditions, etc.)

-Younger people who received the Pfizer vaccine and whose occupation puts them at higher risk of contracting a serious Covid-19 infection

The process that led to those recommendations was laden with controversy. Two FDA officials stepped down after Biden’s announcement and later signed a public letter that the scientific case for boosters was narrower than the president had indicated.

Pfizer had asked the FDA to approve booster shots for all Americans over 16 who had received its vaccine, but the agency’s scientific advisers balked and instead recommended a narrower approval, to which the FDA adhered. The CDC’s own advisers did not formally support a recommendation that people with occupational risk be given a third dose, but CDC Director Rochelle Walensky decided to include those people in the agency’s final recommendation anyway, overruling the advisory panel — an unusual move that kept the CDC in lockstep with the FDA.

After all that, the new federal guidance matches what many experts have said is appropriate, based on the scientific evidence. There is some sign of the Pfizer vaccine waning in effectiveness over time and against the delta variant. But the protection that it provides against hospitalization — the metric that is most closely watched by public health experts — remains strong for most people.

The notable exceptions are older people, who have seen a greater drop in efficacy over time, and people with compromised immune systems, for whom the vaccines are often not as effective to start with. That’s why they are the focus of the new booster guidelines, along with workers in higher-risk settings.

This likely won’t be the last word on booster shots, however. For starters, Moderna has signaled it also believes a booster may be necessary for people who received its vaccine. The Johnson & Johnson vaccine was always somewhat less effective than the Moderna and Pfizer shots, and recent data confirmed its lower effectiveness against hospitalization. That company has also been testing additional doses for people who received its vaccine.

Some experts have also told me that, though there hasn’t been much waning against hospitalization for younger and healthier people, it may end up being necessary for most people to receive a third dose to lock in a longer-term immunity.

But those experts don’t view getting booster shots to young and healthy people as the priority right now, when one in four eligible Americans still hasn’t received any dose of the vaccine at all and much of the world is still far behind the US and Europe in their vaccination rates. More people getting vaccinated, for the first time, is still the best way out of the pandemic, they say.

“I’d argue that it’s more important to get vaccines to unvaccinated populations and offer boosters to high-risk people for now than it is to get a third booster six months out for all,” Angela Rasmussen, a research scientist at the Vaccine and Infectious Disease Organization, told me over email last week.

Why the federal government is recommending booster shots for some, not all

A group of 18 scientists, including the two FDA officials who stepped down, outlined their position on booster shots in a letter published just a few days before the FDA’s advisers reviewed the evidence for themselves.

“COVID-19 vaccines continue to be effective against severe disease, including that caused by the delta variant,” they wrote. “Current evidence does not, therefore, appear to show a need for boosting in the general population.”

One CDC study analyzed new Covid-19 cases and hospitalizations from early May to late July in New York state. The study covers the transition from the “alpha” variant to the delta variant, which had become dominant by the start of July.

The Covid-19 vaccines became somewhat less effective in preventing any illness as the delta variant took over, the CDC researchers concluded. Back in May, vaccines had an estimated 90 percent effectiveness at preventing new infections. But by mid-July, the estimated effectiveness had dropped to just under 80 percent. By that point, vaccinated people were more likely to get infected and feel sick. But the study also found the vaccines remained resilient against the most severe symptoms.

Another CDC study through July examined national data on whether the Pfizer/BioNTech and Moderna vaccines are becoming less effective at stopping severe illness over time. Like the New York study, it found vaccines are extremely good at their most important job. They did not detect a meaningful decline in protection against hospitalization almost six months after patients received a second dose of the vaccine.

A newer CDC study did detect some divergence between the Moderna and Pfizer vaccines in preventing severe illness over time, with the former performing better than the latter. But overall effectiveness for both vaccines remained high for most people.

Research out of the United Kingdom has reached generally the same conclusions: modest waning in effectiveness against any symptomatic illness but little (if any) against the worse outcomes for most people.

The exceptions to those broad findings have been for people who have serious medical conditions, people who are otherwise immunocompromised, and people who are older.

A different CDC study evaluated vaccine effectiveness for nursing home residents, a population particularly vulnerable to Covid-19 and one of the first groups to get vaccinated last winter. That study did find a significant decline in vaccine effectiveness over time against any illness for those Americans, from 75 percent pre-delta to about 50 percent post-delta. So they enjoyed less protection than younger people did from the start, and that protection declined much more than in their younger counterparts.

Likewise, the new UK study found that the most significant drop in the vaccine’s effectiveness against hospitalization was among people 80 years and older, from about 95 percent two to nine weeks after becoming fully vaccinated to about 70 percent more than 20 weeks after. For younger age groups, that protection against severe illness stayed above 90 percent over the study period.

With that in mind, “I think you can make the argument that people who are high risk for severe Covid-19 would benefit from a booster given at least six months after completion of their initial vaccine regimen,” Rasmussen said.

We may end up in a situation where everybody eventually receives a third dose, with more data rolling in that suggests a longer interval between doses does lead to more robust protection. Giving people that third shot could “lock in” long-term immunity, Rasmussen said.

But, for the time being, it’s Americans most at risk who are being put at the front of the line for boosters.