The shift comes just one week after the administration was parsing open questions about the scientific need for a third dose of the Pfizer/BioNTech or Moderna COVID vaccines, which are both based on mRNA technology and require two doses for full vaccination. Those vaccines make up the lion’s share of vaccinated Americans, with a much smaller sliver of the population receiving Johnson & Johnson’s single-dose COVID vaccine that relies on a different kind of technology.

Why the change in thinking? For one thing, we’re still learning about how long immunity conferred by widely-used COVID vaccines last and and how effective they are against emerging strains. Somewhat controversial evidence from Israel, for instance, shows that the Pfizer vaccine’s effectiveness in preventing infections plummets sharply over time to just 39%, although it still remains highly effective (90% or above) in stemming severe illness, the need for hospitalization, or death, even with the Delta variant. Global health officials are keeping a close eye on studies like that, which can come with a wide margin of error and significant lag time since these cases must be closely monitored in real patients over months and years.

The emerging dominance of the Delta variant, which makes up the majority of new reported COVID cases over the past month, is likely also at the back of public health officials’ minds. The highly transmissible strain also leaves the infected with extremely high viral loads of the coronavirus, which means the vaccinated may very well be able to spread it to those who haven’t received COVID shots and are not taking other safety precautions such as masking and social distancing.

Centers for Disease Control (CDC)

“It is one of the most infectious respiratory viruses we know of, and that I have seen in my 20-year career,” said Centers for Disease Control (CDC) director Rochelle Walensky in a briefing last Thursday.

The CDC’s data can take time to pool and doesn’t track all cases of new infections in fully vaccinated people. But companies like Pfizer are keeping their own tabs on the figures in global trials, with the drug giant saying its COVID vaccine’s effectiveness in preventing a new infection that comes with COVID symptoms drops from 95% to 84% between four and six months after full inoculation with two doses.

But many of the most important practical questions are still up in the air. For a third dose, the focus will likely be in the populations at highest risk for getting a nasty coronavirus case, such as the elderly, sick, or immunocompromised. Those populations might be recommended to receive a Pfizer or Moderna booster about nine months after their first dose of an mRNA vaccine in order to keep immunity as robust as possible for as long as possible. Not all Americans will necessarily need a booster, as some public health experts have noted, even if they have a new COVID case that only presents with mild or moderate symptoms, since staving off hospitalization and death is the key goal. But as of now there’s not enough data on how what portion of the vaccinated population are experiencing symptomatic COVID cases.

Regardless, the Biden administration has already bought up enough reserves of the Pfizer and Moderna vaccines to make deploying boosters en masse plausible, and the distribution campaign could theoretically ramp up quickly as Pfizer seeks the Food and Drug Administration’s emergency authorization for a third dose of its COVID shot in August.