Booster shots aren't necessary right now, US health agencies say. Pfizer is seeking authorization anyway.

• Fully vaccinated Americans don't need booster shots right away, US health agencies said on Thursday.
• Pfizer announced earlier that day that it plans to seek authorization for a booster next month.
• The spread of the Delta variant has raised concerns about how long vaccine protection lasts.

Pharmaceutical companies and disease experts increasingly disagree about when booster shots might be necessary.

Pfizer announced Thursday that it plans to seek authorization for a booster shot next month. But in a joint statement later the same day, the Food and Drug Administration and Centers for Disease Control and Prevention said Americans don't need boosters right away.

Pfizer anticipates that a third dose of its existing coronavirus vaccine could be given six to eight months after the first two, the company's research head, Mikael Dolsten, told Bloomberg.

But research so far indicates that coronavirus vaccine protection lasts for the better part of a year, and likely many months beyond that.

Still, the spread of the Delta variant - the most transmissible version of the virus to date - has raised concerns about the degree to which vaccine protection holds up, and whether it could fade more quickly. Preliminary data from Israel's health ministry found that Pfizer's vaccine was 64% protective against coronavirus infections from June to July, when the Delta variant was spreading widely in the country. The vaccine was 94% effective the prior month, before Delta infections surged.

Dolsten told Bloomberg that these findings indicate antibody levels may have faded among vaccinated people in Israel. That could result in more mild breakthrough cases - the term for COVID-19 diagnoses at least two weeks after someone is fully vaccinated.

But many disease experts say that the goal of vaccines has always been to prevent severe disease and death, which the shots still do in the face of Delta. The Israeli data, along with several other studies, show that Pfizer's vaccine largely prevents hospitalization. Two doses were 93% effective at keeping people in Israel out of the hospital from June to July, and 98% the month prior.

"People who are fully vaccinated are protected from severe disease and death, including from the variants currently circulating in the country such as Delta," the CDC and FDA wrote. The statement added that "virtually all COVID-19 hospitalizations and deaths are among those who are unvaccinated."

Preventing mild cases could still curb the virus' spread

There are benefits to preventing mild coronavirus cases, of course. Nobody wants to get sick, and scientists still don't know whether mild breakthrough cases can leave people with long-lasting symptoms the way some natural infections do.

Researchers also increasingly suspect that it might be easier for vaccinated people to spread the Delta variant to others. (Studies done before Delta emerged indicated that Pfizer's and Moderna's vaccines helped reduce asymptomatic transmission.)

But the CDC stopped monitoring non-severe COVID-19 cases among vaccinated people in May, making the answers to these questions elusive.

Experts don't even know for sure yet whether vaccines are significantly less effective in the face of Delta. A recent study found that 95% of blood samples from vaccinated people developed neutralizing antibodies against Delta after two doses of Pfizer's vaccine - a sign that those people would be protected from a symptomatic infection.

A UK analysis, too, found that Pfizer's vaccine was 88% effective against symptomatic COVID-19 caused by Delta. And a Canadian study similarly found that Pfizer's vaccine was 87% effective against symptomatic Delta infections.

However, the same studies indicate that just a single dose of Pfizer's vaccine was either weakly or not at all effective against Delta.

Drug companies' approach: 'Better safe than sorry'

Insider recently spoke with nine experts about booster shots, and several predicted that follow-up doses wouldn't be necessary for another one to five years. Others questioned whether the general public would ever need another round of shots.

But for the most part, pharmaceutical companies have adopted a "better safe than sorry" approach.

"I think for next fall, we, as a community, should rather be two months too early boosting than two months too late," Moderna CEO Stéphane Bancel said at a Goldman Sachs investor conference in June.

Pfizer's early data does seem to suggest that boosters could offer additional immune protection: A third shot appears to increase neutralizing antibody levels five-to-10-fold compared to the original vaccine.

But US health agencies are looking for a more holistic picture of how vaccine protection holds up over time, based on laboratory, clinical-trial, and real-world data. Studies from pharmaceutical companies are just one part of that, the CDC and FDA said on Thursday.

Ultimately, the decision to roll out boosters will depend on the level of vaccine efficacy that health authorities are willing to accept - a threshold that hasn't been determined yet. Research might also still reveal that antibody responses only dip in certain groups of people, such as the elderly, people with preexisting health conditions, or those who never caught the disease. So boosters could wind up being recommended for just a limited group.

"We are prepared for booster doses if and when the science demonstrates that they are needed," the CDC and FDA said.