CHICAGO (NewsNation Now) — Americans fully vaccinated against coronavirus “do not need a booster shot” at this time, the nation’s top health agencies said Thursday, hours after Pfizer announced it would ask for authorization of a third dose of its vaccine.

“People who are fully vaccinated are protected from severe disease and death, including from the variants currently circulating in the country such as Delta. People who are not vaccinated remain at risk,” the joint statement read in part. “Americans who have been fully vaccinated do not need a booster shot at this time. FDA, CDC, and NIH are engaged in a science-based, rigorous process to consider whether or when a booster might be necessary.” 

The Centers for Disease Control and Prevention and the Food and Drug Administration made the unusual move after Pfizer announced it would seek authorization for a third dose of its vaccine, saying that another shot within 12 months could dramatically boost immunity and maybe help ward off the latest worrisome coronavirus mutant — the delta variant.

“The Pfizer vaccine is highly active against the Delta variant,” Dolsten said in an interview. But after six months, he said “there likely is the risk of reinfection as antibodies, as predicted, wane.” Data would be submitted to the FDA within the next month, he added.

Pfizer’s Dr. Mikael Dolsten told The Associated Press that early data from the company’s booster study suggests people’s antibody levels jump five- to 10-fold after a third dose, compared to their second dose months earlier.

Dolsten pointed to data from Britain and Israel showing the Pfizer vaccine “neutralizes the delta variant very well.” The assumption, he said, is that when antibodies drop low enough, the delta virus eventually could cause a mild infection before the immune system kicks back in.

Experts say the delta variant spreads more easily because of mutations that make it better at latching onto cells in our bodies. The CDC estimated Wednesday that the delta variant is now the dominant strain of coronavirus in the U.S., accounting for more than 51% of cases in the country.

In the U.S., case rates have been rising for weeks, and the rate of hospitalizations has started to tick up, rising 7% from the previous seven-day average, Dr. Rochelle Walensky, director of the CDC, told reporters Thursday. However, deaths remain down on average, which some experts believe is at least partly due to high vaccination rates in people 65 and older — who are among the most susceptible to severe disease.

About 151.1 million people, or 58.5 % of U.S. adults, have been fully inoculated with COVID-19 vaccines made by Pfizer Inc/ BioNTech SE, Moderna Inc. and Johnson & Johnson, according to CDC data, with 173.8 million people, or 67.3% million adult Americans, having received at least one dose of a COVID-19 vaccine.

The Associated Press and Reuters contributed to this report.