Over 400,000 Americans received a COVID-19 booster shot over the weekend in response to the Food and Drug Administration’s approval of the Pfizer-BioNTech COVID-19 vaccine, White House coronavirus response coordinator Jeff Zients said Tuesday.

Speaking at a press briefing, Zients said that another one million people have scheduled to get their third dose at a pharmacy in the coming weeks, indicating a “strong start” to the booster rollout.

But as more Americans move to the third layer of protection, millions have not even received their first dose of the coronavirus vaccine.  

“At the same time, our top priority remains first and second shots,” Zients said.

Currently, 64% of Americans have gotten at least one dose of the vaccine while 55% are fully vaccinated. 

This means that over 70 million eligible people remain unvaccinated and are therefore at a greater risk of being hospitalized and dying from COVID and its highly contagious Delta variant, Centers of Disease Control and Prevention Director Dr. Rochelle Walensky said at the Tuesday briefing. 

And for some, the need for a booster shot reinforces their vaccine hesitancy. A new poll revealed that 71% of unvaccinated Americans say that the booster shot is proof that the COVID vaccines don’t work. Meanwhile, about 80% of vaccinated people said they see the booster shots as a good sign.

Walensky explained that “although protection against hospitalization and death remains quite high,” a booster shot is needed because “we are starting to see… some decreased protection against SARS-CoV-2 infection, likely due to both waning and to the Delta variant.”

The booster shot is currently available for those who received a Pfizer primary series, are at least six months after their second dose and are either over the age of 65 or over the age of 18 with an underlying medical condition.

Walensky added that the U.S. has a recent seven-day average of 95,000 COVID-19 cases, 8,500 hospitalizations and more than 1,300 deaths.