More than 400,000 Americans received a Covid-19 booster shot at pharmacies over the weekend after the CDC cleared third doses of Pfizer and BioNTech‘s vaccine to a wide array of Americans, White House coronavirus response coordinator Jeff Zients said Tuesday.

At a press conference, he said nearly one million people are scheduled to obtain their additional dose from a pharmacy in the next few weeks. He also stated that federal and state preparations have made a “strong start”.

He said, “Nevertheless, the top priority is still first and second shots.”

Overall, roughly 2.8 million Americans have received an extra dose since health officials authorized the third shots of Pfizer or Moderna’s vaccines to people with weakened immune systems in August, according to data compiled by the Centers for Disease Control and Prevention.

Federal health officials have been expressing concern about Zients’ remarks, saying that they see a decrease in immunity to infection protection several months after the first dose. According to them, the shots still prove highly effective against serious disease, hospitalizations, deaths and other complications.

On Friday, Dr. Rochelle Walensky, Director of the CDC signed off to a number of recommendations. These included distribution of the shots at older Americans or adults who have underlying medical conditions within six months of their last series.

Walensky also authorized booster shots for people in high-risk institutional and occupational settings. The move was approved by the agency’s Advisory Committee on Immunizing Practices, which had rejected the same proposal just hours before.

President Joe Biden received a booster shot on Monday since his age at 78 made him eligible for a booster under the CDC’s latest guidance. 

Walensky said last week that officials will move “with the same sense of urgency” on recommendations for Moderna and Johnson & Johnson boosters as soon as that data is available.

Moderna submitted its application for booster shots on Sept. 1 and J&J said Wednesday that it also submitted its data showing that an extra dose of its single-shot vaccine raises protection against infection to 94%.