WASHINGTON (NewsNation Now) — The head of the Centers for Disease and Prevention warned Friday that emerging coronavirus variants could stall recent gains against the virus that has ravaged the United States, killing more than 500,000 Americans.

Dr. Rochelle Walensky, director of the CDC, said at a White House COVID-19 team response briefing that the agency’s biggest concern is the spread of virus mutations in the United States. Along with a more transmissible strain first detected in Britain, scientists are tracking new variants appearing in New York and California, which seem to spread more easily.

“We may be done with the virus, but clearly the virus is not done with us,” says Walensky, stressing to continue wearing masks and avoiding gatherings.

“Now is not the time to relax restrictions,” she continued.

The nation’s top infectious diseases expert echoed her concern about the emerging variants.

“Viruses will not mutate if you don’t give them the opportunity to replicate,” Dr. Anthony Fauci said.

Walensky warned that the recent decline in COVID-19 cases may be stalling, a development she called concerning. Cases and hospitalizations have fallen dramatically since the January peak that followed the winter holidays. Deaths have also declined.

This comes as Pfizer Inc and BioNTech SE said Thursday they are testing a third dose of their COVID-19 vaccine to better understand the immune response against new variants of the virus.

Last month, Moderna said while its vaccine is effective against emerging variants of coronavirus that have appeared in the United Kingdom and South Africa, it would test a vaccine booster and an alerted booster against the South Africa variant. While the vaccine is protective against the strains detected as of January, it seemed less effective against the South African variant.

A group of independent advisers to the Food and Drug Administration is also meeting virtually Friday to discuss a recommendation about the J&J vaccine. Once they send their recommendation to the FDA, the agency is expected to make a decision on authorizing the one-shot vaccine within the coming days. President Joe Biden said the promise of a third vaccine receiving approval could come as soon as this weekend.

Biden announced earlier this month that the U.S. will have enough supply of the coronavirus vaccine by the end of the summer to inoculate 300 million people and remains focused for now on making sure every American is inoculated, administration officials say.

Last year, President Biden announced a vaccination goal of 100 million coronavirus shots in his first 100 days in office.

Biden is on track to beat his goal of 100 million injections in his first 100 days in office — though the pace must pick up even further to meet his plans to vaccinate nearly all adults by the end of the summer.

More than 91.6 million vaccine doses have been distributed across the United States, with more than 68.2 million doses administered, according to the CDC. The United States topped more than 500,000 confirmed coronavirus deaths Monday, according to data complied by Johns Hopkins University.