Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, at Thursday’s press briefing.

In his first White House press briefing of the Biden administration, Dr. Anthony Fauci warned that new strains of the virus that causes Covid-19 now in circulation could make the currently available vaccines and antibody therapies less effective.

“We’re paying very, very careful attention to this, and we take it very seriously,” Fauci said.

Fauci was careful not to sound a general alarm. But he brought the issue to widespread attention as the new administration struggles to get control of a botched vaccine rollout, and the country reports roughly 200,000 new Covid-19 cases per day. President Joe Biden, meanwhile, said Thursday that the U.S. is likely to see the death toll from the virus pass 500,000 people next month.

“Right now, from the reports we have—literally, as of today—it appears that the vaccines will still be effective against [the new strains], with the caveat in mind you want to pay close attention to it,” Fauci said Thursday.

Various new strains of the virus, known as SARS-CoV-2, have spread around the world, including one widespread in the U.K. and one found in South Africa. While the U.K. strain is present in many states across the U.S., the South African strain has yet to be detected in America.

Earlier this week, Pfizer (ticker: PFE) and BioNTech (BNTX) scientists published results of a laboratory study that suggested that their vaccine was likely to protect against the U.K. strain. But studies of the South African strain, which have not been peer reviewed, but which were the subject of a Wall Street Journal report Friday, suggest that the current vaccines may be less effective against it.

S&P 500 futures were down 0.7% on Friday morning as Fauci’s cautious words were reported widely in the press.

Fauci said that new studies raised the possibility that the current vaccines may be less effective against the new strain in South Africa.

“What they’re saying is that what we likely will be seeing is a diminution” in the “efficacy of the vaccine-induced antibodies,” he said.

Still, Fauci said that wouldn’t mean that the vaccines wouldn’t work at all. “There’s a thing called a ‘cushion effect,’” he said. While the vaccine may not be able to protect as well against the new strains as it can against the original strain, he said, “you’re still well above the line of not being effective.”

“That’s what we’re seeing, both certainly with the UK, which is [a] very minimal effect,” he said. “We’re following very carefully the one in South Africa, which is a little bit more concerning, but nonetheless, not something that we don’t think that we can handle.”

Fauci said that there is greater concern for the effectiveness of the monoclonal antibody drugs that are being used to treat and, in clinical trials, to prevent, Covid-19 infections. “Since monoclonal antibodies bind to a very specific part of the virus, when there’s a mutation there, it has much greater chance of obliterating the efficacy of a monoclonal antibody,” Fauci said. “And we’re seeing in the much more concerning mutations that are in South Africa—and in some respects, Brazil, which is similar to South Africa—that it is having an effect on the monoclonal antibodies.”

The Food and Drug Administration has given emergency use authorizations to Covid-19 monoclonal antibodies from Eli Lilly (LLY) and Regeneron (REGN). Lilly reported Thursday that its antibody appeared to have been able to reduce the risk of nursing home residents and staff coming down with symptomatic Covid-19 infections. And Regeneron, earlier this month, announced a deal with the U.S. government worth up to $2.6 billion for as many as 1.25 million doses of its Covid-19 antibody.

Shares of both companies were effectively flat in premarket trading on Friday.

Fauci said that the threat of less-effective vaccines was an argument for more vaccination, not less. More vaccinated people, he said, would mean less chances for the virus to mutate. “If you can suppress that by a very good vaccine campaign, then you could actually avoid this deleterious effect that you might get from the mutations,” he said.

Fauci also said that there are plans to update the vaccines if necessary. Last week, BioNTech’s CEO said at investor conference that his company could have an updated version of its vaccine available within six weeks. Pfizer’s chief scientific officer, meanwhile, told Barron’s that the company should “aspire to” getting a new version of the vaccine out in three to four months, if it were needed, with time for preclinical testing and a human trial.

It will be up to regulators to determine how much human testing would be needed for a new version of the vaccine.