White House chief medical advisor Dr. Anthony Fauci said Monday that Americans should continue to get two doses of the Pfizer and Moderna Covid-19 vaccines, despite a recent U.S. study that showed the shots are highly effective after just one dose.

A Centers for Disease Control and Prevention study published last week found a single dose of Pfizer’s or Moderna’s Covid-19 vaccine was 80% effective in preventing coronavirus infections among health-care personnel and other essential workers. Two doses are better than one, federal health officials said, adding that the vaccines’ effectiveness jumped to 90% two weeks after the second dose.

While the 80% figure was great news, Fauci said Monday he is still concerned about the length of protection after a single dose, especially with the emergence of highly contagious variants that have shown the ability to evade the protection of the vaccines.

“When you look at the level of protection after one dose, you can say it’s 80%, but it is somewhat of a tenuous 80%,” Fauci said during a White House news briefing on the pandemic. “When you leave it at one dose, the question is how long does it last?”

Highly infectious Covid-19 variants that have shown some resistance to vaccines also pose a challenge, Fauci said. “You’re in a tenuous zone if you don’t have the full impact” of two doses, he said.

Fauci’s comments come as some health experts and public health officials argue the U.S. should prioritize giving Americans just one dose of the vaccines before moving on to second doses, accelerating the pace of vaccinations across the nation.

Unlike Johnson & Johnson’s vaccine, which requires one dose, Pfizer’s and Moderna’s vaccines require two shots given three to four weeks apart. In the U.K., health officials decided to stretch the time between first and second doses to 12 weeks in an effort to speed up vaccinations.

Fauci has repeatedly said over the past few months that the U.S. should stick to the two-dose regimen.

Dr. Paul Offit, a voting member of the FDA’s Vaccines and Related Biological Products Advisory Committee who reviewed both Pfizer’s and Moderna’s vaccine for emergency use authorization, told CNBC last week that studies have shown immunity appears to be actually more “durable” after the second dose, meaning protection may last longer.

The two-dose vaccine regimen also produces 10 times the amount of neutralizing antibodies, which play an important role in fighting the virus, from the first to second dose, Offit told CNBC.

Secondly, and more importantly, scientists also detected so-called T cells after the second dose, another important part of the immune response that usually provides longer-lasting immunity, he said.

Fauci said Monday he “respects” arguments for a one-dose strategy but added the U.S. currently has enough doses to provide first and second doses for Americans. “Although we always continue to keep an open mind, we consider the route we are on to be the best route,” he said.