Anthony Fauci said he worries about the "durability" of a potential coronavirus vaccine, saying there's a chance it may not provide long-term immunity.

The National Institutes of Health has been fast-tracking work with biotech firm Moderna on a potential vaccine to prevent Covid-19, which has infected more than 6.28 million people worldwide and killed at least 375,987, according to data compiled by Johns Hopkins University.

When asked whether scientists will be able to find an effective vaccine, Fauci said he's "cautiously optimistic," adding that "there's never a guarantee." He cautioned "it could take months and months and months to get an answer" before scientists discover whether the vaccine works. .

officials and scientists are hopeful a vaccine to prevent Covid-19 will be ready in the first half of 2021 — 12 to 18 months since Chinese scientists first identified the coronavirus and mapped its genetic sequence.

Fauci, the nation's leading infectious disease expert, said in congressional testimony last month that he is hopeful scientists would find a workable candidate but warned of potential pitfalls in developing any vaccine. .

"You can have everything you think that's in place and you don't induce the kind of immune response that turns out to be protective and durably protective," Fauci said of a vaccine.