CHICAGO (Reuters) — The United States may not need AstraZeneca’s COVID-19 vaccine, even if it wins U.S. regulatory approval, Anthony Fauci, the nation’s top infectious disease doctor told Reuters on Thursday.

The vaccine, once hailed as another milestone in the fight against the COVID-19 pandemic, has been dogged by questions since late last year, even as it has been authorized for use by dozens of countries, not including United States.

Fauci, director of the National Institute of Allergy and Infectious Diseases and chief medical adviser to the White House, said the United States has enough contracts with other vaccine makers to vaccinate its entire population, and possibly enough for booster shots in the fall.

Asked whether the United States will use the AstraZeneca vaccine doses, he said, “That’s still up in the air. My general feeling is that given the contractual relationships that we have with a number of companies, that we have enough vaccine to fulfill all of our needs without invoking AstraZeneca.”

Late last year, the drugmaker and Oxford University published data from an earlier trial with two different efficacy readings as a result of a dosing error. Then in March, more than a dozen countries temporarily suspended the use of AstraZeneca’s vaccine after reports linked it to a rare blood clotting disorder.

Also in March, a U.S. health agency said data from the company gave an incomplete picture of its efficacy. Days later AstraZeneca published results showing diminished, though still strong, efficacy.

Fauci said that “If you look at the numbers (of doses) that we’re going to be getting, the amount that you can get from J&J, from Novavax from Moderna if we contract for more, it is likely that we can handle any boost that we need, but I can’t say definitely for sure.”