The United States may not need AstraZeneca's COVID-19 vaccine even if it wins regulatory approval, says top infectious disease doctor Anthony Fauci.
The vaccine, once hailed as another milestone in the fight against the pandemic, has been dogged by questions since late last year, even as it has been authorised for use by dozens of other countries.
Fauci, director of the National Institute of Allergy and Infectious Diseases and chief medical adviser to the White House, said on Thursday the United States had 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 AstraZeneca 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 US 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.
"If you look at the numbers (of doses) 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," Fauci said.
Meanwhile, a Californian community that has been a bellwether of the pandemic's rampage across the United States warns an increase more contagious variant cases is worrisome.
"The region's progress in curbing the pandemic remains precarious," Santa Clara County health department said.
"County residents are therefore urged to avoid travel, quarantine if travelling and consistently use face coverings."
The situation in Santa Clara, home to an early virus surge in California last year and the nation's first COVID-19 death, offers a window into the pandemic's progress across the US.
Several states, including Florida and Michigan, are struggling to contain a variant-linked resurgence.
The national 7-day daily case average has increased continuously since March 19.
The average daily number of new infections has jumped 17 per cent, from 55591 on March 19 to 64814 on March 31.
Total cases stand at 30,562,884, including 552,932 deaths.
"We're already seeing surges in other parts of the country likely driven by variants," Santa Clara Health Officer Sara Cody said on Thursday.
"Combined with data we are seeing locally, these are important warning signs that we must continue to minimize spread."