The nation’s top infectious disease expert is standing by his timeline for a possible coronavirus vaccine, reiterating this week that he expects a safe and effective vaccine to be available by early 2021.

Dr. Anthony Fauci, the White House adviser on the coronavirus and the head of the National Institute of Allergy and Infectious Diseases, said Wednesday on “CBS This Morning” that the country should know “by the end of the year” if one of the vaccine candidates currently in advanced trials will be ready on his expected time frame.

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“I have said from the beggining, given the way the trials have emerged now, including the one on hold now, the projection that I’ve made — and will stick by it — is that we’ll likely get an answer if it’s safe and effective by the end of the year, likely November or December,” he said, referring to pharmaceutical company AstraZeneca on Tuesday putting a hold on its experimental coronavirus vaccine study in America, after a participant in the United Kingdom faced an apparent serious adverse reaction, according to a report.

The expert noted that it’s “possible” that a coronavirus vaccine may be ready sooner, but warned that this is “unlikely.”

“The more likely scenario is that we will know by the end of this calendar year and hopefully we’ll be able to start vaccinations in earnest as we begin early 2021,” he noted.

Speaking to the AstraZeneca news, Fauci said it’s routine for late-stage vaccine studies to be put on hold because of side effects, noting that it’s “really one of the safety valves that you have on clinical trials such as this.”

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Fauci added people should remain vigilant about wearing masks, keeping physical distance and avoiding crowds to help prevent future surges.

The United States leads the world in coronavirus cases with 6.3 million and nearly 190,000 deaths.

Fox News' Frank Miles and the Associated Press contributed to this report.