WASHINGTON (NEXSTAR/AP) — Dr. Anthony Fauci is sticking with his projection that a safe and effective coronavirus vaccine may be ready in early 2021. He says it’s possible it could be sooner, but “unlikely.”

The director of the National Institute of Allergy and Infectious Diseases told “CBS This Morning “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.”

Fauci says it’s routine for late-stage vaccine studies to be put on hold because of side effects. A study by AstraZeneca of a potential coronavirus vaccine was recently paused for safety reasons after an illness from a shot in a recipient in Britain.

He says it’s “really one of the safety valves that you have on clinical trials such as this.”

This contradicts with a timeline being pushed by President Donald Trump. He’s hoping to have a vaccine available as early as late October.

Earlier this week, Trump accused Democrats of “disparaging” his aggressive timeline. He leveled the accusation after Democratic vice presidential candidate, Sen. Kamala Harris, said she would trust public health experts and scientists over Trump.

Trump is aiming to have 300 million doses of a coronavirus vaccine in stock by January under a program he calls “Operation Warp Speed” that is working to accelerate development. 

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

The Associated Press contributed to this report.