After a top Trump official shared optimistic news about a potential coronavirus vaccine, the country's leading infectious disease expert "cautioned" people against high expectations.

"The whole process is going to take a year, a year and a half, at least," Fauci told reporters of a potential vaccine on Tuesday.

The Trump administration has faced criticism from lawmakers and public health experts for attempting to downplay the potential impact of the novel coronavirus in the US.

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After President Donald Trump's health secretary on Tuesday shared "important" news about a potential coronavirus vaccine, the country's top expert on infectious disease stepped in and tamped down expectations.

Speaking with reporters, Health and Human Services Secretary Alex Azar said that a potential vaccine for the novel coronavirus had been developed by scientists "within three days," and that on Monday the Food and Drug Administration authorized the "entry of that vaccine into Phase 1 clinical trials."

After Azar's optimistic remarks, Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, stepped in and made it clear that the US is still at least 12 to 18 months from developing a vaccine for the novel coronavirus.

Fauci said he wanted to "caution everybody" that what Azar described is only the first phase of the development of the vaccine.

"The whole process is going to take a year, a year and a half, at least," Fauci said while standing a few feet away from Trump.

This moment came as the Trump administration has faced ongoing criticism from public health experts and lawmakers over its handling of the novel coronavirus, which originated in China but has spread to dozens of countries across the world.

Fauci, who has advised six presidents, has been a voice of clarity as the country braces for a potential pandemic. "I think that this is going to be one of those things we look back on and say boy, that was bad," Fauci said of the novel coronavirus outbreak in the US in an interview with Politico published Tuesday.

Trump appointed Vice President Mike Pence, who has a spotty record when it comes to health crises and science, to spearhead the administration's response. There have been widespread concerns over efforts by the president and his advisers to downplay the impact coronavirus could have in the US amid stark warnings from the Centers for Disease Control and Prevention regarding its potential to spread.

COVID-19, the disease caused by the novel coronavirus, has killed more than 3,100 people and infected more than 92,000.

At least 184 deaths have been reported outside of mainland China, where the vast majority of the cases have been reported, including nine in the US. The US has reported at least 120 cases.