America’s coronavirus expert had a choice to make: Stand beside President Trump at a White House briefing or take questions from billionaire techie Mark Zuckerberg on Facebook.

Dr. Anthony Fauci picked the Facebook gig.

Fauci and Zuckerberg had a 37-minute Facebook Live conversation Thursday about the fast-spreading infection, which is forcing tens of millions of Americans to stay off the job and in their homes.

The men talked about many aspects of COVID-19, from the clinical trials for a vaccine to how to slow community transmissions.

Fauci, who leads the National Institute of Allergy and Infectious Diseases, told Zuckerberg that any vaccine would need to be tested on hundreds — possibly thousands — of people before it could be brought to market.

So far, 45 people have been given a vaccine in the first phase of its testing.

“At the fastest we can go, it’s going to take a year and a half to know if we have a vaccine that we can use,” Fauci told the Facebook CEO.

Later, Fauci cautioned: “This would not be the first time, if it happened, that a vaccine that looked good in initial safety [testing] actually made people worse.”

On Friday, Trump and Fauci sparred over whether an anti-malarial drug would be effective in treating coronavirus. The president called the drug a “game-changer” and Fauci called for more testing.

During the Facebook Live town hall, Fauci stressed that the US is still in the “escalation phase” of the virus, which spreads “too efficiently.”

The native New Yorker told Zuckerberg that he hopes social distancing will stymie the virus, much the way it did with the related virus that causes Severe Acute Respiratory Syndrome.

“What I certainly would like to see is what happened with SARS,” Fauci said. “When public health measures essentially suppressed it, it disappeared and never came back.”