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President-elect Joe Biden warned Monday that the promising COVID-19 vaccine developed by Pfizer won’t be widely available for months.

While hailing the “positive’’ medical breakthrough at his first post-election press conference, the Dem said the vaccine and any other that is developed would still have to go through a “rigorous review and approval process’’ by the government to make sure their effectiveness is “grounded in science.

“It’s clear that this vaccine, even if approved, will not be widely available for many months yet to come,’’ Biden said.

Meanwhile, President Trump was unrestrained in his reaction to news of the vaccine, which was developed by the pharmaceutical giant and the German firm BioNTech.

“STOCK MARKET UP BIG, VACCINE COMING SOON. REPORT 90% EFFECTIVE. SUCH GREAT NEWS!’’ the president tweeted.

Biden, in an earlier statement, had praised “the brilliant women and men who helped produce this [vaccine] breakthrough and … give us such cause for hope.’’

But he said during his later televised appearance, “The challenge before us right now is still immense and growing.

“This crisis claimed nearly a thousand American lives a day and nearly 240,000 deaths so far. In fact, [projections are] still indicating we could lose 200,000 more lives in the coming months before a vaccine can be made widely available.

“So we can’t forego the important work that needs to be done between now and then,” he said.

Biden urged continued mask-wearing and for Americans to keep following other recommended safety precautions amid the pandemic, saying now is not the time to be making any political statements by not adhering to the advice of medical experts.

Trump’s critics have accused him of downplaying the virus and ignoring warnings by health experts of its dangers. He has said he is straddling the line between safety and keeping the economy afloat.

Biden said at the press conference that when a vaccine does become available, he wants it to be free to everyone. He did not take any questions after he spoke

The president-elect — who unveiled a coronavirus task force earlier in the day — spoke after he and Vice President-elect Kamala Harris met with the new advisory group, which includes former Surgeon General Vivek Murthy and ex-Food and Drug Administration Commissioner David Kessler.

Biden called the task force a “transition advisory board’’ and said he would be adding new members.

Pfizer and BioNTech announced earlier Monday that they have developed a vaccine against the virus that is 90 percent effective.

Additional reporting by Ebony Bowden