CNN  — 

President Joe Biden said Tuesday the United States would have enough Covid-19 vaccine doses for every adult American by the end of May, dramatically accelerating his timeline but acknowledging the country must still be vigilant against the virus.

He also announced he would direct states to prioritize teachers in their vaccination plans, reemphasizing a commitment to returning students to classrooms amid a confusing administration stance on when and how classrooms can reopen.

The new date does not mean all Americans would receive shots by May 31; issues with distribution and personnel mean it could take much longer for all doses to be administered.

But the speedier estimate, helped along by a new partnership between major US drugmakers, makes for an optimistic target as millions of Americans wait to receive their first doses. The timeline speeds up by two months the administration’s previous goal of having enough shots by the end of July.

“That’s progress,” Biden said in remarks from the White House, warning there could still be delays in getting shots in Americans’ arms and suggesting the country would not return to full normalcy for much longer.

“There is light at the end of the tunnel, but we cannot let our guard down now to ensure victory is inevitable, we can’t assume that. We must remain vigilant, act fast and aggressively and look out for one another. That’s how we’re going to get ahead of this virus, get our economy going again and get back to our loved ones,” Biden said.

Biden also announced he was directing states to prioritize teachers in their vaccine distribution plans in a bid to reopen schools.

“Let’s treat in person learning like an essential service that it is,” he said, adding his goal was for every teacher or school worker to receive at least the first dose of a vaccine by the end of March.

In his remarks, Biden announced that Merck would help manufacture a recently authorized vaccine developed by Johnson & Johnson, which allowed for the new May goal. The arrangement is unusual and underscores the urgency in manufacturing and distributing enough vaccine doses to inoculate as many Americans as possible.

Biden hailed the partnership as a way to quickly jumpstart the sluggish vaccine production, likening it to cooperation between corporations during World War II.

The White House said it was utilizing the Defense Production Act to help equip two Merck facilities to manufacture the Johnson & Johnson product, including by bolstering “fill-finish” capacity when the doses are places in vials and by increasing availability of the components of the vaccines.

The decision came about after it became clear to administration officials that Johnson & Johnson would fall short of their manufacturing goals.

“It simply wasn’t coming fast enough,” Biden said.

The planned partnership was first reported by The Washington Post.

The US Food and Drug Administration granted emergency use authorization to the Johnson & Johnson vaccine on Saturday, and the US Centers for Disease Control and Prevention recommended it for adults age 18 and older. The vaccine’s distribution is already underway and the first shots occurred Tuesday in Ohio.

Two Covid-19 vaccines from Pfizer/BioNTech and Moderna have already been authorized for emergency use in the United States by the FDA. Unlike those vaccines, Johnson & Johnson’s vaccine only requires one shot.

Merck had been working on developing its own Covid-19 vaccine, but discontinued its effort at the end of January after early studies showed immune responses were inferior to natural infection and other Covid-19 vaccines.

Merck is expected to dedicate two of its facilities to helping Johnson & Johnson, an administration official said, in a rare partnership between two competitors.

Administration officials expressed surprise and disappointment last month when it became clear Johnson & Johnson would have fewer initial doses of its vaccine than originally planned because of production issues at its facilities.

The company said it had about 4 million doses of its vaccine ready to ship “immediately,” and said it should have 20 million ready by the end of March.

“We’ve developed an extensive partnership here in the United States and Europe and other places around the world and we’re very confident in our ability to deliver 20 million doses by the end of March and 100 million doses in the first half of the year en route to a billion doses by the end of this year,” Alex Gorsky, the CEO and chairman of Johnson & Johnson, said Monday on CNN.

The announcement came as Biden and federal health officials warn against relaxing coronavirus restrictions as gains against the pandemic appear to plateau.

“We’re making progress from the mess we inherited. We’re moving in the right direction. And today’s announcements are a huge step in our effort to beat this pandemic. But I have to be honest with you, this fight is far from over,” Biden said on Tuesday. He cautioned that things could continue to get worse as new variants spread and unforeseen setbacks like winter weather hamper vaccine distribution.

“Though we celebrate the news of a third vaccine, I urge all Americans, please keep washing your hands. Stay socially distanced. Wear masks, keep wearing them. Get vaccinated when it’s your turn. Now is not the time to let up,” Biden added.

The President also reiterated his call for all Americans to wear masks for his first 100 days in office, hours after Texas Gov. Greg Abbott and Mississippi Gov. Tate Reeves, both Republicans, announced their states would be lifting mask mandates.

During a White House Covid-19 Response Team briefing Monday, CDC Director Dr. Rochelle Walensky said she is “deeply concerned” about the potential shift in the trajectory of the pandemic.

While the number of new daily cases had been declining, the most recent seven-day average of new cases – at about 67,200 people – represents an increase of a little more than 2% compared to the prior seven days. The most recent seven-day average of deaths has also increased more than 2%.

“Please hear me clearly: At this level of cases with variant spreading, we stand to completely lose the hard earned ground we have gained,” Walensky said Monday.

Walensky said the country can stop the surge of cases in this country by wearing a mask that fits, maintaining social distance, practicing good hand hygiene and avoiding crowds.

“Please stay strong in your conviction,” Walensky said. “Continue wearing your well-fitting mask and taking the other public health prevention actions that we know work.”

CNN’s Jason Hoffman and Kristen Holmes contributed to this report.