In this file photo, the first boxes of Johnson & Johnson's COVID-19 vaccine are loaded for shipment from the McKesson facility in Shepherdsville, Kentucky on March 1, 2021.  (TIMOTHY D. EASLEY / POOL / AFP)

Drug giant Merck will help make rival Johnson & Johnson's single-shot COVID-19 vaccine, US President Joe Biden announced on Tuesday, as health officials warned states against loosening restrictions too early amid an increase in infections from contagious variants of coronavirus.

Biden said the Merck and J&J partnership will mean the US will have enough vaccines for all adults by the end of May, sooner than his previous timeline of the end of July.

Johnson & Johnson's vaccine production has been slower than promised. Under its contract, the company was supposed to deliver 12 million doses by the end of February but had less than 4 million ready to ship when the vaccine was authorized on Saturday

The president also announced his administration is directing states to prioritize vaccinating teachers, child care workers and school staff through the administration's partnership with pharmacies. Biden said he wants all such educators and support staff to have at least one vaccine shot by the end of March.

The president said he will use the Defense Production Act to accelerate the production of equipment, machinery and supplies.

J&J's vaccine production has been slower than promised. Under its contract, the company was supposed to deliver 12 million doses by the end of February but had less than 4 million ready to ship when the vaccine was authorized on Saturday.

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It expects to be able to deliver another 16 million doses by the end of the month－still well short of its previous commitments－but will not ship any next week. The company has said it will be able to provide the full 100 million doses it has agreed to supply by its original midyear deadline.

Variants spreading

As Biden announced the Merck and J&J deal, scientists and health experts are worried about several strains of the coronavirus now in the US.

There are now more than 940 reported cases of coronavirus variants, according to data updated on Tuesday by the Centers for Disease Control and Prevention, or CDC.Some 932 of them are the variant first identified in the UK, B.1.1.7.

The variant B.1.351 was first discovered in South Africa and might partly escape the effects of the vaccines.

The warning about variants comes as more US citizens believe there's no big risk in returning to pre-coronavirus life, a new poll said.

An Axios-Ipsos poll published on Tuesday showed 66 percent of those surveyed said they thought the risk posed by a return to pre-pandemic lifestyles was moderate or large. That's the lowest percentage since October.

The groups least likely to see COVID-19 as a risk were people aged 18 to 29, at 58 percent and Republicans, at 49 percent. But a majority of those vaccinated－76 percent－still see the virus as a high risk.

Healthcare experts are warning it might be too soon to think the COVID-19 pandemic will be over soon.

A steady drop in new cases last month also appeared to be leveling off, and there are fears it could reverse course amid yet another wave of infections.

"If the last week tells us anything, it's that this virus will rebound," said Maria Van Kerkhove, the World Health Organization's technical lead officer for COVID-19.

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Rochelle Walensky, head of the CDC, said: "With these new statistics I am really worried about reports that more states are rolling back the exact public-health measures we have recommended to protect people from COVID-19."