Faced with a surge of novel coronavirus cases in more than 30 states and floundering poll numbers ahead of the November election, President Donald Trump announced Monday that he is reviving daily coronavirus briefings at the White House.

"I was doing them, and we had a lot of people watching, record numbers watching in the history of cable television," Trump told reporters in the Oval Office. "There's never been anything like it. It's a great way to get information out to the public as to where we are with the vaccines, with the therapeutics."

As more states and businesses have mandated the wearing of face masks, Trump on Monday also gave what appeared to be his strongest endorsement yet to wearing a mask, calling it "patriotic" and tweeting an image of himself wearing one.

For months, White House aides have tried to get Trump to wear a face mask, but he has refused and has worn one only once in public: during a July 11 visit to a military hospital. Trump said in a Fox News interview aired this weekend that he opposes a national mask mandate.

CNN reported that a source "familiar with the president's thinking" said Trump has shifted to encouraging mask wearing primarily because of polls showing Democratic presidential candidate Joe Biden leading him by wide margins in some key states.

Trump told reporters that he would probably hold the first of the new series of briefings on Tuesday at 5 pm EDT.

"We have had this big flare-up in Florida, Texas, a couple of other places," Trump said. "I'll get involved and we'll start doing briefings."

As of Monday morning, 31 states have seen more new cases this past week, compared to new cases from the previous week, according to data from Johns Hopkins University.

Trump's advisers had debated a return to the daily briefings that were held in March and April because he often used them to spar with reporters and criticize governors, members of Congress, China and others while frequently making inaccurate statements about the virus.

On April 25, Trump tweeted: "What is the purpose of having White House News Conferences when the Lamestream Media asks nothing but hostile questions, & then refuses to report the truth or facts accurately. They get record ratings, & the American people get nothing but Fake News. Not worth the time & effort!"

But with the Trump campaign finding it difficult to hold large campaign rallies because of the outbreak, the briefing gives him a chance to return to the public spotlight.

Two potential vaccines against the virus from Oxford University and the Chinese company CanSino have triggered immune responses in hundreds of humans without dangerous side effects, according to two studies published Monday in the British medical journal The Lancet.

The CanSino vaccine has also passed safety tests and is heading for an efficacy trial in Brazil.

On Monday, an Oxford group and the British-Swedish pharmaceutical company AstraZeneca reported that their coronavirus vaccine candidate was shown in early-stage human trials to be safe and to stimulate a strong immune response.

The study, involving 1,077 volunteers, was described as promising.

A second report in The Lancet on a Chinese vaccine showed what researchers not involved in the study described as modestly positive results.

The US government has pledged up to $1.2 billion toward the Oxford effort and secured a promise of 300 million doses by October. A European alliance has claimed 400 million doses, while the UK has a claim on 100 million doses.

Large-scale trials of the Oxford vaccine — named ChAdOx1 nCoV-19 — are underway in the UK, Brazil and South Africa. 

The US plans to test it later this summer, along with a handful of other candidates, in clinical trials, each with about 30,000 volunteers.

The two vaccines are among 23 candidates being tested in human trials, according to the World Health Organization. More than 130 others are in preclinical studies.

Florida reported 10,347 new coronavirus cases Monday, marking the sixth consecutive day the state has reported more than 10,000 new cases.

The American Federation of Teachers, the nation's largest teachers union, and its local affiliate, the Florida Education Association, sued Governor Ron DeSantis on Monday to block his emergency order of earlier this month mandating that schools across the state reopen five days a week starting in August.

They accused the Republican governor of violating a state law requiring that schools be "safe" and "secure". Florida is home to five of the country's 10 largest school districts.

In California, the number of coronavirus cases and hospitalizations in Los Angeles County reached a new high Sunday, and Los Angeles Mayor Eric Garcetti said that the city was "on the brink" of imposing new restrictions to prevent the spread of the virus, but he did not elaborate.

Health officials in Los Angeles County also said that the latest COVID-19 numbers also indicate that the disease has become most prevalent in the relatively young: 53 percent of the new cases occurred in people under 41.

New York Governor Andrew Cuomo threatened to roll back the reopening of New York City's bars and restaurants after a weekend that saw crowds in some parts of the city partying outside.

"We cannot allow those congregations to continue. If it happens, I'll tell you what's going to happen: We're going to have to roll back the opening plan, and we're going to have to close bars and restaurants," he said at his briefing. "I'm telling you; we are right on the line."