The Trump administration recommended on Tuesday giving a coronavirus vaccine to everyone over 65 in the United States, in an attempt to accelerate lagging distribution as Covid-19 deaths have soared to their highest levels since the pandemic began.

The Department of Health and Human Services announced the new guidelines at a briefing. Axios earlier reported the new guidelines.

More than 375,000 people have died from Covid-19 in the United States since the start of the pandemic. In recent days, the number of daily deaths in the country has topped 4,000.

As of Monday, about 9 million people have received at least one dose of a Covid-19 vaccine, according to the Centers for Disease Control and Prevention, far short of the federal government’s original goals. At least 151,000 people in the United States have been fully vaccinated, as of Jan. 8, according to a New York Times survey of all 50 states.

Health officials also recommend that the vaccines be given to all adults with pre-existing conditions that make them more likely to develop serious illness from the virus, such as diabetes, chronic lung or heart disease, high blood pressure and cancer. Currently, the vaccines are largely being distributed to people in the highest-risk categories, including frontline health care workers and older people in nursing homes.

In addition to the eligibility changes, health officials are also adding more community centers and pharmacies to the list of places where people can be vaccinated.

H.H.S. will also no longer hold back vaccine doses to ensure that those who receive a first dose will have a second dose in reserve. Instead, all existing doses will be sent to states to provide initial inoculations. Second doses are to be provided by new waves of manufacturing. Both vaccines authorized in the United States so far require two doses: 21 days apart for the one developed by Pfizer and BioNTech, and 28 days apart for the one from Moderna.

That change comes just days after aides to President-elect Joseph R. Biden Jr. said his administration would make a similar adjustment by using more of the already procured vaccines for initial doses. Mr. Biden’s team said it would aim to distribute the doses more quickly at federally run vaccination sites at high school gyms and sports stadiums, and mobile units to reach high-risk populations.

The idea of using existing vaccine supplies for first doses has raised objections from some health workers and researchers, who worry that frontloading shots will raise the risk that second injections will be delayed. Clinical studies testing the vaccines showed the shots were effective when administered in two-dose regimens on a strict schedule. And while some protection appears to kick in after the first shot, experts remain unsure of the extent of that protection, or how long it might last without the second dose to boost its effects.

But others have vocally advocated for explicit dose delays, arguing that more widely distributing the partial protection afforded by a single shot will save more lives in the meantime.

The new recommendations come after some states have already begun vaccinating people 65 and older, leading to long lines and confusion over how to get a shot. Health experts and officials have faced difficult choices as they decided which groups would be prioritized in the vaccine rollout. While the elderly have died of the virus at the highest rates, essential workers have borne the greatest risk of infection, and the category includes many poor people and people of color, who have suffered disproportionately high rates of infection and death.

Despite the bumpy rollout, Gov. Ron DeSantis of Florida, who prioritized people 65 and older from the start, said he believed making all older people eligible was always the right thing to do.

The initial guidelines “would have allowed a 20-year-old healthy worker to get a vaccine before a 74-year-old grandmother,” he said on Tuesday at a news conference in the sprawling retirement community of The Villages. “That does not recognize how this virus has affected elderly people.”

In New York, which began vaccinating people 75 and older and more essential workers this week, Gov. Andrew M. Cuomo said that the state will accept the new federal guidance to prioritize those 65 and older, though he criticized the administration for not clearly defining who should be considered “immunocompromised.”

The new guidance will make more than 7 million New Yorkers eligible for the vaccine, Mr. Cuomo said, though the state only receives 300,000 doses a week.

“The federal government didn’t give us an additional allocation,” he said. “At 300,000 per week, how do you effectively serve 7 million people, all of whom are now eligible, without any priority?”

New Yorkers 65 and older are immediately able to schedule appointments on the state’s website, according to Melissa DeRosa, a top Cuomo aide, who added that the state was working with the C.D.C. on who is considered immunocompromised.

New guidelines released on Monday by the Centers for Disease Control and Prevention now note that while people should get their second shots “as close to the recommended 3-week or 1-month interval as possible,” there is “no maximum interval between the first and second doses for either vaccine.”

The update perplexed experts, who said that while other, previously licensed vaccines that involve multiple doses can be administered months or even years apart, no evidence yet exists to clearly support this strategy for Covid-19. “They will need to back this up with data,” said Marion Pepper, an immunologist at the University of Washington.

Dr. Leana Wen, an emergency physician at the George Washington School of Public Health, echoed the call for an explanation. With skepticism of vaccines already hindering the rollout of some shots, “the last thing we want to do is give the impression that there are shortcuts being taken in the approval process.”

Health officials in Britain are now allowing intervals between the first and second doses of Pfizer’s vaccines of up to 12 weeks. Last week, the World Health Organization said the injections could be given up to six weeks apart. The agency’s Strategic Advisory Group of Experts on Immunization “considers the administration of both doses within 21 to 28 days to be necessary for optimal protection,” said Saad Omer, a vaccine expert at Yale University who helped draft the WHO’s position on the matter.

In response to queries about dose delays, representatives from Pfizer and Moderna have repeatedly pointed to the company’s clinical trials, which tested dosing regimens of two shots, separated by 21 days for Pfizer, and 28 days for Moderna.

“Two doses of the vaccine are required to provide the maximum protection against the disease, a vaccine efficacy of 95 percent,” Steven Danehy, a spokesman for Pfizer, said earlier this month. “There are no data to demonstrate that protection after the first dose is sustained after 21 days.”

Representatives from the C.D.C. did not respond immediately to requests for comment.

Dr. Mark Ghaly, California’s secretary of health and human services, said at a news conference on Monday that the state was working to distribute vaccines to those who need them and want them — without allowing wealthy people to cut the line.

Mr. Newsom said the state was allowing a broader range of workers to administer vaccines, including pharmacists and dentists, and was rolling out a public awareness campaign in 18 languages.

“People have said, ‘Well, what about sending in the National Guard?’” he said of the groups administering vaccines. “Well, we have the National Guard out there.”

He also said there were urgent efforts to “vaccinate the vaccinators.”

Three Democratic members of Congress have tested positive for the coronavirus, and say they believe their infections are linked to their time spent in a secure location with colleagues who did not wear masks during last week’s siege of the U.S. Capitol.

Representative Brad Schneider, Democrat of Illinois, said he received a positive test result Tuesday morning after driving home to Illinois, and that he did not have symptoms. Like Representatives Bonnie Watson Coleman of New Jersey and Pramila Jayapal of Washington, two Democrats who had announced positive tests on Monday, he directly blamed a group of House Republicans who refused to wear masks while sheltering in a secure location during the Capitol siege.

“Today, I am now in strict isolation, worried that I have risked my wife’s health and angry at the selfishness and arrogance of the anti-maskers who put their own contempt and disregard for decency ahead of the health and safety of their colleagues and our staff,” Mr. Schneider said.

He called for lawmakers who ignore public health guidance to be sanctioned “and immediately removed from the House floor by the Sergeant-at-arms for their reckless endangerment of their colleagues.”

Capitol Hill has long struggled to contain the spread of the virus, and within hours of the beginning of the 117th Congress on Jan. 3, lawmakers began announcing positive test results.

Now lawmakers, aides, police officers and reporters who fled to secure locations during the siege have been warned that they might have been exposed to the virus while sheltering from the mob.

On Sunday, Representative Chuck Fleischmann, Republican of Tennessee, who was also in protective isolation at the Capitol during the siege, said that he had tested positive for the virus after being exposed to his roommate, Representative Gus Bilirakis of Florida, also a Republican.

Mr. Fleischmann told the local news station WRCB that he was notified Wednesday that Mr. Bilirakis had tested positive, but did not receive the notification amid the riot. He said he did not know how many other lawmakers he had come in contact with.

Democrats, already frustrated by resistance from their Republican colleagues to wearing masks, accused maskless Republicans in the secure House location of reckless indifference.

“It angers me when they refuse to adhere to the directions about keeping their masks on,” Ms. Watson Coleman said in an interview. “It comes off to me as arrogance and defiance. And you can be both, but not at the expense of someone else.”

Ms. Jayapal said on Twitter that she had tested positive “after being locked down in a secured room at the Capitol where several Republicans not only cruelly refused to wear a mask but recklessly mocked colleagues and staff who offered them one.”

Ms. Jayapal, who said she had begun quarantining immediately after the siege on the Capitol, also said that any member of Congress who did not wear a mask should be removed from the floor by the sergeant-at-arms and fined.

“This is not a joke,” she said in a statement. “Our lives and our livelihoods are at risk, and anyone who refuses to wear a mask should be fully held accountable for endangering our lives because of their selfish idiocy.”

This year’s Masters tournament in April will be attended by a limited number of spectators, the Augusta National Golf Club announced Tuesday. The club, which prohibited fans from the event two months ago, did not specify how many fans would be allowed in 2021, adding that spectators would be permitted if “it can be done safely.”

The 2020 Masters was postponed from its usual April date to November because of the coronavirus pandemic and was contested with protocols that included virus testing before the event for all players, caddies, club members, staff and other personnel, including a reduced number of media members.

Fred Ridley, the club chairman, said in a statement issued Tuesday that similar health standards would be instituted for this year’s tournament, which is scheduled to be contested from April 8 to 11. The club, based in Augusta, Ga., made the announcement as the state reported 16 new coronavirus deaths and 7,957 new cases on Jan. 11. Over the past week, there has been an average of 9,604 cases per day, an increase of 55 percent from the average two weeks earlier.

“Following the successful conduct of the Masters Tournament last November with only essential personnel, we are confident in our ability to responsibly invite a limited number of patrons to Augusta National in April,” Ridley said. “As with the November Masters, we will implement practices and policies that will protect the health and safety of everyone in attendance.”

The Augusta National statement said the club was in the process of communicating with all ticket holders and that refunds will be issued to those patrons not selected to attend.

Another new coronavirus variant has been detected in four people who traveled to Japan from Brazil.

Japan’s health ministry said that the people who arrived this month at Tokyo’s Haneda Airport had tested positive for the coronavirus and that it was a separate variant with similarities to those detected in Britain and South Africa. It is also distinct from another variant recently identified in Brazil, according to experts who have analyzed the data.

Makoto Shimoaraiso, an official with Japan’s Cabinet Secretariat and Office for Covid-19 Preparedness and Response, said on Tuesday that the country was consulting with the World Health Organization.

It is not unusual for viruses to accumulate mutations or for new variants to emerge. But scientists are calling for greater surveillance of variants, particularly after those from Britain and South Africa proved to be more contagious.

Mr. Shimoaraiso said epidemiologists were not sure whether the variant identified in Japan was more infectious or likely to cause more severe illness.

According to Japan’s health ministry, one of the passengers infected with the new variant, a man in his 40s, was admitted to a hospital after having breathing difficulties. Of the other cases, a woman in her 30s and a teenage boy are experiencing sore throats and fever, and a teenage girl is asymptomatic.

In recent weeks, scientists have raised concerns about a coronavirus variant first detected in December in South Africa, noting that this version of the virus may spread more quickly than its cousins, and perhaps be harder to quash with current vaccines.

Their worries are compounded by skyrocketing Covid-19 cases in the United States and another highly infectious new variant that is driving a surge in Britain.

Scientists still have a lot to learn about these variants, but experts are concerned enough to warn people to be extra-vigilant in masking and social distancing. Here’s what you need to know:

• The British variant has been found in about 50 countries, including the United States, where dozens of cases have been identified. The South African variant has spread to about 10 countries but has yet to be detected in the United States.

• Both variants carry genetic changes in the virus’s spike protein — the molecule used to unlock and enter human cells — that could make it easier to establish an infection. Researchers estimate that the British variant is about 50 percent more transmissible than its predecessors. Julian Tang, a virologist at the University of Leicester, said that researchers didn’t yet have a good estimate for how much more contagious the South African variant is.

• There is no evidence that any of the new variants are more deadly on their own, but an uptick in the spread of any virus creates ripple effects as more people become infected and ill. That can strain already overstretched health care systems and undoubtedly lead to more deaths.

• It is unlikely that either variant will completely evade the protective effects of the new Covid vaccines. A recent study, not yet published in a scientific journal, found that the Pfizer-BioNTech vaccine is still effective against a virus carrying a mutation common to both new variants.

  The South African variant does carry genetic changes that could make vaccines less effective: One mutation appears to make it harder for antibodies produced by the immune system to recognize the coronavirus, which means they may be less effective at stopping the variant. But it is “important to note that doesn’t mean vaccines won’t be functionally protective,” said Angela Rasmussen, a virologist affiliated with Georgetown University.

  Vaccines use multifaceted immune responses, and while some antibodies may be confused by the variant, others probably won’t be. In addition, antibodies are only one sliver of the complex cavalry of immune cells and molecules that battle infectious invaders.

  Also, if the virus accumulates more genetic changes, many of the authorized vaccines, including Pfizer’s and Moderna’s, can be adjusted fairly quickly.