The vaccine developed by the University of Oxford and AstraZeneca not only protects people from serious illness and death but also substantially slows the transmission of the virus, according to a new study — a finding that underscores the importance of mass vaccination as a path out of the pandemic.

The study by researchers at the University of Oxford is the first to document evidence that any coronavirus vaccine can reduce transmission of the virus.

Researchers measured the impact on transmission by swabbing participants every week seeking to detect signs of the virus. If there is no virus present, even if someone is infected, it cannot be spread. And they found a 67 percent reduction in positive swabs among those vaccinated.

The results, detailed by Oxford and AstraZeneca researchers in a manuscript that has not been peer-reviewed, found that the vaccine could cut transmission by nearly two-thirds.

Matt Hancock, the British health secretary, hailed the results on Wednesday as “absolutely superb.”

“We now know that the Oxford vaccine also reduces transmission and that will help us all get out of this pandemic,” Mr. Hancock said in an interview Wednesday morning with the BBC.

The results, he said, “should give everyone confidence that this jab works not only to keep you safe but to keep you from passing on the virus to others.”

The Oxford and AstraZeneca researchers also found that a single dose of the vaccine was 76 percent effective at preventing Covid-19. The data measured the three months after the first shot was given, not including an initial three-week period needed for protection to take effect.

The encouraging results, lend support to the strategy deployed by Britain and other countries to prioritize providing as many first doses of vaccines as possible, setting aside concerns that people will get their second doses later than initially planned.

The latest data do not have bearing on the debate over whether to further space out the doses of the two vaccines authorized in the United States, those from Pfizer-BioNTech and Moderna, since the data on AstraZeneca’s candidate cannot be generalized to other vaccines.

Some scientists have called on the United States to follow the lead of Britain and other countries that have opted to delay the second doses of vaccines by up to 12 weeks. But U.S. federal officials have resisted, saying such a move would not be supported by the data from clinical trials of the two vaccines currently available across the nation. Tuesday’s results could amplify pressure on U.S. health officials to delay second doses of AstraZeneca’s vaccine, though it has not yet been authorized by the country.

The vaccine appeared more effective when the interval between the two shots was longer than the originally intended four-week gap, the Oxford and AstraZeneca researchers found. Among clinical trial participants who got two standard-strength doses at least three months apart, the vaccine was 82 percent effective, compared to 55 percent effective when the doses were given less than six weeks apart.

A vaccination strategy that spaces out doses by three months “may be the optimal for rollout of a pandemic vaccine when supplies are limited in the short term,” the researchers wrote.

The newly released study builds on data issued late last year, which found that the vaccine was 62 percent effective when given as two standard-strength doses. In those initial findings, the vaccine’s efficacy was much higher, at 90 percent, when the first dose of the vaccine was given at half-strength.

Oxford and AstraZeneca researchers initially attributed the different levels of effectiveness to the lower strength of the initial dose. But they gradually reached a different conclusion: the amount of time between doses was the more likely explanation.

In the United States, the Food and Drug Administration is waiting on data from a clinical trial that enrolled about 30,000 participants, mostly Americans. Results from that study are expected later this month.

The study is expected to arm AstraZeneca with enough safety data to allow it by around early March to seek authorization to provide the vaccine for emergency use.

The United States has agreed to buy 300 million doses of AstraZeneca’s vaccine, but neither the company nor the federal government has said when and in what quantities those doses will be available after the vaccine is approved.

Almost half of U.S. states have begun allowing teachers to be vaccinated as officials decide which groups should be given higher priority for early protection against the coronavirus, a New York Times survey shows. By this week, 24 states and Washington, D.C., were providing shots to teachers of kindergarten through high school students.

How quickly states give shots from a growing but still limited vaccine supply to teachers has become a central element of the heated debate about how best to reopen school systems, just as more contagious virus variants are emerging and spreading.

In some states where many teachers are already teaching with in-person classes, teachers are not yet eligible for vaccines. And for many places where classes are mainly remote, vaccinating teachers has been a first step to returning children to classrooms, though it is not the only factor.

“This discussion is not about if we return, but how we return,” Stacy Davis Gates, a leader of the Chicago Teachers Union said recently amid a standoff in that city over whether students younger than high school — and their teachers — should return to classrooms immediately. “And how we return is with the maximum amount of safety that we can obtain in an agreement.”

Last week, the Centers for Disease Control and Prevention weighed in with a striking message: Children should return to classrooms because it’s safe for them to do so.

The agency said the “preponderance of available evidence” indicated that in-person instruction could be carried out safely as long as mask-wearing and social distancing were maintained. Its researchers found “little evidence that schools have contributed meaningfully to increased community transmission” when proper safety precautions were followed.

There was an important caveat: Local officials also must be willing to impose limits on other settings — like indoor dining, bars or poorly ventilated gyms — to keep infection rates low in the community at large.

The debate remains far from settled, and teachers’ unions across the country have pressed for teachers to be given high priority for receiving a vaccine.

Oregon began vaccinating K-12 teachers last month, giving them an earlier spot in line than some residents of 75 years or older, who are only eligible for shots in certain counties. Gov. Kate Brown, a Democrat, said the move was part of her plan to bring students back into the classroom during this school year.

“For every teacher who is back in the classroom, they help 20, 30, 35 students get their lives back on track,” Ms. Brown said. “They help ensure 20, 30, 35 kids have access to mental health support. They make sure 20, 30, 35 kids get breakfast and lunch several days a week. And they allow families to know their children are in good hands when they go to work.”

As part of Gov. Mike DeWine’s goal to bring Ohio students back to in-person learning either full- or part-time by March 1, Ohio began vaccinating teachers in certain counties this week. Union officials have praised that decision, but say it is not the sole answer to getting back to school safely. Children will not yet have shots, they note, nor will all adults in schools.

“Even when educators are able to be vaccinated, it will remain critically important to continue following all C.D.C. guidance to keep our schools safe and open for in-person instruction when possible,” Steve DiMauro, president of the Ohio Education Association, said in a news release.

Whether the threshold is 65 or 80, people who meet their state’s age requirement are eligible to get a Covid-19 vaccination shot in every U.S. state but one.

The exception: Rhode Island, the only state still in the first phase of its vaccination campaign, which restricts access to health care workers and residents of long-term care facilities.

If Barbara Diener, 65, had known she still wouldn’t be eligible for the vaccine in Rhode Island — where she and her husband moved to be closer to family — she said they might not have moved there in November from Georgia, where anyone 65 or older can get a shot.

“We moved here three months ago from Georgia, where everyone I know has been vaccinated,” said Ms. Diener, a travel agent. “We’re a little disappointed that the state of Rhode Island, being as small as it is, just can’t get it together.”

The state health director, Dr. Nicole Alexander-Scott, said last week that Rhode Island was expanding eligibility slowly because the state wanted to make sure that the people most at risk were vaccinated first.

“In addition to how many people you vaccinate, who you vaccinate matters,” Dr. Alexander-Scott said. “And that’s what distinguishes Rhode Island, and how we are taking this thoughtful approach.”

Ms. Diener and her husband both came down with Covid-19 last March, and her husband was hospitalized for two weeks. They want to get inoculated as soon as they can. But since neighboring states are limiting vaccinations to their own residents, they have little choice but to wait.

“My doctor is in Massachusetts, because we’re so close in proximity, but I can’t get it in Massachusetts either,” Ms. Diener said.

Rhode Island’s health department said last week that it was starting to vaccinate a small number of especially vulnerable people 75 and older — people who are part of a registry indicating that they need extra assistance during an emergency. In about two weeks, the department plans to start making doses available to everyone in that age group.

State residents ages 65 to 74 are forecast to become eligible at the end of the month, according to the health department’s projections.

Dr. Alexander-Scott said the state was being strategic because of its “very limited supply” of vaccine.

The state has administered about 54 percent of its available vaccine doses so far, and has given at least one of the two required shots to 6.5 percent of its population, a lower proportion than most other states.

In New York City, despite its many major hospitals and research institutions, only about 55 coronavirus cases a day on average last month were sequenced and screened for more contagious variants.

That amounted to just 1 percent of the city’s new cases, a rate far below the 10 percent that some experts say is needed to understand the dynamics of the pandemic in New York at a time when variants that may blunt the effectiveness of existing vaccines have led to surging cases in Britain, Brazil and South Africa.

By the end of February, New York City health officials hope to have a more robust surveillance program in place that would involve sequencing the genomes — that is, examining the genetic material for mutations — of about 10 percent of new virus cases, according to Dr. Jay Varma, a senior public health adviser to City Hall.

With an average of more than 5,000 new cases a day in recent weeks, that would provide a good picture of which variants are present in New York and how widely they are proliferating, Dr. Varma said.

But the effort to reach that benchmark is underscoring how, a year into the pandemic, local, state and federal officials have often been slow to mobilize resources for public health needs.

“Trying to get laboratories that are primarily focused on things like human genome sequencing specifically for cancer or other conditions to shift their interest to work on pathogens sometimes takes effort,” he said.

The capacity to sequence the genomes of as many virus samples as necessary already exists, scattered across the city, though it has been largely untapped.

“Every institution is doing their own thing,” Professor Adriana Heguy of New York University’s Grossman School of Medicine, whose team has been sequencing some 96 samples a week and whose research last year helped establish that New York’s pandemic arrived via Europe, not China.

With its major academic medical centers and research institutions, there are far more sequencing machines in New York than would be needed to check the coronavirus genomes from every positive case, were anybody inclined to do so.

“Our machines could handle thousands or hundreds of thousands,” said Dr. Neville Sanjana, a scientist with a lab at the New York Genome Center in Lower Manhattan. “So the capacity is just not the issue.”

The issue for research laboratories — strangely enough, amid a pandemic that has probably infected more than a quarter of New Yorkers — is access to samples. In New York, there is no high-volume pipeline of positive virus samples from hospitals or testing sites to research laboratories to conduct genetic surveillance.

McClurg, in Missouri’s rural Ozarks, is more a crossroads than a town, and it’s home to a particular strain of old-time dance music that is not played in precisely the same way anywhere else.

Pre-pandemic, in an abandoned general store, Alvie Dooms, 90, and Gordon McCann, 89, would play rhythm guitar. Nearly a dozen more musicians would join in on fiddle, mandolin, banjo and upright bass. Their tunes had names like “Last Train Home,” “Pig Ankle Rag” and “Arkansas Traveler.”

The McClurg jam, as the Monday night music and potluck fest was known, endured for decades, the last gathering of its kind in the rural Ozarks. But the coronavirus pandemic has silenced the instruments, at least temporarily. And the suspension has led to worry: What will become of this singular musical tradition?

“Because it’s ear music, it’s a little bit fragile,” said Howard Marshall, 76, a retired professor at the University of Missouri and a fiddler himself. “I’m not playing it exactly like the next chap will play it.”

“I’m one of the younger ones, and I’m 74,” said Steve Assenmacher, a bass player who lives just up the hill from the McClurg Store and acts as its caretaker.

When the pandemic led Missouri officials to limit in-person gatherings last spring, the musicians pledged to find a way to keep the sessions going. Last May, they moved to a barn adjacent to the store and seated visitors at the entrance, in the open air.

The jams continued for most of the year. But in mid-November, as alarmed hospital officials warned that their facilities were nearing capacity and the frigid temperatures became unpleasant, the jam session was called off indefinitely.

Dr. Marshall said the internet has guaranteed that many of the songs will endure. It’s the stories behind the songs and institutional knowledge that will disappear if jams like McClurg cease to exist.

Even if the musicians aren’t stricken with Covid-19, he said, an extended pause is precious time — because they can “get rickety” with age.

And that’s exactly what is happening with the McClurg elders.

Mr. McCann suffered a second stroke in November.

And Mr. Dooms, who has survived three major heart attacks in his life, said that his “lungs ain’t no good.”

Once the winter has passed, Mr. Assenmacher hopes to welcome musicians back to the open-air barn. But, he said, until the musicians have been vaccinated and public health officials declare widespread immunity, the old general store in McClurg will remain closed.

global roundup

Iran announced on Tuesday that its first batch of Covid-19 vaccine, Russia’s Sputnik V, would arrive on Thursday. It coincided with a study published on Tuesday in The Lancet, a British medical journal, that the vaccine was safe and effective.

The news could not come at a better time for Iran. Vaccines, like everything else in the Islamic Republic, have been politicized and subject to disinformation from the top down.

Iran’s supreme leader has declared a ban on importing American- and British-made vaccines, saying they could not be trusted. The Iranian Health Ministry canceled a batch of donated Pfizer doses and said it would import vaccines from Russia and China and develop a vaccine with Cuba.

For weeks, senior Iranian health officials have publicly argued over whether it was safe to inoculate Iranians with the Russian vaccine because it had not been approved by the World Health Organization or any Western medical agency.

Before the Lancet study said that Sputnik V had a 91.6 percent efficacy rate against the coronavirus, several high-profile Iranians had expressed skepticism about it, including Iran’s top infectious disease official and the head of Parliament’s health committee. And 98 physicians from Iran’s largest union of medical workers wrote a letter last week to President Hassan Rouhani urging him not to purchase the “unapproved and unsafe” Sputnik vaccine.

Other health officials pushed back, defending the Russian vaccine, including the health minister and his former spokesman.

On Twitter, a hashtag trending in Persian was #buysafevaccines, as many Iranians declared that they would reject Russian- and Chinese-made vaccines.

In other news from around the world:

• New Zealand’s drug regulator said on Wednesday that it had provisionally approved the Pfizer-BioNTech vaccine but added 58 conditions, most of which require the manufacturer to supply extra data. Pfizer said last week that the first of the 1.5 million vaccines on order were expected to arrive before the end of February. New Zealand’s director-general of health, Ashley Bloomfield, has said that the country will be “ready to start vaccinating people as soon as a vaccine arrives.”

• The state of Victoria, Australia, which includes the city of Melbourne, declared the coronavirus “technically eliminated” on Wednesday — for the second time — after a month in which no new cases were recorded. The state government also announced a rollout plan for the Pfizer-BioNTech vaccine that is scheduled to begin in late February. Frontline health workers will be among those included in the first phase, and nine vaccination hubs will be established at the state’s major hospitals. “This is a significant day in the response of the Victorian community to the pandemic,” said Martin Foley, the state’s health minister. “It turns a corner.”

• Prime Minister Justin Trudeau of Canada said on Tuesday that his government had signed a tentative deal with Novavax to produce the company’s coronavirus vaccine at a government facility in Montreal, once the drug and the site are approved by domestic regulators. Canada has a separate agreement to purchase 52 million doses of the Maryland-based company’s drug, but its regulators have so far only approved the Pfizer-BioNTech and Moderna vaccines. Novavax is expected to deliver results from its Phase 3 clinical trials in the United States in March, and to deliver 100 million doses for use there later this year.

A military judge on Tuesday indefinitely postponed the arraignment of three prisoners at Guantánamo Bay who were scheduled to make their first court appearance in 17 years of detention, finding that the coronavirus pandemic made it too risky to travel to the Navy base.

The Indonesian prisoner known as Hambali, who has been held since 2003 as a former leader of a Southeast Asian extremist group, and two people accused of being his accomplices were scheduled to appear at the war court on Feb. 22. But Col. Charles L. Pritchard Jr., the military judge who was to travel to Guantánamo this week, ruled that “the various counsels’ belief that their health is at significant risk by traveling” to the base in Cuba was reasonable.

Colonel Pritchard is the most recent military judge to join the Guantánamo military commissions bench, and the latest to postpone a proceeding as too risky in nearly a year of cancellations because of the coronavirus. Pretrial hearings in the death-penalty case against five men accused of plotting the Sept. 11, 2001, attacks have been delayed for a year.

The judge, court staff and lawyers bound for the hearing began a quarantine in the Washington area over the weekend before a charter flight to the base on Thursday.

Once there, the passengers were to be individually quarantined for 14 days under a plan devised by prosecutors to protect those living on the base of 6,000 residents and at the prison from the threat of infection.

The case had been dormant throughout the Trump administration, but on the second day of the Biden administration, a senior Pentagon official appointed under the Trump administration who had been put in charge of the military commissions approved the prosecutions.

The defendants include Mr. Hambali, who is charged as Encep Nurjaman and accused of being the former leader of the extremist group Jemaah Islamiyah, and those accused of being his accomplices, Mohammed Nazir Bin Lep and Mohammed Farik Bin Amin, who are Malaysian.

The three men were captured in Thailand in 2003 and are charged with conspiring in the 2002 nightclub bombings in Bali, which killed 202 people, and the 2003 Marriott Hotel bombing in Jakarta, which killed at least 11 people and wounded at least 80. They spent their first three years in the C.I.A.’s secret prisoner network before they were transferred to Guantánamo for trial in 2006.