The vaccines made by Pfizer-BioNTech and Moderna set off a persistent immune reaction in the body that may protect against the coronavirus for years, scientists reported on Monday.

The findings add to growing evidence that most people immunized with the mRNA vaccines may not need boosters, so long as the virus and its variants do not evolve much beyond their current forms — which is not guaranteed. People who recovered from Covid-19 before being vaccinated may not need boosters even if the virus does make a significant transformation.

“It’s a good sign for how durable our immunity is from this vaccine,” said Ali Ellebedy, an immunologist at Washington University in St. Louis who led the study, which was published in the journal Nature.

The study did not consider the vaccine made by Johnson & Johnson, but Dr. Ellebedy said he expected the immune response to be less durable than that produced by mRNA vaccines.

Dr. Ellebedy and his colleagues reported last month that in people who had survived Covid-19, immune cells that recognize the virus remained in the bone marrow for at least eight months after infection. A study by another team indicated that so-called memory B cells continue to mature and strengthen for at least a year after infection.

Based on those findings, researchers suggested that immunity might last years, possibly a lifetime, in people who were infected and later vaccinated. But it was unclear whether vaccination alone might have a similarly long-lasting effect.

After an infection or a vaccination, a specialized structure called the germinal center forms in lymph nodes. This structure is an elite school of sorts for B cells.

The broader the range and the longer these cells have to practice, the more likely they are to be able to thwart variants of the virus that may emerge.

After infection with the coronavirus, the germinal center forms in the lungs. But after vaccination, the cells’ education takes place in lymph nodes in the armpits, within reach of researchers.

Dr. Ellebedy’s team found that 15 weeks after the first dose of vaccine, the germinal center was still highly active in all 14 of the participants, and that the number of memory cells that recognized the coronavirus had not declined.

“The fact that the reactions continued for almost four months after vaccination — that’s a very, very good sign,” Dr. Ellebedy said. Germinal centers typically peak one to two weeks after immunization, and then wane.

“Usually by four to six weeks, there’s not much left,” said Deepta Bhattacharya, an immunologist at the University of Arizona. But germinal centers stimulated by the mRNA vaccines are “still going, months into it, and not a lot of decline in most people.”

Dr. Bhattacharya noted that most of what scientists know about the persistence of germinal centers is based on animal research. The new study is the first to show what happens in people after vaccination.

The results suggest that a vast majority of vaccinated people will be protected over the long term — at least, against the existing variants. But older adults, people with weak immune systems and those who take drugs that suppress immunity may need boosters; people who survived Covid-19 and were later immunized may never need them at all.

Exactly how long the protection from mRNA vaccines will last is hard to predict. In the absence of variants that sidestep immunity, in theory immunity could last a lifetime, experts said. But the virus is clearly evolving.

A third dose of the Covid-19 vaccine developed by AstraZeneca and the University of Oxford generated a strong immune response in clinical trial volunteers, Oxford researchers reported on Monday.

The finding indicates that the AstraZeneca vaccine could be an option should third shots end up being needed, for example, to extend immunity. To date, the vaccine has been given as two doses, typically between four and 12 weeks apart.

The new data, detailed in a preprint manuscript that has not yet been peer reviewed, came from 90 study volunteers in Britain who were among the earliest to receive the shots in a clinical trial last year. This past March, they were given a third dose, roughly 30 weeks after their second.

Laboratory analyses showed that the third dose increased levels of antibodies to the virus in the volunteers to a point higher than seen a month after their second dose — an encouraging sign that the third shot would be likely to bring greater protection if the effectiveness of two doses waned over time.

“We do have to be in a position where we could boost if it turned out that was necessary,” Prof. Andrew Pollard, an Oxford researcher who has led studies of the vaccine, said in a news conference on Monday. “I think we have encouraging data in this preprint to show that boosters could be used and would be effective at boosting the immune response.”

Scientists and policymakers do not yet know whether booster shots may be needed.

Scientists reported Monday that the vaccines made by Pfizer-BioNTech and Moderna set off a persistent immune reaction in the body that may protect against the coronavirus for years, but it isn’t clear if the same is happening with other vaccines, including AstraZeneca.

Emerging coronavirus variants could also accelerate the need for booster shots. If third shots are deemed necessary in the coming months, their availability could be severely limited, especially in poorer countries that are lacking enough supply to give first doses to their most vulnerable citizens.

Earlier this month, the National Institutes of Health announced that it has begun a new clinical trial of people fully vaccinated with any of the three authorized vaccines in the United States. The goal is test whether a booster shot of the vaccine made by Moderna will increase their antibodies against the virus. Initial results are expected later this summer.

The AstraZeneca vaccine has won authorization in 80 countries since last December but is not approved for use in the United States, which already has more than enough doses of its three authorized vaccines to meet demand. The shot has been the backbone of the struggling Covax program to provide vaccines to poor countries, accounting for more than 88 percent of the doses shipped out to middle- and low-income nations through last week.

AstraZeneca announced on Sunday that the first volunteers had been vaccinated in a separate study assessing a new version of the vaccine designed to protect against the Beta variant of the virus first seen in South Africa. Some study results suggested that the original version of the AstraZeneca vaccine may not be effective against that variant. Professor Pollard said the study would compare the effects of a third dose of the original vaccine against those of boosting volunteers with the new Beta-targeted vaccine.

Australia on Monday faced a grim and unfamiliar challenge: simultaneous outbreaks in several parts of the country — most notably in Sydney — fueled by the spread of the highly infectious Delta variant.

In the outbreak centered in Sydney, which has sent the city into at least a two-week lockdown, cases grew by 18 on Monday and now stand at 130. Other states across Australia also reported new cases and toughened restrictions, and an expansion of Australia’s lagging vaccination program was announced after an emergency cabinet meeting.

But even as anxiety about the outbreak intensified, many Australians found something to lighten the mood: a tale of two naked men caught violating lockdown rules in the woods of a national park, where they fled after a deer startled them out of their au naturel sunbathing on a secluded beach.

The stay-at-home orders introduced on Saturday allow for exercise. Fleeing wildlife nude did not qualify. The men, who were not identified, both received fines of 1,000 Australian dollars ($ 758). Mick Fuller, the police commissioner for New South Wales, which includes Sydney, was not pleased.

“It’s difficult to legislate against idiots,” Commissioner Fuller said at a news conference.

The news traveled quickly across the country to Western Australia, where the state premier, Mark McGowan, who recently closed his state’s borders to anyone from New South Wales, made clear that it was a Sydney thing.

“It wouldn’t happen here in W.A.,” he said. He added that he was worried about at least one of the suspects, saying: “I hope the deer’s OK.”

The two men were found by emergency responders, separately, at about 6 p.m. on Sunday, after one of them called for help, according to a police statement.

One of the men, 30, was found near a remote road in Royal National Park with simply a backpack. The other, 49, was partially clothed. They appeared to have violated the public health orders by leaving home for leisure.

And so, in a moment marked by rising anger at the government for failing to buy and distribute more vaccines, when Australia’s largest city stands to see its economy lose hundreds of millions of dollars from the lockdown, the news of a free-spirited buck-naked couple fleeing a buck — or was it a doe? — allowed for, well, something to smile about.

But there was worry, too, as tens of thousands of people in New South Wales rushed to get tested for the coronavirus, with more than 300 locations around Sydney having been identified as visited by people who were infectious.

That has raised concerns that the outbreak is far from over. Still, no deaths from the new outbreak have been recorded; no one in Australia has died from the virus at all this year. Two people are in intensive care, and officials encouraged everyone who is eligible for a vaccine to line up for one — and otherwise, stay home.

Global Roundup

Prime Minister Pedro Sánchez of Spain announced on Monday that British visitors would have to present a negative Covid-19 test or proof of full vaccination, bowing to concerns about a massive influx of summer tourists from Britain, which has been grappling with the Delta variant of the disease.

Last week, the British government added Spain’s Balearic Islands to its “green list” of countries and territories from which British visitors can return without quarantining, providing a major lift to the islands’ tourism-dependent economies.

But the authorities on the islands then asked Spain’s central government for tougher screening measures for arrivals from Britain. Sensitivities were also raised after an outbreak among hundreds of Spanish students who were visiting Mallorca, the largest of the islands, to celebrate the end of their academic year.

Spain lifted restrictions on British visitors on May 24, just as Germany, France and some other European countries reintroduced quarantine rules for the British in order to avoid the spread of the Delta variant. Since then, Germany and France have pushed for a British quarantine obligation to be applied across the European Union, but so far to no avail, as countries like Spain rely heavily on British visitors in the summer tourism season.

In other news from around the world:

• Italy said on Monday that people were no longer required to wear masks outside, joining Spain and France in relaxing the rules as cases dropped. Masks must still be worn indoors and in crowded areas. In Rome, many still wore masks on Monday, citing concerns about the Delta variant, but some took advantage of the new rules. “It feels like freedom,” said Francesca Tronconi, a tour guide, as she crossed Piazza Navona with her mask around her arm.

• Young people in Greece will be offered an incentive to get vaccinated, Prime Minister Kyriakos Mitsotakis said on Monday, in the form of a “freedom pass” with 150 euros, about $ 180, in prepaid credit to be spent on tourism, culture and travel. The pass will be available from July 15 for people ages 18 to 25 who have had at least one shot. “It is a thank you to youngsters for their patience and persistence and an incentive to get inoculated,” Mr. Mitsotakis said.

• All passenger flights from Britain to Hong Kong will be banned starting Thursday to prevent the spread of the Delta variant, the city’s government said in a statement on Monday. The authorities have added Britain to an “extremely high-risk” list.

Some 4,000 federal offenders who were part of a mass release last year of nonviolent prisoners to help slow the spread of the coronavirus could soon return to prison — not because they violated the terms of their home confinement, but because the United States appears to be moving past the worst of the pandemic.

In the final days of the Trump administration, the Justice Department issued a memo saying inmates whose sentences lasted beyond the “pandemic emergency period” would have to go back to prison.

But some lawmakers and activists are urging President Biden to revoke the rule and use his executive power to keep the prisoners on home confinement or commute their sentences entirely, arguing that the pandemic offers a glimpse into a different type of punitive system in America, one that would rely far less on incarceration.

Mr. Biden has vowed to make overhauling the criminal justice system a crucial part of his presidency, saying his administration could cut the prison population by more than half and expand programs that offered alternatives to detention.

While the White House has yet to announce a decision about those on home confinement, the administration appears to be following the direction of the Trump-era memo.

Andrew Bates, a spokesman for Mr. Biden, said in a statement that the president was “committed to reducing incarceration and helping people re-enter society,” but he referred questions about the future of those in home confinement to the Justice Department.

The White House revisits the emergency declaration every three months, leaving the former prisoners in a constant state of limbo. The next deadline is in July.

Bangladesh will return to national lockdown by the end of the week, responding to a wave of infections that on Monday brought its highest single-day death toll of the pandemic so far.

The looming restrictions, imposed in a series of steps, have sent tens of thousands of migrant laborers in Dhaka, the capital and largest city, scrambling to get to their villages in scenes reminiscent of neighboring India’s migrant exodus last year.

The garment industry, which employs 4.5 million people and makes up 80 percent of the country’s exports, will remain open. But other businesses were instructed to limit their operations to minimum levels of required staffing, and almost all public transportation systems are either closing or already closed.

Residents of Dhaka expect to be largely confined to their homes after Thursday, the first day of what the government has called a “hard lockdown,” though how strictly the measures will be implemented remains to be seen. The government has said the army, police, and border guard will be deployed for strict enforcement.

Bangladesh had slowed the spread of the virus with sporadic restrictions and reduced movement while trying to keep much of the economy open. But a fast-spreading wave now, with barely 3 percent of the population vaccinated, has forced officials to take more drastic measures.

The country reported 119 deaths on Monday, the highest daily toll since the pandemic began, while the test positivity rate was over 20 percent. Bangladesh has officially reported a total of nearly 900,000 infections and 14,172 deaths from the virus, though experts believe the true numbers are much higher.

The current lockdown has been gradual. The government stopped trains and long distance buses last week. It also imposed lockdowns in seven districts surrounding Dhaka, aiming to avert a surge there. Shopping malls are closed, and restaurants are limited to takeout orders only.

The full lockdown, initially expected to last one week, begins on Thursday. All transportation systems except for auto-rickshaws will be shut.

The government has instructed garment factory owners to arrange transportation for their workers during previous rounds of restrictions. When the public transportation was shut in April to slow the spread of the virus, factory owners who did not arrange transportation were accused of violating the order, and workers had to walk for miles twice a day to get to work.

As the latest lockdown approached, ferry stations in Dhaka have been swamped by people trying to cross the river to the southern districts.

Though Congress approved billions in aid for small companies to help them keep paying their employees during the pandemic, it wasn’t reaching the tiniest and neediest businesses.

Then two small companies came out of nowhere and found a way to help those businesses.

They also helped themselves. For their work, the companies stand to collect more than $ 3 billion in fees, according to a New York Times analysis — far more than any of the 5,200 participating lenders.

One of the companies, Blueacorn, didn’t exist before the pandemic. The other, Womply, founded a decade ago, sold marketing software. But this year, they became the breakout stars of the Paycheck Protection Program.

Blueacorn and Womply aren’t banks, so they couldn’t actually lend any money. Rather, they acted as middlemen, charging into a gap between what big banks wouldn’t do and what small banks couldn’t do.

From late February to May 31, when the program ended, the companies processed 2.3 million loans. Most were for less than $ 17,000, and the vast majority went to solo ventures, which are more likely to be run by women and people of color.

All that hustle had downsides, however, including widespread customer service failures. And some lenders now have regrets about signing rushed deals that delivered most of the profit to their partners.

As the country’s vaccination campaign slows and doses go unused, it has suddenly become clear that one of the biggest challenges in reaching mass immunity will be persuading skeptical young adults of all backgrounds to get vaccinated.

Federal officials expressed alarm in recent days about low vaccination rates among Americans in their late teens and 20s, and have blamed them for the country’s all-but-certain failure to reach President Biden’s goal of giving 70 percent of adults at least an initial dose by July 4.

The straightforward sales pitch for older people — a vaccine could very possibly save your life — does not always work on healthy 20-somethings who know they are less likely to face the severest outcomes of Covid.

As public officials race to find ways to entice young adults to get vaccinated, interviews across the country suggest that no single fix is likely to sway these holdouts. Some are staunchly opposed. Others are merely uninterested. And still others are skeptical.

But pretty much everyone who was eager for a vaccine already has one, and public health officials now face an overlapping mix of inertia, fear, busy schedules and misinformation as they try to cajole Gen Z into getting a shot.

Public health experts say vaccinating young adults is essential to keeping infection numbers low and preventing new case outbreaks, especially as the more infectious Delta variant spreads.

Since vaccines became available six months ago, health departments have focused with varying degrees of success on urging groups identified as reluctant — including people living in rural communities, African American residents, conservatives — to get vaccinated.

But in recent days, public health officials have identified young adults as a significant challenge for a country where fewer than a million people a day are receiving a vaccine, down from an April peak of more than 3.3 million.

In a federal report released last week, just over one-third of adults ages 18 to 39 reported being vaccinated, with especially low rates among Black people; among people 24 or younger; and among those who had lower incomes, less education and no health insurance.