NEW YORK — Just over two years ago, a middle-aged lawyer arrived at a Westchester County hospital in New York City, struggling to breathe. What ailed him remained a mystery for days until, on March 2, he tested positive for the new coronavirus.

Within two weeks, subway ridership was down by half. Within three weeks, the state banned social gatherings of any size. Many New Yorkers — the ones with jobs deemed nonessential — stayed home.

Then the ambulance sirens began.

New York City had become the global epicenter of the pandemic. Scenes from overwhelmed hospitals in Brooklyn and Queens replaced earlier ones from Wuhan, China, and northern Italy. Until the virus struck, Mayor Bill de Blasio and his health commissioner, Dr. Oxiris Barbot, had offered optimistic pronouncements of how New York would fare.

That first wave infected nearly one-quarter of the city and killed more than 22,000 residents, according to city data. Over the next two years, New York City would face three more waves, killing 16,000 others and infecting several million more.

The pandemic is not yet over. A subvariant of omicron, BA.2, now makes up a growing proportion of cases in the New York region, according to the Centers for Disease Control and Prevention. Still, New York’s experience of the past two years will be studied for decades to come.

Here are a few of the lessons.

The City Failed to Act Quickly

With growing dread, public health researchers at the Health Department’s headquarters watched a big screen showing emergency room visits across the city, sorted by symptoms. Over the first few days of that March, flulike symptoms were rising.

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Around March 5, the department’s top infectious disease official, Dr. Demetre Daskalakis, and his team were growing convinced the new coronavirus was spreading rapidly, largely undetected.

But when Daskalakis and his boss, the health commissioner, Barbot, presented the data to de Blasio and his advisers, it set off few alarms.

At the time, the city had fewer than a dozen cases confirmed through coronavirus testing. The official case count, however, was not a measure of the outbreak, but of limited test availability because of the CDC’s botched rollout of a test. Fewer than 100 city residents had been tested.

The situation offers a clear lesson: When facing a dangerous new pathogen with pandemic potential, one must sound the alarm early and clearly.

On March 11, the NBA suspended its season and Gov. Andrew Cuomo said the St. Patrick’s Day parade would be postponed.

Less than 40 minutes after the NBA announcement, the mayor emailed his advisers, directing them to learn how other countries were curbing the virus.

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But crucial time had been lost.

The shutdown worked

Public health experts and researchers had long studied the 1918 influenza pandemic, the deadliest in modern history, for lessons. A consensus emerged: In the United States, cities that acted quickly to thin out crowds and close schools, theaters or other venues fared better during a devastating fall wave that year.

Such restrictions could reduce the total number of infections during a wave. They could also “flatten the curve” — reducing the number of people sick at once — so hospitals would not be overwhelmed.

China had stamped out its initial outbreak through lockdowns and by sealing off Wuhan. New York’s shutdown measures, once they began, were much less severe. Still, they managed to cut short the first wave, public health researchers say. Had they been implemented sooner, these experts add, the toll of the first wave would have been lower.

On March 12, Cuomo barred gatherings larger than 500 people. By March 15, de Blasio shuttered bars and restaurants and announced public schools would close the next day. By March 22, the state prohibited all social gatherings.

Hospital admissions peaked the week of March 29 before declining rapidly during April. One study estimated New York City’s “lockdownlike measures” were associated with a greater than 50% reduction in coronavirus transmission.

Even though shutdowns suppress transmission, cases may once again spike when restrictions are lifted. But that did not occur in New York, which gradually lifted restrictions over months. The city’s experience shows lockdowns can prevent many people from getting infected until a vaccine is available or doctors learn how to treat the disease more effectively — especially if other measures, such as mask-wearing, remain in place.

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Safety net hospitals needed more help

The economic and racial inequalities that mark life in New York amplified the virus’s toll. COVID-19 has been especially deadly for those with diabetes, hypertension and obesity — conditions often linked to socioeconomic status. The virus hit hardest in poor and immigrant communities, where crowded homes were more common and social distancing and working from home were rarely options.

The hospitals in these neighborhoods — like Elmhurst Hospital in Queens and Brookdale Hospital in Brooklyn, where patients are predominantly Black or Hispanic — have fewer resources than the big hospital systems in Manhattan. And some were quickly stretched to the breaking point as desperately ill patients filled emergency rooms. Nurses had more patients than they could possibly care for. At some safety net hospitals, patients were three times as likely to die as patients at big Manhattan institutions.

Yet reinforcements were scarce or slow to arrive.

“There was no system in place to immediately share the load,” Dr. Howard Zucker, then the state health commissioner, later said.

After the first wave, the state largely left it up to hospitals to coordinate patient transfers when virus cases are surging. Often big hospitals have partnered with smaller ones.

But two years into the pandemic, hospitals tend to have fewer nurses than before, a result of burnout and higher-paying jobs elsewhere.

Self-help was better than waiting for the CDC

After early shortages of testing kits, plenty were available by the summer of 2020. But large commercial labs were now being inundated with samples. Results were often delayed by days or more than a week, often making them useless for people urgently wanting to know if they might be contagious.

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So the city partnered with a robotics company to open the Pandemic Response Lab, able to process tens of thousands of tests daily.

That did not fix another problem: the long lines of people waiting to get their noses swabbed at clinics during surges. But where the city has established its own public health infrastructure — lessening its reliance on the federal government or the marketplace — it has helped.

For instance, with new variants proliferating by early 2021, the United States had no large-scale system for screening coronavirus samples for mutations and tracking variants. So New York City quickly stitched together its own program, which offered insights about the spread of variants around the city.

Contact tracing provided unexpected benefits

As the first wave of the virus ebbed, New York City hired 3,000 people to call every positive case, and they reached more than 900,000 people between June 2020 and December 2021.

Of those, about half provided the name of at least one contact, and the tracers reached out to about 800,000 additional New Yorkers, asking them to quarantine.

But this huge tracing effort had limited effectiveness in curbing the fast-moving respiratory virus. By the time tracers reached people, they typically had been contagious for days. And it seemed many people named only a fraction of their contacts, if any.

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But a call from a tracer did help people in other ways. Tracers provided information about the virus to the sick in multiple languages. They instructed people on how to isolate and offered a broad list of services. These services grew over time, from free meal and medicine delivery to dog-walking. For those who could not isolate at home, the city offered free hotel rooms.

In all, 360,000 people received help through these services, according to city data.

New York did not plan well for vaccine distribution

One month after New York City received its first vaccine shipments in December 2020, thousands of doses sat for days or weeks in freezers, despite enormous demand.

Cuomo had for weeks stuck to rigid guidelines that prioritized high-risk health care workers and residents and staff members at nursing homes. But those vaccinations were going more slowly than expected, even as virus cases surged.

Under pressure to speed things up, the governor made millions of New Yorkers older than 65 eligible, though New York had nowhere near enough doses for them all. That set off a confused rush for appointments.

Older New Yorkers were forced to navigate glitchy websites, and family members spent hours refreshing websites for slots.

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Vaccine mandates worked

Still, New York’s vaccination effort improved greatly over time. Today, an estimated 87% of adults in the city are fully vaccinated, and 97% have had a least one dose, surpassing national averages.

Sweeping vaccine mandates were a significant factor in achieving those rates.

De Blasio had initially been reluctant to make coronavirus vaccines mandatory. The city deployed vaccination vans to neighborhoods with low rates and even offered free at-home vaccinations.

But many people avoided the vaccine, for varied reasons: anxiety about side effects, distrust of government, the reach of conspiracy theories.

As the delta variant proliferated in the summer of 2021, de Blasio began to apply pressure.

The city required all public schoolteachers to get vaccinated. Then the entire city workforce came under a similar mandate. De Blasio’s moves were mirrored by the state, which required health care workers to get vaccinated.

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The new policies provoked lawsuits and protests. Thousands of health care workers statewide left their jobs, and thousands of municipal workers were placed on unpaid leave. But those numbers were dwarfed by the many holdouts who finally got vaccinated. By March 9, 2022, 97% of the city’s 370,000 workers had received at least one dose, up from 84% when the mandate was first announced in October 2021.

Last September, the city also began requiring proof of vaccination to enter restaurants or other indoor venues, ranging from museums to movie theaters.

By December, just before leaving office, de Blasio announced a sweeping mandate that all New York City employers must require in-person employees to be vaccinated.

Closing schools had mixed results

After closing schools during the first wave, de Blasio was determined to reopen public schools in September 2020, making New York the first big city in the country to do so.

Some teachers protested. But schools did reopen that October, on a hybrid schedule. About 300,000 of the city’s 1 million schoolchildren showed up.

As students returned to their desks, 6 feet apart, the city began to administer virus tests to a fraction of in-person students and teachers. There appeared to be less transmission at school than in the community as a whole.

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After city officials closed schools for a second time, in November 2020, when the city hit a 3% positivity rate, officials later said they regretted it. By the spring, the city began opening many schools five days a week. This school year, a remote option for schools was eliminated.

Still, as the virus has mutated, the dynamics of transmission and the risk to children have grown more uncertain. Young children are far less likely to be vaccinated than adults. And with masks now optional, another layer of mitigation has been removed.

What will happen the next time?

The key lesson of the deadly first wave in March 2020 turned out to be the same as in 1918: Decisive action early was needed to reduce transmission. But that is a difficult task, requiring leaders to act on incomplete information and reorder daily life before catastrophe strikes.

Experts agree this moment is an ideal opportunity to reinvest in public health programs, which have been defunded over decades. Counties and cities could have more health workers providing education and care in poorer neighborhoods, and pandemic surveillance methods could be strengthened.