Until 2020, few Americans needed to think about how viruses spread or how the human immune system works. The pandemic offered a painful crash course. Sometimes, it seemed that the science was evolving as quickly as the virus itself.

So The New York Times asked experts to revisit the nightmare. Of the most significant public health measures introduced during Covid, which have held up scientifically, and which turned out to be wrongheaded?

The question is particularly important now, because pandemics that could upend American lives are inevitable. One candidate has already surfaced: bird flu.

Perhaps the biggest lesson learned, several experts said, is that recommendations during any pandemic are necessarily based on emerging and incomplete information. But during Covid, federal agencies often projected more confidence in their assessments than was warranted.

Next time, the scientists said, officials should be more forthright about the uncertainties and prepare the public for guidance that may shift as the threat comes into clearer focus.

Rather than promote preventive measures as infallible solutions, they should also acknowledge that no single intervention is perfect — though many imperfect measures can build a bulwark.

If you venture out in a “huge, heavy rainstorm, your umbrella alone is not going to keep you from getting wet,” said Linsey Marr, an expert in airborne viruses at Virginia Tech.

“You need your umbrella; you need your boots; you need your waterproof pants and jacket; and you would probably try to avoid the puddles,” she said.

Vaccines

A victory, but officials oversold the benefits at first.

The mRNA vaccines were, in a sense, victims of their own unexpected success in clinical trials in 2020. Those results were spectacular: The shots warded off symptomatic illness caused by the original version of the coronavirus at miraculous rates.

But government officials had to walk back their enthusiasm as breakthrough infections with the Delta variant surged in the summer of 2021. Americans were told to get boosters. Then again, and again.

Federal health officials should have acknowledged at the start that the long-term effectiveness was unknown, said Natalie Dean, a biostatistician at Emory University.

Mistrust over the safety and effectiveness of the Covid vaccines is now taking a toll on other immunizations, including those targeting childhood diseases like measles.

“Making claims early on that this was going to prevent all infections was, I think, a little bit of an overpromise” that eventually undermined public trust, said Saskia Popescu, an infection prevention expert at the University of Maryland.

Still, the vaccines saved an estimated 14 million lives just in the first year after their introduction.

Airborne Spread

Surfaces were not the problem. Indoor air was.

Disagreements among scientists about how the coronavirus traveled had profound ramifications for how Americans were told to protect themselves.

Early on, health officials insisted that the virus was spread through large droplets that were coughed or sneezed out by an infected person onto other people or objects. The “fomite” theory led to protocols that made little sense in retrospect.

Remember the plexiglass barriers during the Presidential debates? The face shields? Schools closed for cleaning days midway through the week. People were scrubbing down groceries and mail.

“The whole hygiene theater was terribly unfortunate,” said Michael Osterholm, an infectious disease expert at the University of Minnesota. It wasted millions of dollars and gave people a false sense of security.

Health agencies took months to admit that the virus was carried aloft by tiny droplets, called aerosols, that could be exhaled, traveling long distances indoors. Sadly, that insight initially led to another overreaction.

Some states closed down beaches and parks, and forbade interactions outdoors, even though “there’s good scientific evidence that outdoor events are lower risk,” Dr. Dean said.

Eventually, understanding that the virus was primarily floating indoors prompted the Biden administration to earmark funds for improved ventilation in schools.

Masking

It worked if you used the right masks, correctly.

As the pandemic spread in the United States, masking morphed from a public health intervention into a cultural flashpoint.

Assuming that the coronavirus traveled like the flu and worried that hospitals might not have enough resources, federal heath officials at first told the public that masks were not needed.

That advice was suddenly reversed once scientists learned that the coronavirus was airborne. Even so, officials initially recommended cloth masks — which are not very effective at keeping out airborne viruses — and did not endorse more protective N95 respirators until January 2022, well after much of the public had stopped using cloth masks.

Dozens of studies have shown that when used correctly and consistently, N95 masks or their equivalents can prevent infected people from spreading the virus and protect wearers from contracting it.

Unfortunately, several flawed studies and the politics of personal freedom created a culture war surrounding the use of masks, especially by children, said Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health.

In the event of another respiratory outbreak, “I feel quite anxious that a whole constituency has already discarded masks,” he said.

Children in Asia routinely wear masks, especially during respiratory virus and allergy seasons, some experts noted.

“I wish we could infuse more infection prevention into especially elementary schools during respiratory virus season,” Dr. Popescu said. “It seemed like a really great way to get children back in schools.”

Herd Immunity

A chimera. We never got there.

For nearly two years after the pandemic began, experts talked of reaching herd immunity once enough of the population had acquired protection either by being ill or getting vaccinated.

That was a mistake, experts said. Herd immunity is only possible if immunity is sterilizing — meaning it prevents reinfections — and lifelong. Immunity to most viruses is neither.

Seasonal coronaviruses change rapidly enough that people undergo repeated infections throughout their lives, said Jeffrey Shaman, an epidemiologist at Columbia University who insisted early on that the new coronavirus might also cause reinfections.

Once vaccines arrived, officials at first presented the shots as a way to stay safe from the virus forever, rather than as a means to lessen the severity of infections.

“There was a lot of confusion and misconceptions about herd immunity — that the toothpaste was going back in the tube somehow,” Dr. Dean said.

School Closures

Necessary at first. Questionable as time went on.

Few aspects of the pandemic provoke as much rancor as school closures. In many parts of the country, test scores never recovered and absenteeism has become an intractable problem.

But experts said it was the right decision to close schools in the spring of 2020, when a poorly understood pathogen was sweeping across the country. Ideally, schools would have reopened that fall, but with measures — improved ventilation, testing, masks — to mitigate the risks.

“And of course, we didn’t really have any of those things,” Dr. Hanage said.

By early fall in 2020, it was clear that schoolchildren were not driving community transmission significantly. Still, many schools stayed closed for months longer than they needed to, forcing children to muddle through remote learning and causing some to fall irrevocably behind.

“It’s a really difficult one to Monday-morning quarterback,” Dr. Shaman said.

“We don’t have the counterfactual, that alternative scenario to see how it really would have played out.”

If bird flu turns into a pandemic, it would be foolish to base school policies on how the coronavirus behaved, he and others warned. Other respiratory viruses, like the flu, tend to be deadlier among young children and older adults.

“We have every reason to think that a future flu pandemic would be far more dangerous to young people than Covid was,” Dr. Hanage said. “I think we should talk about what we could do to mitigate transmission in schools.”

Lockdowns

They slowed the virus, but the price was high.

The pandemic destroyed local businesses, sent unemployment rates soaring and increased household debt. Many people now feel that lockdowns were to blame for much of the damage — and that their harms outweighed any benefits.

Many scientists see it differently. “The economy got shut down by just the pure force of the pandemic,” said Dr. Osterholm.

No American state’s policies neared the strictness of those in China, India, Italy or Jordan — where people were not allowed to leave home at all — and much of the work force and societal activities continued because they were deemed essential, he noted.

By the end of May 2020, indoor dining and religious services had resumed in much of the country, if they had been paused at all, although many cities continued to institute temporary bans as virus levels rose and fell.

The shutdowns may have been unpopular in part because they were introduced with no clear explanation or end in sight.

Instead, Dr. Osterholm said, health officials could have instituted a “snow day” concept. People stayed home when hospitals were overwhelmed, as they do when roads are snowed under, but their behavior returned to normal when the situation eased.

The shutdowns eased the burden on hospitals and slowed the transmission of the virus, buying time to develop a vaccine. Studies from multiple other countries have also shown that stay-at-home orders and restrictions on mass gatherings were the most effective measures for curbing transmission of the virus within communities.

“Whatever people did in 2020, before folks were vaccinated, saved millions of lives,” Dr. Hanage said. “If we had done nothing, truly done nothing at all, things would have been much, much worse.”