Is it now reasonable to discuss the end of the pandemic? Yes, but with caveats.

For more than a year, everyone has wondered when this dreadful pandemic will end. The answer has always been “not for a long time.” That answer, however, has been overtaken by events - at least in the United States.

The end of the pandemic may not be near, exactly, but it’s no longer rash, impolitic or scientifically dubious to broach the topic.

New coronavirus infections in the United States have dropped to their lowest rate since mid-September and, if trends continue, will within days be lower than they have been in nearly 11 months. The numbers are good across the coronavirus data dashboard. More than half of adults have had at least one shot of vaccine, and the shots are remarkably effective at preventing severe illness and death.

The federal government has looked at those trends and on Thursday revised the guidelines for vaccinated people, telling them they can stop wearing masks under most circumstances, indoors as well as outdoors. Coast to coast, communities are flinging wide-open the doors to their restaurants, businesses and stadiums.

Speaking Thursday afternoon at the White House, President Joe Biden came close to declaring outright victory over the virus.

“When your country asked you to get vaccinated, you did. The American people stepped up. You did what I consider to be your patriotic duty,” Biden said. “That’s how we got to this day.”

Still, pandemics start quickly and end slowly. And there is a huge caveat: Infectious-disease experts have repeatedly made clear that the coronavirus,known officially asSARS-CoV-2, is unlikely to be eradicated.

The only disease-causing virus in humans driven to extinction is smallpox. Others have been eliminated in geographical areas but lurk somewhere on the planet and can be reintroduced. Some measures to combat the novel coronavirus - including booster shots and perhaps mask-wearing and social distancing during winter, when respiratory viruses spread more easily - could remain part of our lives.

But the pandemic as we know it - a massively disruptive, lethal and terrifying health emergency that for months and months has been killing at rates comparable to cancer - could soon begin a gradual fade into memory.

That, at least, is the current, rather nuanced and potentially confusing consensus of scientists and public health officials. They remain concerned about the threat of variants that may yet emerge with an array of mutations that permit the virus to evade the human immune response. But so far, the vaccines have remained effective.

The seven-day average for daily infections dropped below 36,000 on Thursday, about half the number reported in mid-April during a moderate spring increase in cases, and barely a tenth of the number from mid-January during the winter wave. Hospitalizations are at the lowest level since early October. Deaths remain at more than 600 a day on average, but also are gradually dropping as the infection pipeline is squeezed, and have not been this low since June 10. So far this month, the nation’s capital has reported no deaths from covid-19, the disease caused by the virus, on seven different days.

U.S. officials have gained confidence that the virus will be brought under control in coming months. Cases are likely to decline sharply by July, according to some models released recently by the Centers for Disease Control and Prevention.

The nation is benefiting from widespread vaccination in combination with natural immunity from coronavirus infections. The contagion may soon experience exponential decay - the opposite of its explosive spread last spring and again in the late fall and winter.

“Covid-19 as a pandemic will end,” Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in an interview. “But it won’t end uniformly throughout the world for obvious reasons, because many countries will not have capabilities to make it end. But as a pandemic, or as a countrywide epidemic, it absolutely will end in this country.”

Achieving that depends on getting at least 70 percent of adults vaccinated, he said. If that happens, Fauci said, by the time autumn arrives, “I think we’re going to get out of the epidemic stage and much more into the control stage.”

This remains a long-duration event, and there’s not likely to be an easily defined end to the pandemic. Experts think the crisis will ratchet down incrementally, with people able to resume normal activities in steps - something already happening as more people venture into restaurants or attend social gatherings.

“There’s not going to be a day we won. There’s not going to be Pandemic End Day,” said Justin Lessler, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health and co-author of the recent CDC report suggesting high vaccination rates could drive infections to low levels this summer.”It’s going to be a gradual process. There’s going to be new variants.”

But Lessler said the optimistic scenarios laid out in his report look more likely to become reality than the more pessimistic scenarios in the analysis.

“It does look like, for the most part, vaccination is winning,” he said.

There are multiple caveats that cloud this sunny outlook.

First: The vaccination goal is not a given. Polls suggest a significant portion of the public does not want to be inoculated.

Second: Mutations in the virus could erode the efficacy of vaccines. So far, variants that have emerged possess limited ability to evade vaccine-induced immunity. That includes B.1.1.7, first seen in the United Kingdom and now dominant in the United States, and a new variant spreading in India, B.1.617, amid a catastrophic outbreak. That variant is rapidly seeding itself globally. The virus continues to evolve, and as it collides with human immunity, it may evolve in ways that allow it to outwit the immune system.

Third: The U.S. population is not fully safe from the coronavirus (or any other virus) until the whole planet is safe. The global picture is alarming. Less than 25% of the global population has been infected so far, according to Maria Van Kerkhove, an epidemiologist with the World Health Organization. The coronavirus has plenty of fuel still available, and new introductions of the virus into the United States are possible for a long time to come.

Fourth: Many experts believe the coronavirus will behave like influenza and other seasonal respiratory viruses, and flare anew come winter. That would be the fifth distinct wave of this pandemic (counting the big surges last spring and in the winter, and the smaller bumps last summer and this spring).

With fewer people vulnerable, a wave this coming winter would probably be less brutal than last time around. But much depends on whether people would be willing to take precautions again, such as wearing masks.

“How can you convince the public, after a good summer, that winter is going to be bad?” said Ali Mokdad, an epidemiologist with the University of Washington’s Institute for Health Metrics and Evaluation.

Fifth: A pandemic is a psychological, cultural and political event as well as a biological one. This has been a global trauma, with almost everyone affected and some grievously so. Many people will not feel past the pandemic until they are confident there is zero chance they could get infected with the coronavirus - a tough standard, given the nature of invisible pathogens.

Others may say it’s over when they don’t have to wear a mask anymore - something that as of Thursday became closer to reality for vaccinated people in the United States.

Since the earliest days of the pandemic, the concept of herd immunity has been a frequent - and frequently confusing - topic of discussion. The threshold of herd immunity is when there is so much collective immunity to a virus that it cannot easily infect people who are still vulnerable or generate new chains of infection. Fauci points to a herd of wildebeest: If there are enough strong, densely massed animals, they can usually protect the young or elderly or sickly animals in their midst from attacks by roaming predators.

No one knows exactly what percentage of a population needs to be immune to achieve herd immunity. Nor does reaching that threshold suddenly end the pandemic. It’s not an off switch.

Moreover, rising levels of immunity begin having an effect even before the herd immunity threshold is reached. This is happening now in the United States.

Early last year, at the start of the pandemic, the virus circulated among populations with no immunity at all. Now, the virus is slamming, again and again, into brick walls of immunity. There are still tens of millions of vulnerable people out there, so the virus is still finding victims. But the immunity landscape has changed.

The herd immunity threshold is not a static, immutable number. The threshold depends on many factors, including what measures a population is taking to limit viral spread, and the degree to which a pathogen is transmissible. Mutations in variants can raise the threshold for herd immunity. Seasonality probably plays a role.

“We don’t know what the herd immunity threshold is for SARS-CoV-2,” Fauci said. “Don’t get hung up on a number. . . . I absolutely know as a fact that the more people you get vaccinated, you are sooner or later going to start seeing a significant and likely precipitous decline in the number of cases in the community.”

Van Kerkhove, the WHO epidemiologist, said it is impossible to predict when the pandemic will be over. The WHO regularly meets to assess whether to maintain its designation of a Public Health Emergency of International Concern, first announced in January 2020. But Van Kerkhove said it is possible to end the pandemic with enough aggressive measures to limit viral spread.

“We have the tools at hand now that can control covid,” she said.

Fauci said the first big step in ending the epidemic phase of a contagion is to get it under control. The next step would be to eliminate it from the population. The ultimate goal - achieved only with smallpox - is to eradicate it from the planet.

“We are now in an epidemic phase of covid-19 in the United States. We will end the epidemic phase, and we will likely get somewhere between control and elimination, more likely closer to control,” Fauci said. “In order to have elimination, you’ve got to have the whole world vaccinated.”

The Washington Post’s Jacqueline Dupree contributed to this report.