Schools in some parts of the United States have started to close down and numerous districts are postponing plans to reopen in the face of skyrocketing community covid-19 cases, setting back efforts to try to reopen campuses closed since this past spring when the coronavirus pandemic began.

Though the latest covid-19 surge is being blamed by health experts on social gatherings and not on schools, officials in Chicago, Philadelphia, Boston, Topeka, San Diego, Sacramento, Minneapolis, D.C. and other districts have put off plans to soon reopen school buildings for the first time in the 2020-21 school year. Instead students will keep learning remotely at home, with no set date to return to school.

In other places — including Texas, Utah, Michigan, Georgia and Indiana — some districts are temporarily closing schools that already opened, often because of pandemic-caused staffing shortages. In Georgia’s Walker County, for example, all schools are closing at least through Thanksgiving because more than 100 students and teachers tested positive for the coronavirus. Scores of schools are closing in Utah because of exceptionally high community covid-19 rates.

In New York City, the largest school district in the country with 1.1 million students, Mayor Bill de Blasio warned parents on Friday to “get ready” with plans for their children because schools, which opened last month for the 2020-21 school year, could close as early as Monday as covid-19 rates are rising in the city.

While the available evidence shows that so far, coronavirus transmissions in schools have not been significant, the overall daily infection rates across the country are now at their highest since the start of the pandemic — and health experts have warned that the worst is yet to come.

On Friday, the United States reported more than 177,000 new coronavirus cases, a record high for the third straight day. More than 10.7 million coronavirus cases and 243,000 fatalities have been reported in the United States since February, when the earliest U.S. deaths were confirmed and forced schools around the country to close.

Districts had hoped to reopen schools in August or September for the 2020-21 school year, but most did not as the pandemic persisted and health experts raised fears that fall and winter would see a surge in cases of the deadly disease.

When some districts did open (while others stayed all-remote), many opted for a “hybrid” model; students who wanted to could spend some time in school each week while at home doing remote learning the rest of the time. The amount of time students are in school buildings varies; in some cases, it is five days a week while in others it can be only one. Families were given an all-remote learning option, and many took it.

Now some states are in covid-19 crisis with ballooning rates; New Mexico Gov. Michelle Lujan Grisham (D) said Friday her state is at a “breaking point” and she imposed restrictive statewide measures. She did not, however, shut down schools already open but did say that no districts could start the hybrid model during the restrictions. Other districts are trying to stay on plan; Baltimore still plans to bring students back into schools next week, although it has significantly cut back the number of campuses.

Though the transmission rate in New York City schools is only 0.16 percent, De Blasio said Friday during an appearance on Brian Lehrer’s radio show that the covid-19 positivity rate across the city was rising and had reached a seven-day rate of 2.83 percent. De Blasio earlier had said that schools would close if the city rate surpassed 3 percent.

“So, if we get to that point, Brian, let’s say it is over the weekend, we’ll immediately alert parents that school would then be closed in that instance as early as Monday,” he said. “… This is not something any parent you know wants to have to deal with, but we should get ready and parents should have a plan for the rest of the month of November. I think that’s the safe way to think about it — have an alternative plan for beginning as early as Monday for whatever will help them get through this month if school is not open.”

Asked by Lehrer why schools could close with such a low in-school transmission rate, the mayor said: “Because we have to always focus on health and safety first. Folks who, you know, want to offer their critique, that’s fine. But I’m telling you, you know, we’ve had to manage this process from the beginning, focused on health and safety.”

The strategy of closing schools amid rising covid-19 rates in the United States is in contrast to Europe, where cases are soaring and countries are closing or restricting access to restaurants, stores and other places of business — but not schools or child-care centers.

Some health experts in the United States have said that schools should try to stay open in light of the available evidence on low transmission rates on campuses. Celine Gounder, an infectious-disease expert who is a member of President-elect Joe Biden’s new covid-19 advisory task force, said Friday on CNBC’s “Squawk Box” that state officials should institute tight restrictions in areas where covid-19 rates are high — but only in places known to be high-risk for transmission of the coronavirus, such as bars and gyms.

“I think of this as a dimmer switch, not an on-and-off light switch,” she said. “I think we need to close only those things that really are contributing to the spread, and really try to … as much as possible remain open, like schools, if they’re not contributing to the spread.”

Ashish K. Jha, dean of Brown University’s School of Public Health and former director of the Harvard Global Health Institute, tweeted this past Sunday that schools can stay open if the federal government gets more active in a pandemic response, Congress provides more relief aid, more tests are given, and everybody wears masks and avoids indoor gatherings. He wrote: “We can keep schools open, save lives, prevent health care system from collapse.”