When many students in the United States go back to in-person learning this fall, parents and school administrators may have to contend with an unexpected infectious disease problem: more colds than usual.

That’s the caution coming from researchers in Hong Kong, who published a study last week detailing a spike in common colds after students returned to classrooms in the fall following nearly a year of remote learning. Specifically, the researchers reported almost seven times more large outbreaks of acute upper respiratory infections (involving 20 people or more) compared with those recorded in 2017, 2018 and 2019 combined.

“Normally, we don’t think of them as a real public health challenge,” Benjamin Cowling, one of the study’s authors and a public health researcher and biostatistician at the University of Hong Kong School of Public Health, said of common colds. But it can be tough to distinguish the symptoms of a cold from those of COVID-19, especially in kids. And if colds start spreading through schools in the United States, children may be sent home until they have been tested for the coronavirus. They may even have to return to remote learning.

Here’s what happened to students abroad, and what it might mean for kids in the United States.

What the Researchers Found

In late January 2020, as the spread of the coronavirus increased, schools in Hong Kong closed and began teaching students remotely. Schools reopened in late May, but then closed again in July through late September because of a surge of cases of COVID-19. During the last week of October, after schools reopened once more, the Hong Kong Centre for Health Protection received reports of a surge in respiratory infections in kindergartens, child care centers, nursery schools, primary schools and secondary schools.

By the end of November, schools had reported 482 outbreaks of the common cold, mostly in children up to and including primary school age. Researchers defined these types of outbreaks (which were classified differently from the 81 larger ones) as at least three students in a class developing two or more symptoms — cough, runny nose, fever or sore throat — within the span of four days.

Some experts think that the spike in cases may have simply been a result of more people paying attention to every cough, sniffle and runny nose last fall. “These children were likely being tested at a far higher rate than would normally occur outside of a pandemic,” said Dr. Ilan Schwartz, an infectious disease physician at the University of Alberta in Canada.

But Cowling said that the spike in colds that occurred in Hong Kong in the fall seemed much higher than the increase that was seen in years past. “It did seem like there were an unusual number of children in hospital with rhinoviruses,” he said, referring to the viruses that cause most common colds. And the doctors he has spoken with have told him the infections “seemed to be a bit more severe” than usual, too.

In a correspondence published in The Lancet in December, researchers from the United Kingdom noted something similar — cold viruses spiked in adults in the two weeks after children returned to school last fall, emphasizing that children are the primary drivers of transmission of cold viruses to adults.

What Could Have Been Going On

At this point, immunologists can only speculate as to why colds surged after schools reopened in Hong Kong.

What really surprised Cowling was that colds spread despite the strict virus-prevention measures at schools. In Hong Kong, students spend only half a day at school, and they avoid eating lunch together. They also wear masks all day and sit at distanced desks. “Somehow, the common colds were breaking through those measures,” he said.

It’s possible that COVID-19 mitigation measures do not work that well for colds. In a study published last April, Cowling and his colleagues found that surgical masks weren’t as effective at blocking rhinoviruses as they were at blocking coronaviruses and influenza viruses. In other words, “maybe the virus can survive the journey through the mask,” Cowling said.

Rhinoviruses are also hardier than coronaviruses because they have strong outer envelopes, which means that they can survive on surfaces for longer, and can also be difficult to remove from hands. A small study from 2010 found that after researchers deliberately contaminated 16 participants’ hands with rhinoviruses, only five had no detectable virus on their hands after washing them with soap and water.

When students with cold viruses on their hands touch doorknobs or other frequently handled surfaces, those viruses might be able to transfer to other kids’ hands (and ultimately into their bodies through eyes, noses or mouths) when they touch them minutes or hours later — which, research suggests, is not the main route of transmission of the coronavirus.

Some experts say it’s also possible that weakened immune systems may have been to blame. Since kids likely had not been exposed to as many colds and other viruses while at home, it’s plausible that they could have lost some of their immune defenses, making them especially susceptible to colds when they returned to school.

When people catch colds, their bodies mount various kinds of immune responses against the invading pathogen. One type of immune response, known as the innate immune response, is fast-acting and expansive. “When these viruses are sensed, the body turns on a really broad defense. And that defense is effective against many different viruses, not just the virus that turned it on,” said Dr. Ellen Foxman, an immunobiologist at the Yale School of Medicine.

What Parents Can Do

Interestingly, the first two weeks after schools start in the fall are when cold viruses typically peak each year. “If you think about it, that’s after a summer break, when kids haven’t been in school,” Foxman said — when many kids haven’t been exposed to other viruses. Once kids suffer through an initial fall cold, though, infection rates often inch back down to normal. This may happen in the United States when schools reopen, too, she said.

Still, there are steps parents can take to mitigate the spread of colds when kids head back to class. Encourage your children to wash their hands at school with soap and water, lathering for at least 20 seconds, ideally before and after they eat, after using the bathroom and after blowing their noses, coughing or sneezing. Teach them to cough and sneeze into their elbows rather than into their hands. And if they wear face masks at school, ensure that their masks are fitted properly so that they don’t have to touch them and their faces frequently to adjust them.