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A curious thing happened when Hong Kong reopened schools after closing them because of the Covid-19 pandemic. It bears watching here.

Hong Kong closed its schools to in-person learning from late January 2020 to late May — and then again in early July, when more Covid cases were detected. Within a few weeks of schools reopening in October, they started to see large numbers of kids getting sick, despite mandatory mask-wearing, additional spacing between desks, and other measures to lower the risk of spread of SARS-CoV-2, the virus that causes Covid-19.


But the children weren’t infected with the virus. Nor did they have influenza, which would have been another possibility. They were infected with rhinoviruses — one of the most common causes of the common cold.

Researchers believe the surge in illness was no accident — but rather a consequence of children congregating after so many months of social distancing. In short, they may have been more susceptible to respiratory viruses because they likely had fewer exposures to people outside their households and thus fewer chances to contract them and build up immunity.

The findings were published recently in Emerging Infectious Diseases, the journal of the Centers for Disease Control and Prevention.


“I can imagine places where schools have been closed for a long time are going to have the same experience as Hong Kong,” said Ben Cowling, a professor of infectious diseases epidemiology at the University of Hong Kong and the report’s senior author. “That when the schools go back there’s suddenly going to be a lot of rhinoviruses going around, a lot of kids getting sick with colds, and then their parents getting it off them and then panicking that it could be Covid.”

A number of viruses that cause cold and flu-like illnesses have gone to ground since Covid-19 started to spread broadly around the world. There was virtually no flu activity during the Southern Hemisphere’s winter in July and August; so far this winter only about 1,400 people in the United States have tested positive for flu. By this time last year, more than 174,000 people had tested positive for flu — more than a hundred-fold difference. And at this time last year, 105 children had died from flu in the country; this year, that tragic toll is down to a single child.

(Flu cases that are diagnosed by a test are the tip of the iceberg, even in a normal season.)

Likewise, infections caused by respiratory syncytial virus, or RSV, are down. RSV causes cold-like illness that is normally mild; but in young children and older adults, the infection can be severe. In a normal year, about 58,000 children under the age of five are hospitalized with RSV and about 14,000 adults over the age of 65 die from it.

Rhinoviruses, on the other hand, mostly cause mild colds; the more than 200 viruses in the rhinovirus family are estimated to be responsible for about a third of all colds. And, if Hong Kong’s experience is any indication, they appear not to be contained by the measures in place to minimize spread of Covid.

In total, there were 482 rhinovirus outbreaks reported by schools over about a one-month period from late October to late November. The vast majority were in primary schools, kindergartens, nursery schools, and child care centers.

Population “susceptibility to rhinoviruses and other respiratory viruses, including influenza viruses, might have been increasing over time because persons were likely less exposed to the viruses when intense social distancing measures, including school dismissals, were implemented in response to the Covid-19 pandemic,” the authors of the new paper wrote. “This would have increased transmission potential when schools resumed.”

The authors, who are all from the University of Hong Kong, noted that a similar phenomenon had been observed by British researchers, who reported a sharp spike in rhinovirus infections in adults starting about two weeks after children returned to school in the U.K. last September.

Cowling and colleagues had earlier reported that the amounts of human coronaviruses and influenza viruses emitted by infected people are greatly reduced if they are wearing surgical masks, but the same is not true for rhinoviruses. That and the fact that rhinoviruses are hardy — they may be better able to withstand surface cleaning than coronaviruses and influenza viruses — may help to explain why they continue to circulate while other respiratory viruses have declined in incidence, the researchers said.

Covid control measures likely don’t fully explain the phenomenon, said Ed Belongia, director of the Center for Clinical Epidemiology and Population Health at Wisconsin’s Marshfield Clinic.

Belongia noted that during the 2009 H1N1 flu pandemic, seasonal flu viruses and RSV virtually disappeared for a time, even though mask-wearing was not common outside of some parts of Asia back then and many of the social distancing tools currently being employed were not part of the pandemic response.

“We don’t really understand what’s going on here in terms of these different patterns of virus circulation. We have obviously not very much experience with pandemics and what pandemics do to circulation of other viruses,” Belongia said.

The Hong Kong researchers noted that not only were there lots of rhinoviruses cases, but there were more severe infections than is normally seen, with some of the children needing hospital care.

“It did seem to be more than usual,” Cowling said. “I think it’s to do with that loss of immunity. Not only were a lot more children susceptible, maybe they were more susceptible.”

The same phenomenon could play out with influenza when flu viruses resume circulating, Cowling and others are warning. The lack of exposure to flu viruses for more than a year could leave a lot of people more susceptible to the viruses when Covid-containment measures are eased.

“If I had to gamble on it, then I would guess that we are likely to get a more severe epidemic in the coming winter — assuming restrictions are fully lifted by then,” John Edmunds, of the London School of Hygiene and Tropical Medicine and a member of the British government’s Sage committee of scientific advisors, told the Daily Telegraph recently.

Cowling agreed. “Once measures are relaxed, once people try to get back to normal, we’re going to have the biggest flu season on record,” he predicted.

  • Thank you for this article, I feel that it is so important for our children to be able to build up their natural immunity to their environment. And as was appointed out to me that isolating our children is causing more harm than good. Well done!

  • What a stupid article. How to expect kids to wear mask properly when adults doesn’t even wear mask properly. Just because one wear mask does not mean one will not get sick if the mask is not worn properly.

    And for Covid19, it can get thru the eyes, which is worst. Mask alone is not enough.

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