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

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.

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.

“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.”.

By this time last year, more than 174,000 people had tested positive for flu — more than a hundred-fold difference.

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.

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.

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.

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

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