Outbreaks of the common cold could become more commonplace as schools reopen for in-person learning, according to a new report from the Centers for Diease Control and Prevention (CDC). 

In the report recently published in Emerging Infectious Diseases, the journal of the CDC, researchers looked at school reopenings in Hong Kong as an indication of what may happen in the U.S. as students move away from remote learning. In Hong Kong, schools closed down due to COVID-19 from late January-May 2020. They reopened briefly but closed back down in July amid a surge in cases. 

However, when the schools and child care centers reopened in October, cases of the common cold surged among children — despite the mandatory use of face masks and other measures in place to curb the spread of COVD-19. 

The researchers suspect that the children were more susceptible to common-cold-causing rhinoviruses once they returned to in-person learning because they had spent the majority of the year away from others outside their household, decreasing the number of chances they had to be exposed to rhinoviruses and ultimately build up immunity. 

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"A large number of common cold outbreaks in Hong Kong schools and childcare centers during October–November 2020 led to territory-wide school dismissals," they wrote in the report. "Increased susceptibility to rhinoviruses during prolonged school closures and dismissals for coronavirus disease and varying effectiveness of nonpharmaceutical interventions may have heightened transmission of cold-causing viruses after school attendance resumed."

The outbreaks — there were 482 reported between October 25 and November 28, the CDC said — largely occurred in primary schools, kindergartens, child care centers, and nursery schools. 

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"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. This would have increased transmission potential when schools resumed," the report’s authors wrote, noting that in September, schools in England experienced a similar occurrence. Two weeks after fully reopening schools, "a substantial increase in the detection of rhinoviruses among adults was recorded, possibly driven by transmission among children."

Researchers aren’t sure why common cold outbreaks occurred despite the wide range of infection control measures in place when children returned to school. 

"Although in general transmission modes may be similar for different respiratory viruses, how much each mode contributes to transmission of a specific virus remains unclear; therefore, the effectiveness of certain nonpharmaceutical interventions might differ between viruses," they hypothesized. "For example, face masks were shown to be efficacious in blocking the release of coronaviruses and influenza viruses, but not rhinoviruses, in exhaled breath."

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They also noted that rhinoviruses are more resistant to certain disinfectants than coronaviruses and influenza viruses are, which may also help to explain the outbreaks. 

"Our findings highlight the increased risk posed by common cold viruses in locations where schools have been closed or dismissed for extended periods during the COVID-19 pandemic," they concluded.