CDC reports on upper respiratory infections in schools and childcare centers after COVID-19 dismissals in Hong Kong

Feb. 23, 2021

A large number of common cold outbreaks in Hong Kong schools and childcare centers during October to November 2020 led to territory-wide school dismissals, released the Centers for Disease Control and Prevention (CDC) in a research letter. Increased susceptibility to rhinoviruses during prolonged school closures and dismissals for COVID-19 and varying effectiveness of nonpharmaceutical interventions may have heightened transmission of cold-causing viruses after school attendance resumed. 

Many countries implemented school closures and dismissals in 2020 as a public health measure to reduce spread of COVID-19, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In Hong Kong, schools were dismissed after the Lunar New Year holiday in late January 2020 and remained dismissed until late May; during early July to late September, schools were dismissed again in response to a surge in cases of COVID-19.

During the dismissal periods, most school campuses remained open to staff but lessons were delivered online. A large number of outbreaks of acute upper respiratory tract infections (URTIs), likely rhinovirus infections, were identified during October to November 2020 in reopened primary schools, secondary schools, kindergartens, childcare centers, and nursery schools in Hong Kong.; these outbreaks led to further territory-wide school dismissals for younger children.

In the last week of October 2020, the Hong Kong Centre for Health Protection began receiving reports of URTI outbreaks in kindergartens, childcare centers, nursery schools, and primary schools. Outbreaks of URTIs in schools continued to increase rapidly in the following weeks. A school URTI outbreak was defined as >3 students in the same class each developing >2 symptoms of respiratory tract infection within four days. Various measures were implemented in response to these URTI outbreaks. Initially, schools with outbreaks were advised to dismiss affected classes for >3 days; this guideline was expanded to dismissal of entire schools for >7 days beginning November 18. SARS-CoV-2 testing was also conducted for students in affected classes and all staff in these schools.

URTI outbreaks caused by the respiratory viruses responsible for common colds (i.e., other than influenza viruses) occurred in Hong Kong schools despite a wide range of infection control measures being in place. Staff and students wore face masks at all times; lunch hours were cancelled, desks were spaced out, and group activities were limited. 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). For example, face masks were shown to be efficacious in blocking the release of coronaviruses and influenza viruses, but not rhinoviruses, in exhaled breath. In addition, enveloped viruses (e.g., coronaviruses and influenza viruses) are less resistant to lipophilic disinfectants than nonenveloped viruses (e.g., rhinoviruses)). This difference might have played a role in URTI outbreaks in Hong Kong related to rhinoviruses but not influenza viruses, even though individual persons and schools had practiced frequent cleaning and disinfection. These 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.

Ms. Fong is a research postgraduate student at the School of Public Health, University of Hong Kong. Her research interests are the transmission and control of respiratory viruses among children, particularly in school settings.

CDC has the letter.

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