An Influenza A(H3N2) Virus Outbreak During the COVID-19 Pandemic, Kingdom of Cambodia, 2020

Abstract

Background: Coinciding with the COVID-19 pandemic, global influenza virus circulation decreased, possibly due to widespread community mitigation measures. Cambodia eased some COVID-19 mitigation measures in June and July 2020. On Aug 20th a cluster of respiratory illnesses among residents of a pagoda included persons that had tested positive for influenza A, and none that were positive for SARS-CoV-2.Methods: On Aug 25, a response team deployed. Persons with influenza-like illness (ILI) were asked questions on demographics, illness, personal prevention measures, and residential arrangements. Respiratory swabs were tested for influenza and SARS-Cov-2 by RT-PCR; viruses were sequenced. Sentinel surveillance data were analyzed to assess recent trends in influenza circulation in the community.Findings: Influenza A(H3N2) viruses were identified in sentinel surveillance in Cambodia in July 2020, prior to the reported pagoda outbreak. Among the 362 pagoda residents, 73(20.2%) ILI cases were identified; 40 were tested with 33/77(82.5%) confirmed positive for influenza A(H3N2). All 40 were negative for SARS-CoV-2. Among 73 residents with ILI, none had been vaccinated against influenza, 47(64%) clustered in 3 of 8 sleeping quarters; 20(27%) reported often wearing a mask; 27(36%) reported often washing hands; and 11(15%) reported practicing social distancing. All viruses clustered within clade 3c2.A1 close to strains circulating in Australia in 2020.Interpretation: Circulation of influenza viruses began in the community following a relaxing of national COVID-19 mitigation measures, and prior to this outbreak in a pagoda with limited social distancing. Continued surveillance and influenza vaccination remain needed to limit the impact of influenza globally.Funding Statement: This investigation as a public health response undertaken by the Cambodian MoH, with technical support from the U.S. Centers for Disease Control and Prevention, the World Health Organization, and Institute Pasteur of Cambodia. Work at Institute Pasteur in Cambodia was supported, in part, by the Office of the Assistant Secretary for Preparedness and Response, US Department of Health and Human Services (grant number 114IDSEP 140020-01-00; http://www.asideproject.org/).Declaration of Interests: None to declare.Ethics Approval Statement: This investigation was undertaken as part of a public health outbreak response and was considered a non-research investigation by the Cambodian MOH. This activity was also reviewed and determined to be a non-research activity by the US CDC, and undertaken in a manner consistent with applicable federal law and CDC policy.