Local Outbreak of COVID-19 in Shunyi District Attributed to an Asymptomatic Carrier with a History of Stay in Indonesia — Beijing Municipality, China, December 23, 2020

SUMMARY: What is known about this topic? Patients with coronavirus disease 2019 (COVID-19) infection can be categorized by severity: asymptomatic infection, mild illness, moderate illness, severe illness, and critical illness. The rate of transmission to a specific group of contacts (the secondary attack rate) may be 3–25 times lower from people who are asymptomatically infected than from those with symptoms. The incubation period is 2–14 days. What is added by this report? An individual with asymptomatic infection shed live virus that started a 42-case outbreak in Shunyi District of Beijing in December 2020. The individual had been quarantined for 14 days in a designated quarantine hotel in Fuzhou after entering China from Indonesia. During quarantine, he had 5 negative throat swab tests and 2 negative IgM serum tests. The investigation team determined that the incubation periods of 31 confirmed cases ranged from 2 to 23 days. What are the implications for public health practice? The frequency of nucleic acid testing should be increased for international entrants and people who are close contacts of confirmed COVID-19 cases. The investigation team recommend expanding sample types for nucleic acid testing to include anal swabs and residential environmental sampling; serum antibody testing can be used as a reference indicator. Parallel testing by two institutions at key points in time, such as the end of isolation, should be implemented.