This study provides data on clusters, and argues that more attention should be paid to clusters as we move to prevention and control of COVID-19. It also stresses that self-isolation may be key.

COVID-19 outbreaks seem to begin in clusters. Researchers collected data on 11 COVID-19 clusters in Qingdao City from Jan 29, 2020 to Feb 23, 2020. These clusters (including 44 cases) accounted for 73% of all confirmed cases. Three cases were infected from exposure to confirmed cases. The average incubation period was 6 days.

The number of cases per cluster ranged from 2 to 11 cases, with a median of 4. The clusters had a median duration of 6 days. The cumulative number of exposed people was 583, with a median of 54 per cluster.

In terms of transmissibility, each initial case led to 1-10 other cases in per cluster, with an average of 4. Family clusters were involved in all 11 clusters. Co-living was the main exposure mode in clusters (100%), followed by dinner together (36%), and seeing a doctor at the same time (27%).