A new study published in The Lancet Infectious Diseases suggests that people with travel links to China, Iran, and Italy accounted for nearly two-thirds of initial COVID-19 cases in affected countries.
Data on the early spread of SARS-CoV-2 are limited and the global epidemiology of confirmed cases outside of mainland China will likely be a point of scientific inquiry for some time due to complex transmission dynamics.
Investigators publishing in The Lancet sought to move this area of epidemiological research forward by compiling information in publicly available reports from the pre-pandemic period leading up to March 10, 2020.
Reports were gathered via public health agency websites, transcripts of press conferences, and social media feeds of government agencies.
Travel history, exposures to other cases, and case characteristics were noted.
In each of 99 affected countries outside of mainland China, 75 (76%) had recent travel to affected countries. According to investigators, 60 (61%) had travelled to China, Italy, or Iran.
Among 1200 cases examined with age or sex information available, 874 (73%) were early cases. Early cases were defined as within the first 100 a country records.
Among 762 early cases with age information, the median age was 51 years (IQR 35—63); 25 (3%) of 762 early cases occurred in children younger than 18 years.
Additionally, 21 (2%) of these 1200 cases were among health care workers.
Out of 101 clusters identified during the pre-pandemic period, the most commonly identified transmission setting was households (76 [75%]; mean 2·6 cases per cluster [range 2—7]).
The second and third most common sources of an outbreak appeared to be in non-health-care occupational settings (14 [14%]; mean 4·3 cases per cluster [2—14]), and large scale community gatherings (11 [11%]; mean 14·2 cases per cluster [4–36]).
The investigators point out that although close household contact was an overwhelmingly large source of early clusters, clusters in occupational or community settings tended to be large. This means the results should not be taken as an endorsement or rejection of particular outbreak control policies by the authors.