Nearly two-thirds of early coronavirus cases globally linked to Italy, China and Iran: CDC
Travel from just a few countries with substantial virus transmission may have seeded additional outbreaks around the world before the characterization of Covid-19 as a pandemic
Of the first confirmed Covid-19 cases in each affected country outside China, nearly two-thirds had a travel history linked to just three countries — Italy, China or Iran. One in four of these first cases originated in Italy and one in five in China, according to an analysis by the US Centers for Disease Control and Prevention (CDC).
The research team used publicly available global case data to describe travel exposure and case cluster characteristics among early coronavirus cases in different countries. Web-based surveillance of the global spread of Covid-19 reveals that during the first 11 weeks of the coronavirus outbreak, from December 31, 2019, to March 10, 2020, three-quarters of affected countries outside mainland China reported their first Covid-19 case in people who had recently traveled to an affected country. Almost two-thirds of these first cases were linked to travel to Italy (27%), China (22%) or Iran (11%).
"Our findings suggest that travel from just a few countries with substantial SARS-CoV-2 (the virus that causes Covid-19) transmission may have seeded additional outbreaks around the world before the characterization of Covid-19 as a pandemic on March 11, 2020," says Dr Fatimah Dawood from the CDC, who co-led the research, which has been published in The Lancet Infectious Diseases journal.
The authors examined publicly available online reports from national ministries of health and other government agency websites, social media feeds and press releases daily to identify newly confirmed cases of coronavirus reported during the first 11 weeks. Countries with at least one case were classified as affected. Early cases were defined as the first 100 cases reported in each country and later cases as those after the first 100 cases. The researchers analyzed travel history for the first case reported in each country outside China, case characteristics (such as age, sex, exposure), and cluster frequencies and sizes.
The team identified 32,459 Covid-19 cases from 99 countries and locations outside mainland China during the study period. The authors note that the first confirmed case in each country might not have been the first true case of infection in some countries, since early case detection efforts varied substantially.
Travel history of the first reported case in each affected country varied by world region. Travel to Italy was linked with half or three of the first six reported cases in Africa, over a third (36%) or 16 out of 45 cases in Europe and 38% or five out of 13 cases in America. Travel to China accounted for 83% (10 out of 12) of the first reported cases in the Western Pacific and over half (57%) or four out of seven in Southeast Asia. Seven (44%) first-reported cases in the Eastern Mediterranean region had travel links to Iran.
Among 1,200 cases from 68 countries with age or sex data available, 874 (73%) were early cases, with an average age of 51 years. Just 3% of cases (25 out of 762 with age information available) occurred in children younger than 18 years. In total, 2% (21 out of 1,200) of early cases occurred in healthcare workers. "Consistent with case characteristics reported from China and the US, almost all cases in this analysis occurred among adults (over 18 years), with only 3% occurring among children (less than 18 years)," says the study.
Before the outbreak was declared a pandemic, 101 clusters involving 386 cases were identified in 29 countries. Household transmission was reported in three-quarters (76 out of 101) of clusters, with an average of 2.6 cases in each cluster. By contrast, the 11 clusters related to community gatherings such as tour groups, faith-based groups and dinner parties had on average 14.2 cases per cluster. The 14 clusters reported in non-healthcare occupational settings had on average, 4.3 cases per cluster. While there were many clusters of household transmission among early cases, clusters in occupational or community settings tended to be larger, supporting a possible role for physical distancing to slow the spread of Covid-19, say researchers.
The authors emphasize that additional studies with detailed epidemiological and clinical data, and ideally with systematic testing of suspected cases, including among special populations (such as healthcare workers, children and pregnant women), could further the understanding of Covid-19 and inform preparedness and response measures for the current pandemic.
"Four large clusters in our analysis, and large outbreaks reported elsewhere, have been linked with transmission in faith-based settings, highlighting the need to partner with faith-based organizations when designing and implementing community mitigation efforts. Six healthcare-associated clusters were also identified, underscoring the need for strict infection prevention and control practices and monitoring healthcare workers for signs of illness," writes co-author Dr Philip Ricks from the CDC.