Study: Travel to 3 countries may have seeded COVID-19 pandemic

Aircraft cabin
Aircraft cabin


US Centers for Disease Control and Prevention (CDC) researchers who conducted web-based infection surveillance during the first 11 weeks of the COVID-19 pandemic have found that nearly two-thirds of the first confirmed cases in each affected country outside of mainland China had a history of travel to Italy, China, or Iran.

Findings of the study, published yesterday in The Lancet Infectious Diseases, show that from Dec 31, 2019, to Mar 10, 2020, 32,459 cases were identified in 99 countries outside of mainland China. Seventy-five of the 99 countries reported their first cases in people with a travel link to another country, with 27% of those cases originating in Italy, 22% in China, and 11% in Iran.

Travel to Italy was linked with three of the first six (50%) reported cases in Africa and 16 of 45 (36%) of the earliest cases in Europe and 5 of 13 (38%) in the Americas. Travel to mainland China made up 10 of 12 (83%) of early cases in the Western Pacific and 4 of 7 (57%) in Southeast Asia. Seven of the 16 first-reported cases (44%) in the Eastern Mediterranean region had travel links to Iran.

Of 1,200 coronavirus cases from 68 countries with age or sex data, 874 (73%) were early cases. Only 25 of 762 cases (3%) with age data occurred in children younger than 18 years; median age was 51 years. Twenty-one of 1,200 (2%) of early cases occurred in healthcare workers. Fifty-two (4%) of the 1,200 COVID-19 patients died.

Sources of coronavirus clusters

Before the outbreak was declared a pandemic, researchers had identified 101 clusters involving 386 cases in 29 countries. Seventy-six of 101 clusters involved household coronavirus transmission, with, on average, 2.6 cases per cluster. In comparison, the 11 clusters related to tour groups, faith-based groups, and dinner parties had, on average, 14.2 cases per cluster. The 14 clusters occurring in non-healthcare workplaces had, on average, 4.3 cases. Healthcare settings were the site of six clusters, while travel in taxis, airplanes and a cruise ship and train were linked to nine clusters.

The study findings also showed the relatively late detection of COVID-19 in Africa, where only 6 out of 46 countries (13%) in the region were reporting cases by the time the World Health Organization (WHO) characterized the outbreaks as a pandemic on Mar 11. In contrast, 13 of 35 countries (37%) in the Americas, 45 of 54 countries in Europe (83%), 16 of 23 Eastern Mediterranean countries (70%), and 7 of 11 (64%) countries in Southeast Asia had reported cases.

Data suggest that travel from just a few countries with significant coronavirus spread may have seeded outbreaks around the world even before COVID-19 was characterized as a pandemic, the researchers said, while cautioning that because nearly all cases in the study were reported by middle- and high-income countries in Asia and Europe, the data couldn't offer a complete global picture of early global coronavirus epidemiology.

Physical distancing, surveillance

Although many early COVID-19 cases were part of small household coronavirus clusters, larger clusters mainly took place in workplaces and community settings, supporting the importance of physical distancing in curbing disease transmission, the authors said.

They said that the multiple large clusters linked to faith-based settings highlight the need to work with places of worship when developing or implementing community public health measures. Likewise, they said that health systems need to work on early recognition of infection, control of disease transmission through infection-control protocols, and healthcare worker surveillance.

"Additional studies with detailed epidemiological and clinical data, and ideally with systematic testing of suspected cases, including among special populations (eg, health-care workers, children, and pregnant women), could further our understanding of COVID-19 and inform preparedness and response measures for the current pandemic," they wrote.

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