Travel to three countries may have seeded pandemic

July 31, 2020

U.S. 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, according to a news report from the Center for Infectious Disease Research and Policy (CIDRAP) at the University of Minnesota.

Findings of the study, published in The Lancet Infectious Diseases, show that from December 31, 2019, to March 10, 2020, there were 32,459 cases 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 percent of those cases originating in Italy, 22 percent in China, and 11 percent in Iran.

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

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.

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.

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