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Researchers from Imperial College London in the UK have found that nearly 66% of coronavirus Covid-19 cases exported from mainland China may have gone undetected.

The team is based at the WHO Collaborating Centre for Infectious Disease Modelling.

For assessment of the relative sensitivity of Covid-19 international surveillance, researchers compared the average number of people travelling from Wuhan each month to major foreign destinations with the number of coronavirus cases identified internationally.

Commenting on the findings, report co-author Dr Sangeeta Bhatia said: “Based on these data, we then estimate the number of cases that are undetected globally and find that approximately two-thirds of the cases might be undetected at this point. Our findings confirm similar analyses carried out by other groups.”

The researchers note that exported cases differ in clinical symptoms severity, which makes them difficult to detect.

Based on the volume of flight passengers from Wuhan, some countries have identified significantly fewer cases than expected.

Report co-author Dr Natsuko Imai added: “We are starting to see more cases reported from countries and regions outside mainland China with no known travel history or link to Wuhan City.

“Our analysis, which extends and confirms previously released analysis by other groups using flight volumes from Wuhan City and the reported number of Covid-19 cases, demonstrates the importance of surveillance and case detection if countries are to successfully contain the epidemic.”

Comparisons with Singapore alone estimated that 63% of cases are undetected.

Also, comparisons with Singapore, Nepal, Finland, Sweden, Belgium, Sri Lanka, India and Canada show that 73% of cases may be undetected.

These undetected cases increase the risk of more human-to-human spread.