Chennai: Travel restrictions are effective in countries with low numbers of Covid-19 cases, or that have strong travel links with nations experiencing high rates of the viral infection.
The study, published in The Lancet Public Health journal, also suggests that travel restrictions may have been most effective during the early stages of the pandemic, and the measures are unlikely to be effective when the virus is already spreading rapidly within a country.
“We recognise that these measures carry a high economic and social cost, so it is important that governments use travel restrictions in a targeted way,” said Professor Mark Jit from the London School of Hygiene and Tropical Medicine, who led the study.
“Before introducing restrictions, they should take into account local infection figures, epidemic growth rates, and the volume of travellers arriving from countries heavily-affected by the virus,” Jit explained.
The researchers used detailed flight data to compare the number of expected Covid-19 cases arriving from international flights with the number of infections arising from transmission within individual countries.
They produced estimates of international travellers in May and September 2020 based on two scenarios.
One scenario used flight data for the same months in 2019, assuming no reduction in travel numbers, and the other scenario was based on the expected reduction in passenger numbers.
Numbers of Covid-19 cases and infection rates were estimated using a mathematical model that adjusts recorded cases to take account of asymptomatic and unreported infections.
Results were determined based on how imported Covid-19 cases would affect local epidemic growth rates, using country-specific reproduction number, or R number, estimates.
Where imported cases accounted for more than 10 per cent of infections within individual countries, they were considered to have a major impact on growth of the epidemic, the study found.
The work estimated that when imported cases accounted for less than 10 per cent, their impact on the growth of the epidemic is usually small, while those below 1 per cent would have an almost undetectable effect on epidemic size.
The researchers noted that had there been no travel restrictions or reduction in travel volumes in May 2020, the imported COVID-19 cases would account for more than 10 per cent of infections in the majority of countries included in the analysis, the researchers said.
According to the estimates based on expected passenger numbers in September 2020, travel restrictions would have contributed to more than 10 per cent of infections in only 37 countries, less than 10 per cent in 125 countries and less than 1 per cent in only 44 countries.
The findings indicate that international travel restrictions were most effective at limiting local transmission of the virus during earlier stages of the pandemic, said the researchers, adding this is because imported cases led to outbreaks in countries with very few or no existing cases.