International travel restrictions may only be effective at controlling the spread of Covid-19 when applied in a targeted way, according to research published in The Lancet Public Health journal.
The new study also suggests that travel restrictions can be effective in countries close to a tipping point for exponential growth but not in those where it is already spreading rapidly among the population.
“We recognise that these measures carry a high economic and social cost, so it’s important that governments use travel restrictions in a targeted way,” said from Mark Jit from the London School of Hygiene and Tropical Medicine in the UK.
“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 added.
The authors used detailed flight data to compare the number of expected Covid-19 cases arriving from international flights (assuming no travel restrictions) 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.
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 (102/136 countries included in the analysis).
By September 2020, had there been no travel restrictions or reduction in travel volumes, imported cases would account for more than 10 per cent of infections in only a small number of countries (56/162 countries).
The findings indicate that international travel restrictions were most effective at limiting local transmission of the virus during earlier stages of the pandemic. This is because imported cases led to outbreaks in countries with very few – or no – existing cases.
The authors conclude that recommendations about international travel restrictions should not be applied uniformly.
Countries must first consider local infection figures and epidemic growth rates, as well as the volume of travellers arriving from countries heavily-affected by Covid-19.