Travel restrictions must be used in a targeted way to effectively control local COVID-19 transmission, according to research published in the Lancet Public Health.
The findings suggest travel restrictions can be effective in countries close to a tipping point for exponential growth (reproduction number, 0.95-1.05) but not in areas where it is already spreading rapidly among the population.
The study, led by the Centre for Mathematical Modelling of Infectious Diseases at the London School of Hygiene & Tropical Medicine, reports that in May 2020, imported cases are likely to have accounted for a high proportion of total incidence in many countries, contributing >10 per cent to the total incidence in 102 of 136 countries when assuming travel volumes similar to the same period in 2019, and in 74 countries when assuming estimated 2020 travel volumes.
Imported cases in September 2020 would have accounted for ≤10 per cent of the total incidence in 106 of 162 countries and less than 1 per cent in 21 countries, when assuming no reductions in travel volumes.
With estimated 2020 travel volumes, imported cases in September 2020 accounted for ≤10 per cent of total incidence in 125 countries and <1 per cent in 44 countries. Of these 44 countries, 22 had epidemic growth rates far from the tipping point of exponential growth, making them the least likely to benefit from travel restrictions.
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.
For instance, in September 2020, the measures would be effective in New Zealand and China because the virus had been suppressed to such low levels in both countries that the expected number of imported cases is similar to the local rate, meaning arrivals could trigger a new local wave of infections.
The authors conclude that 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.
