Travel restrictions are unlikely to be effective in countries already experiencing high numbers of coronavirus cases and should be used in a “targeted way”, a new study suggests.

Researchers from the London School of Hygiene and Tropical Medicine and the University of Hong Kong assessed the global impact of travel restrictions on the pandemic.

They found that countries with low numbers of Covid-19 cases, or those with strong travel links to other destinations experiencing high rates of infection, were most likely to benefit from such measures.

Restrictions can also be effective in countries that are close to a tipping point for exponential growth – where the reproduction number, or R value, is between 0.95 and 1.05, the peer-reviewed study found.

However, it suggested that the measures were less likely to be effective in countries where the virus was already spreading rapidly among the population.

Study leader Professor Mark Ji, from the London School of Hygiene and Tropical Medicine, said it was important governments use travel restrictions in a “targeted way” as the measures “carry a high economic and social cost”.

He said: “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.”

Every country in the world imposed some form of travel restriction by late April this year, the study, which was published in The Lancet Public Health journal, said.

Researchers used 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.

Results were determined based on how imported coronavirus cases would affect local epidemic growth rates, using country-specific R value estimates.

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

Where imported cases accounted for more than 10% of infections within a country, they were considered to have a major impact on the epidemic’s growth.

But when imported cases accounted for less than 10%, their impact on the growth of the epidemic was usually small, while those below 1% would have an almost undetectable effect on the epidemic’s size, researchers said.

Imported coronavirus cases would have accounted for more than 10% of infections in 102 out of 136 countries analysed if no travel restrictions had been in place in May, the study said.

Had there been no travel restrictions in September, imported cases would only account for more than 10% of infections in 56 out of 162 countries.

The findings indicate that international restrictions were most effective at limiting transmission of the virus during earlier stages of the pandemic.

This was because imported cases led to outbreaks in countries with very few existing cases, the study said.

The authors concluded that recommendations about international travel restrictions should not be applied “uniformly” and should take into account an individual country’s circumstances.