“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.

Four in 10 Americans, and six in 10 French people say they will not get vaccinated against the virus, even if the shot is free

The most common complications associated with Covid-19 were pneumonia, respiratory failure, kidney failure, and sepsis or systemic inflammation, consistent with other studies.The researchers also found associations with a range of other lung and cardiovascular conditions, such as collapsed lung, blood clotting disorders and heart inflammation, although the risk of these was relatively low

The authors said their findings may raise important questions about the reliability of seroprevalence studies, since the rapid waning of antibody titters may lead to an underestimation of how many people may have been previously infected in a given population.

“The decrease in antibodies after infection also raises the question of how long antibodies elicited by vaccination will last, and whether frequent boosting will be needed to maintain protection,” the authors noted.