This marked a sharp reversal during a decades-long rise in global life expectancy, according to hundreds of researchers sifting through data for the US-based Institute for Health Metrics and Evaluation (IHME).

“For adults worldwide, the pandemic has had a more profound impact than any event seen in half a century, including conflicts and natural disasters,” said Austin Schumacher, IHME researcher and lead author of the study published in the The Lancet.

During 2020-2021, life expectancy declined in 84% of the 204 countries and territories analysed, “demonstrating the devastating potential impacts” of new viruses, he said in a statement.

Mexico City, Peru and Bolivia were some of the locations where life expectancy fell the most.

But there was some good news: half a million fewer children under the age of five died in 2021 compared with 2019, continuing a long-term decline in child mortality.

IHME researcher Hmwe Hmwe Kyu hailed this “incredible progress”, saying the world should now focus on “the next pandemic and addressing the vast disparities in health across countries”.

And despite the pandemic, people still live far longer than they used to: between 1950 and 2021, the average life expectancy at birth has risen by 23 years, from 49 to 72, the researchers said.

Covid-19 was responsible for 15.9 million excess deaths during 2020-2021, either directly from the virus or indirectly due to pandemic-related disruptions, the researchers estimated. That is a million more excess deaths than previously estimated by the World Health Organization.

Excess deaths are calculated by comparing the total number of deaths with how many would have been expected if there had not been a pandemic.

Barbados, New Zealand and Antigua and Barbuda were among the countries with the lowest rate of excess deaths during the pandemic, partly reflecting how isolated islands were often spared the full brunt of Covid-19.

The study also showed how the populations of many ageing, well-off countries have started to decrease, while numbers continue to grow in less-wealthy countries.

This dynamic “will bring about unprecedented social, economic and political challenges, such as labour shortages in areas where younger populations are shrinking, and resource scarcity in places where population size continues to expand rapidly,” Schumacher cautioned.

“Nations around the world will need to cooperate on voluntary emigration,” he added.