Global life expectancy dropped by 1.6 years between 2019 and 2021, a sharp reversal from past improvements, according to a research published in The Lancet journal.

One of the first to fully evaluate demographic trends in the context of the first two years of the COVID-19 (coronavirus) pandemic, the study has published findings, which researchers said, could have implications for health systems, economies and societies around the world.

The findings also included those of an ageing worldwide population.

The study found that life expectancy declined in 84 per cent of countries and territories during this time. This demonstrated the "devastating potential impacts of novel pathogens", the researchers said.

Places such as Mexico City, Peru and Bolivia experienced some of the largest drops, the study found.

The researchers also found a marked increase in adult mortality rates globally during the pandemic in 2020 and 2021, reversing past decreasing trends.

However, child mortality continued to drop amid the COVID-19 pandemic, albeit more slowly than in earlier years, with half a million fewer deaths among children under five years in 2021 compared to 2019, they found.

The researchers said stark differences in child mortality persisted regionally, with one in every four children who died globally living in South Asia and two in every four children who died living in sub-Saharan Africa.

The estimates reported here will enable key stakeholders to better understand and address the profound changes that have occurred in the global health landscape following the first two years of the COVID-19 pandemic and longer-term trends in the future, they said.

The research, coordinated by the Institute for Health Metrics and Evaluation (IHME), University of Washington (UW), US, presented updated estimates from the Global Burden of Disease Study (GBD) 2021.

GBD 2021, the newly published most recent round of GBD results, is one of the first studies to fully evaluate demographic trends in the context of the first two years of the pandemic.

Measuring mortality, excess mortality from the COVID-19 pandemic, life expectancy and population, the researchers estimated that global death rate jumped among people aged over 15 years -- by 22 per cent in men and 17 per cent in women -- from 2019 to 2021.

Roughly 131 million people died globally from all causes in 2020 and 2021 combined, of which the pandemic caused about 16 million, either directly by infecting or indirectly by bringing about social, economic or behavioural changes, they estimated.

The team identified high mortality during the pandemic in places previously less recognised or reported such as Jordan and Nicaragua. The high pandemic-related excess deaths in these countries were not apparent in previous all-age excess mortality estimates, they said.

Conversely, they found Barbados, New Zealand and Antigua and Barbuda to be among those places experiencing some of the lowest excess mortality from the pandemic.

The pandemic did not, however, completely erase historic progress made in enhancing life expectancy at birth, which rose by nearly 23 years between 1950 and 2021, they said.

Assessing population trends, the researchers found that in 2021, the global population almost touched 7.9 billion, with 56 of 204 countries and territories peaking and subsequently shrinking.

The decline in global population was observed to begin in 2017 following years of stagnation, which accelerated during the pandemic.

Many lower-income countries, however, continued to see rapid population growth, with the largest share coming from Sub-Saharan Africa (39.5 per cent) and South Asia (26.3 per cent), they said.

The researchers said that further, the global population is ageing with the number of people aged 65 years and older now increasing faster than the number of people younger than 15 years in 188 of 204 countries and territories.

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