More than 28 million extra years of human life were lost in 2020, a year marked by the global spread of the coronavirus, according to a study released Wednesday that further underscored the immense human toll that the pandemic has wrought.

The international study, published in the BMJ journal, examined changes to life expectancy in 37 upper-middle to high-income countries where researchers said reliable data was available. Led by an Oxford University public health professor, the study also considered years of life lost, a metric that measures the degree of premature mortality among the dead, by comparing the ages of the deceased to their life expectancies.

The authors said the measure was more precise in calculating the impact of the pandemic than, for instance, just looking at excess deaths, a metric that does not distinguish between the death of a 17-year-old and that of a 70-year-old. The researchers used life expectancy between 2005 and 2019 as a benchmark for their study.