The figure the CDC reported last week is known as “period life expectancy.” It measures the average length of time a hypothetical American who, from birth to death, is exposed to the mortality rates observed in the current period. That means the 2020 statistic assumes that Covid will be killing people forever at the same rate as it did last year—an implausible scenario.

The common-sense definition of life expectancy is captured in what demographers call “cohort life expectancy”—the life expectancy of an individual born in a given year using actual mortality rates in each period of his life so far and predicted mortality rates for future periods. Cohort life expectancy for new cohorts has increased slowly over time and is expected to continue increasing as medical knowledge advances.

The decline in period life expectancy owing to the Covid pandemic is likely to prove (you’ll pardon the expression) short-lived. The pandemic’s indirect effect on the more meaningful cohort life expectancy is likely to be positive since the recent gains in medical knowledge and vaccine technology can be expected to extend life expectancy for incoming cohorts as it leads to better treatments for all types of diseases.