China surpasses U.S. in life expectancy(twitter.com) |
China surpasses U.S. in life expectancy(twitter.com) |
Those are derived by making a Markov model where people who are aged N years have a certain probability of surviving to N+1 years otherwise they transition to the "dead" state.
What they do is incorporate the probability that "a person in 2021 survives from 14 to 15" and "a person in 2021 survives from 71 to 72" whereas the person who really survived from 14 to 16 in 2021 is going to survive, if they do from 71 to 72 in 2078.
In the case of a pandemic year this calculation assumes that the pandemic is going to go on forever and have a chance to kill you every year without any acquired immunity, vaccination, the disease going out of circulation, etc.
(Period) life expectancy at birth summarizes the current health status of a population, nothing more, nothing less. It doesn't make a predictions about what happens in 2078.