Over the months that followed, the unvaccinated share of mortality fell even further, to 38 percent in May 2022. The share of deaths among people vaccinated and boosted grew significantly as well, from 12 percent in January 2022 to 36 percent in April. Those levels held roughly steady throughout the duration of the summer, during which time just about as many boosted Americans were dying as the unvaccinated. The share of deaths among older adults kept growing: In April, 79 percent of American deaths were among those 65 and older. In November, 90 percent.
As many Twitter discussions about the “base rate fallacy” have emphasized, this is not because the vaccines are ineffective — we know, also from the Centers for Disease Control and Prevention data, that they work very well. Estimates of the effectiveness of updated bivalent boosters suggest they reduce the risk of mortality from Covid in Americans over the age of 12 by more than 93 percent compared with the population of unvaccinated. That is a very large factor.
But it isn’t the whole story, or vaccinated older adults wouldn’t now make up a larger share of Covid deaths than the unvaccinated do. That phenomenon arises from several other factors that are often underplayed. First is the simple fact that more Americans are vaccinated than not, and those older Americans most vulnerable to severe disease are far more likely to be vaccinated than others.
It is also partly a reflection of how many fewer Americans, including older ones, have gotten boosters than got the initial vaccines: 34 percent, compared with 69 percent. The number of those who have gotten updated bivalent boosters is lower still — just 12.7 percent of Americans over the age of 5.
Finally, vaccines are not as effective among older adults because the immune system weakens with age. It’s much harder to train older immune systems, and that training diminishes more quickly. In Americans between the ages of 65 and 79, for instance, vaccination reduced mortality risk from Covid more than 87 percent, compared with the unvaccinated. This is a very significant reduction, to be sure, but less than the 15-fold decline observed among those both vaccinated and bivalent-boosted in the overall population. For those 80 and above, the reduction from vaccination alone is less than fourfold.
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