Question: I keep reading news articles like this one that mention “age-adjustment” as related to racial/ethnic differences in COVID hospitalization rates, death rates, etc.  I’m not sure that I actually understand what “age-adjustment” is.  Would you explain? 

Answer: Age-adjustment is a process researchers use to try and make apples-to-apples comparisons between diverse populations.  In the United States, the age structure of each racial/ethnic group is quite different (Figure 1).  If you want to make comparisons across racial/ethnic groups for outcomes that vary by age– like disease prevalence or mortality– you have to take population composition (age distribution) into account.  Enter age-adjustment.  

Age-adjustment can be direct or indirect, but for brevity and context I’m going to describe the basic concept of direct age-adjustment (also called direct standardization).  Direct standardization is the process of applying age-specific rates (of disease/death/etc.) from our population of interest to a standard population.  That may still sound confusing, but believe me, it’s not!  Let me further explain using an example from our recent analysis of racial/ethnic disparities in COVID case fatality rates (Figure 2).  Reminder: case fatality rates are the proportion of cases that result in death. 

On the left side of Figure 2, you’ll see monthly crude case fatality rates by race.  On the right side, you’ll see the same rates after age-adjustment.  Strikingly different, right?!  That’s because the left side– the crude rate– is misleading.  Without age-adjustment, the differences between racial/ethnic groups are being driven by population composition, NOT actual disease outcomes.  The right side shows the real case fatality differences between racial/ethnic groups. To arrive at the age-standardized rate visualized on the right, we first measured case fatality rates for each 10-year age/race group.  We then applied these rates to a standard population– here, all COVID cases in the US.  As recently as August, Blacks were still dying of COVID-19 at 1.24x the rate of whites.  Age-adjusted case fatality rates reveal dramatically different racial and ethnic disparities that are hidden by crude case fatality rates. Such adjustment is imperative for understanding COVID-19’s toll.

Figure 1. Population pyramids, Hispanics and non-Hispanic Whites, 2013 (from Pew Research Center)

Figure 2. Crude and age-adjusted COVID-19 Case Fatality Rates by Race, March-August (from Fabic, Choi, Bishai)