How does population age structure affect overall case fatality ratios for COVID-19?

The first graph shows coronavirus deaths in Italy up to 26 March 2020 by sex and age. The overall case fatality rate for lab confirmed cases is 11.1% and 70% of deaths are male (closer to 80% below age 80, and it drops to 63% for 80 and above, because fewer men than women survive to their 80s. This is a much higher apparent case fatality rate than other countries and it is often mentioned as an explanatory factor that Italy has the oldest population in Europe. How much do differences in age structure of populations affect overall crude case fatality rates?

A paper published 2 days ago in the Lancet used data from China to estimate infection fatality ratios by age for all COVID-19 infections after adjustments for censoring (recent cases for which there has not been enough time for deaths to occur), demography, differential testing rates by age, and underascertainment. The second graph shows the resulting infection fatality ratios (as fractions -not per cents) by age group, corresponding to an overall infection fatality ratio of 0.65%. This is much lower than the crude confirmed case fatality ratio of 2.3%.

Note: These case fatality rates relate to tested cases if all age groups had the same testing rate as 50-59 year olds (the age group with highest testing rate as proportion of population). The infection fatality ratio refers to total infections including an estimate for non-tested cases that are not diagnosed.

I did a “what if” calculation to see how the overall case fatality rate would change if the population of China (where 17% of people are aged 60 or older) had the age structure of the Italian population (where 30% of people are 60+) or that of various other countries, including Nigeria (with 4.5% aged 60+ typical of African countries).  The third graph shows the resulting overall case fatality rates.  If all else was equal, including age specific infection fatality ratios, having the Italian age distribution  would approximately double the Chinese ratio, and having that of Nigeria would halve it.

Note: these do not represent real infection fatality ratios in countries. They are predicted overall ratios for all infections if age-specific infection and fatality rates are the same as those of mainland China.

Across European countries, the variation of population age structure by itself would cause relatively small variations in overall case fatality rates. Presumably other factors such as smoking levels, cardiovascular disease prevalence, health system responsiveness, and intensive care respirator supply would be more important.  Apparent case fatality rates calculated from COVID-19 deaths divided by lab-confirmed cases are not comparable across countries for a number of reasons. In particular, overall testing rates may vary across countries, with varying proportions of community and hospital samples, and the testing rates may vary in different ways across age groups.

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