File:Alternative version of Figure 3 in main paper showing the age-standardised proportion of population at increased risk and high risk of severe COVID-19 by country and region.png

structure and disease prevalence. This information is important when calculating the numbers that might need to be shielded or vaccinated but does not allow direct comparison of risks at equivalent ages in different countries. In this alternative version (see below), circles have been added to show the age-standardised share of the population at high risk (black circles) and increased risk (open circles). These assume each country has the same WHO standard reference population.17 A low age-adjusted population at risk in countries with older populations (eg, Japan, Europe and Puerto Rico) helps to confirm that older age is the main reason why these countries have a high unadjusted population at risk. Similarly, a high age-adjusted population at risk in African countries with high HIV prevalence (eg, eSwatini, Lesotho) and small island nations with high diabetes prevalence (eg. Fiji, Mauritius) explains why these countries have a high unadjusted population at risk, despite having younger populations. Differences in demography can mask important differences in age-specific risks that may be revealed by age-standardisation. For example, in eSwatini and New Zealand the population at high risk is 5% in both countries, but when risks are compared for equivalent age groups (within the spreadsheet tool) the age-specific risks in eSwatini are more than double those in New Zealand (consistent with eSwatini having a higher age-adjusted population at high risk ie, 8% vs 3%). Thus, although younger populations will tend to have a lower share of the population at risk than older populations, their risk at equivalent ages could still be higher.