This chart traces the evolution over time of the share of countries that have started administering boosters in the total number of countries within each country group of the World Bank income classification.
The definition of the country universe matters considerably for this chart. We consider a total of 196 countries. This universe is based on the latest World Bank’s income classification updated every July 1 (see full details in the background section below) and reflects a further narrowing down to only include members and/or observers of UN or UN specialized agencies (which excludes 21 HICs and 1 UMIC, which are mostly overseas territories and dependencies). The full methodology is discussed in detail in the background notes copied below.