When Cambodia rolled out COVID-19 vaccines, lines stretched down entire streets and people left their shoes out to save their places as they sheltered from the sun. But three months into its campaign, just 11% of the population had received at least one dose. In far wealthier Japan, it took two weeks longer to reach that level.

Now both countries boast vaccination rates that rank among the world’s best. They are two of several nations in the Asia-Pacific region that got slow starts to their immunization campaigns but have since zoomed past the United States and many nations in Europe.