A Japanese government delay in rolling out Covid-19 booster shots left it more vulnerable than other rich countries when the Omicron variant brought a surge of deaths, say experts, local governments and a former vaccine czar.

The issue could mean political trouble for Prime Minister Fumio Kishida as nearly 30 percent of the population is aged 65 or older, and so at greater risk from the coronavirus without the protection of the booster.

Kishida's predecessor...