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 per cent of the population is aged 65 or older, and so at greater risk from the coronavirus without the protection of the booster.

Although Japan was comparatively slow to launch its initial vaccination campaign, it ramped it up quickly and by November had the highest vaccination rate within the Group of Seven rich countries, according to a Reuters report.

But then the health ministry stuck to a protocol for an eight-month wait between the first vaccination course and the booster, even as other countries cut the waiting times and local governments, including Tokyo, urged a faster roll-out.