TOKYO - Japan's population level immunity to Covid-19 has reached about 90 per cent in major population areas after a recent Omicron wave, though that level of protection is likely to diminish in a matter of months, according to a study published on Tuesday.

That level of so-called "herd immunity" reflects partial protection imparted from both natural infection and vaccination, according to the Tokyo Foundation of Policy Research, which estimated the levels for 12 of Japan's most-populated prefectures.

People in Tokyo, Osaka and the southern prefecture of Okinawa got most of their immunity through contagion amid high case counts in those areas, particularly during a seventh wave of infections that peaked last month, the researchers found.

About 65 per cent of Japan's population have received at least one Covid-19 vaccine booster shot, compared to about 33 per cent in the United States, based on government data.

Japan last week began distributing booster shots formulated to target the Omicron strain of the virus.

Japan currently requires a five-month interval for booster shots, though that may be too long to offer protection to elderly and vulnerable groups should a projected eighth wave emerge toward the end of the year, the researchers wrote.