Over half of Japan's population has received their third COVID-19 vaccine dose, with 86.9 percent of those aged 65 and above vaccinated, government data showed Monday.

But vaccination rates among younger people still remain low relative to the general population, with only 30.1 percent and 33.2 percent of those in their 20s and 30s, respectively, having received their booster shots.

COVID-19 vaccines have proven effective in stemming the spread of the Omicron variant of the virus despite initial doubts, and health experts are encouraging people to keep up to date with their shots.

Shigeru Omi, the Japanese government's top COVID-19 adviser, receives his third shot of coronavirus vaccine at a mass vaccination center in Tokyo's Otemachi area on Feb. 5, 2022. (Kyodo)

Between April 4 and 10, infections per 100,000 among those in their 20s stood at 766 for the unvaccinated, but dropped to 306 for the fully vaccinated, and to 141 for those who had received their third shot, according to the health ministry. A similar trend was also observed in other age groups.

"The results show that (the vaccine) can prevent infections in any age group. Young people are also at risk of long COVID if they get infected, so we want them to get their third shots," said Takaji Wakita, head of the National Institute of Infectious Diseases who also leads the expert panel of the Ministry of Health, Labor and Welfare.

The booster shot vaccination rate in countries like Singapore and Britain is higher. Singapore's rate stood at 73 percent as of Saturday.

A study at Nagasaki University in southwestern Japan found that among those aged 16 to 64, a third vaccine dose was 69 percent effective in preventing an outbreak of the Omicron strain, compared with 43 percent for those who had only received up to two doses.

Studies at Chiba University Hospital and Kobe University have also yielded results showing a certain degree of effectiveness of third doses.

According to the health ministry, side effects accompanying third shots of both U.S.-manufactured Moderna and Pfizer vaccines reported by medical institutions were less than 0.01 percent of the total number of doses administered as of March 20.

An analysis of side effects among health care workers by the ministry included fever, headaches and fatigue, but most disappeared within a few days. Side effects lasting beyond 10 days were rare.

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