A Japanese health ministry panel approved Wednesday the administration of third booster shots of U.S. pharmaceutical giant Pfizer Inc.'s COVID-19 vaccine to people aged 18 and over, starting with medical personnel from December.

The move came as overseas studies showed antibodies that protect against the respiratory disease decrease over time in all age groups, with the effectiveness of vaccines lasting around six months.

Supplied photo shows a tray of Pfizer Inc.'s COVID-19 vaccines. (Photo courtesy of Pfizer Inc.)(Kyodo)

After health care workers, individuals who were inoculated with two doses of the Pfizer vaccine will become eligible for the booster shots in January.

The government is planning to start making booster shots available at workplaces from March.

For now, Japan will refrain from administering Pfizer booster shots to those aged between 12 and 17 due to a lack of data on the vaccine's safety and efficacy. The country, however, will consider lowering the age for third shot recipients after safety is confirmed as the rollout progresses.

The panel made the decision as Pfizer only conducted studies on the booster shoot's effectiveness in those aged 18 or older while considering cases in the United States and other countries already administering the booster shots.

Pfizer's clinical studies found that a third dose of its vaccine increased the antibodies against the coronavirus to 3.3 times the levels seen after the second dose.

In another trial, the vaccination became 95.6 percent effective against symptomatic infection when a booster shot was administered about 11 months after a second dose.

A U.S. research study found the efficacy of the Pfizer vaccine in people aged between 16 and 44 declined from 89 percent within a month of a second dose being received to 39 percent five months later.

The vaccine's efficacy fell from 87 percent to 50 percent in people aged between 45 and 64, and from 80 percent to 43 percent in those 65 and over, the study showed.

In order to secure vaccine supply, Japan has signed contracts to receive an additional 120 million doses from Pfizer and an additional 50 million doses from Moderna Inc. from the beginning of 2022.

The country will also secure 150 million doses of the vaccine developed by U.S. pharmaceutical firm Novavax Inc. starting in early 2022.

The ministry's panel will discuss the use of Moderna's vaccine as a booster shot in late December at the earliest.

As of late October, over 70 percent of Japan's population had been fully inoculated against COVID-19, with the country ranked among the top three in the Group of Seven nations after an initially slow vaccine rollout.

Japan's vaccination program was launched in February, starting with health care workers. It was expanded to the elderly in April and later to other members of the public.