Local governments will be allowed to administer mix-and-match COVID-19 vaccine booster shots six months after a second dose — instead of the initially proposed eight months — now that studies have shown the vaccines’ effectiveness against infection wanes by around 50% over the period of half a year.

A health ministry panel on Monday, however, kept unchanged its standard timeline for an eight-month interval between the second and third shots, putting it in line with the practice in many European nations, in order to give time for preparation. But municipalities can give the booster shot at the six-month point if they deem it necessary, the panel said.

The shortened timeline for booster shots came after a study by Yukio Ohsawa, a professor at the the University of Tokyo’s graduate school of engineering, showed that failing to accelerate the rollout of boosters may lead to a wave of infections that could far surpass that of the fifth wave over the summer, with daily cases in Tokyo alone potentially topping 10,000. Daily cases in Tokyo reached a record 5,908 on Aug. 13.

New cases nationwide, however, would continue their extended decline from the fifth wave’s peak in late August and head toward zero early next year if a booster is given as soon as the vaccine’s efficacy wanes enough that infections become more likely, the research showed.

The government plans to administer boosters for messenger RNA vaccines — manufactured by Moderna Inc. or Pfizer Inc. and BioNTech SE — to all fully vaccinated individuals. So far, only the Pfizer booster has been authorized, for people age 18 and older at least six months after the second shot, and it can be given regardless of whether the initial vaccine was from Pfizer, Moderna or AstraZeneca PLC.

The government began delivering supplies of Pfizer’s boosters this week. The same dose as the initial two Pfizer shots will be used, as is the case in the U.S.

Takeda Pharmaceutical Co. submitted an application to use Moderna’s vaccine as a booster last week. If approved, a workplace vaccination program using Moderna’s booster is expected to start from March.

The government says all eligible residents should get the vaccine, and the health ministry highly recommends a booster for elderly people and those with medical conditions who are at a higher risk of developing a serious illness. Nursing and health care workers who are at a higher risk of exposure to the coronavirus are also strongly advised to get a booster.

“Though a two-shot vaccination gives an efficacy of over 90% against serious illness, hospitalization and death, data in Japan has shown that the rate of breakthrough infections begins to rise gradually after six months,” said Tetsuo Nakayama, a project professor at Kitasato Institute for Life Sciences and director of the Japanese Society of Clinical Virology.

The sooner fully vaccinated individuals get a booster shot the better, especially elderly people and others who have a higher risk of developing serious COVID-19 symptoms, Nakayama said.

But he also added that there’s not going to be a significant difference whether they receive it six months or eight months after the second dose, and that healthy people may not need a booster shot at all.

Booster shots are set to be administered to health care workers from Dec. 1, followed by elderly people and the rest of the general public next year. While the government initially planned to finish its yearlong free coronavirus vaccination program at the end of February, it is now set to be extended until Sept. 30 due to the addition of the third dose.

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