Vaccinations that kicked off Wednesday in Japan are administered through intramuscular injection — the delivery of medication deep into the muscles — which is widespread in Western countries but not so in Japan.

Clinical trials have been conducted using intramuscular injections, in which the needle is inserted into the arm at a 90-degree angle. In Japan, subcutaneous injection, or a shallower insertion under the skin, is more prevalent. But if the COVID-19 vaccine is administered subcutaneously, efficacy and safety can be compromised.

Although those who have been administered the shot may feel a dull pain after a while, medical experts urge people not to worry since it probably will not last long.

The shot will be administered to the deltoid muscle in the upper arm, and the manufacturer advises against injecting subcutaneously or into the veins.

In Japan, most vaccines, including flu shots, are given through subcutaneous injections, in which the needle is inserted at a 30- to 45-degree angle. Intramuscular injection, meanwhile, administers the shot deeper below the subcutaneous tissue into the muscle. Subcutaneous injections became prevalent in Japan following reports of abnormalities in muscle movement after infants and toddlers were administered shots using intramuscular injections.

Mahito Mine, director of the Japan Pediatric Association, said people will not feel that much pain when they get the shot.

But he also added that some may feel pain, develop a fever or have trouble using the arm several hours afterwards due to the response of the immune system. Most symptoms disappear by the end of the day, but in rare cases, they may last for a few days.

“Please call your family doctor or a municipal hotline if you are worried,” Mine said.

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