People who have been inoculated against COVID-19 need to remain cautious until they acquire immunity and watch for any delayed adverse effects from the vaccination, according to experts, even though the two vaccines currently available in Japan have proved to be highly effective.
"Even after vaccination, those who have been inoculated are urged to remain vigilant about their health and take steps to prevent getting infected," one expert said.
The Pfizer-BioNTech and Moderna vaccines both have 95% efficacy in preventing the onset of COVID-19 and are expected to be highly effective inducing immunity two weeks after the second dose. A single dose is less effective, and one out of 20 people will not develop sufficient immunity even after receiving two doses.
Experts in the United States and other countries say that those who have been inoculated no longer need to wear a mask in general because "herd immunity" has nearly been reached as inoculation rates are high.
However, the vaccination rate in Japan is low.
"It's important even for vaccinated people to continue taking preventive measures, such as by practicing social distancing, until many people have received the second dose," Prof. Hiroyuki Kunishima of St. Marianna University School of Medicine said.
The Moderna vaccine began being administered Monday at large-scale vaccination sites, and there have been reports of an adverse effect dubbed "Moderna arm" in which redness and swelling occur after vaccination. The reaction does not happen immediately and usually goes away after a few days.
In clinical trials, 92% of people felt pain after being inoculated with the Moderna vaccine. In a study of 12 people who developed redness or swelling in their arm from four to 11 days after inoculation, five experienced redness more than 10 centimeters in diameter, but all of them recovered within two to 11 days.
"It's rare that adverse reactions appear in a delayed fashion, and it's not clearly known why," said Tetsuo Nakayama, a specially appointed professor of clinical virology at Kitasato University. "Having said that, no serious cases have been reported, so we should not overreact."
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