Coronavirus vaccination is seen taking place in Minato Ward, Tokyo, on June 21, 2021. (Mainichi/Koichiro Tezuka)

       TOKYO -- Over 60% of people in Japan have had two COVID-19 vaccine shots, and new positive cases numbers are rapidly decreasing. But infections are spreading again in other countries further along with their vaccine rollouts than Japan. Can a sixth wave of coronavirus infections be prevented by acquiring so-called herd immunity, which makes it difficult for infected people to spread the disease?

       "Even if we do our best to vaccinate, we won't immediately be able to bring it down to zero cases," Shigeru Omi, head of the government's coronavirus countermeasures subcommittee, said in reference to coronavirus vaccinations' limits at a House of Representatives Committee on Health, Labor and Welfare meeting held on Sept. 15 during the Diet recess.

       A state of herd immunity is a situation in which an epidemic stops because a certain percentage of the population is immune to a pathogen such as a virus, meaning others are less likely to be infected.

       Initially, the Japanese government thought herd immunity could be achieved if 60 to 70% of the total population could be made immune to the coronavirus through vaccinations. This percentage is generally derived from the "basic reproduction number," the average number of people one infected person transmits the pathogen to.

       The coronavirus's basic reproduction number was said to be two to three for the conventional strain. But now the delta strain, which is reportedly twice as infectious, has spread across the world. According to calculations by the U.S. Centers for Disease Control and Prevention that were included in an internal document reported by The Washington Post, the delta strain's basic reproduction number is about 5 to 9.5 people on average.

       Atsuo Hamada, a special professor at Tokyo Medical University and an infectious diseases expert, said, "If these figures are right, we cannot achieve herd immunity unless we have at least 80 to 90% of the population with immunity via vaccinations. The hurdle has been raised considerably compared to conventional strains."

       The government's pessimistic view on acquiring herd immunity derives from the fact that highly vaccinated countries are experiencing severe infection situations. Singapore's two-dose vaccination rate is about 80%, but still new COVID-19 case numbers increased in September after early August saw some strict behavioral restrictions relaxed, including lifting a ban on eating out.

       According to Our World in Data, a statistics site run by researchers at the U.K.'s Oxford University and others, on Oct. 2, Singapore's average new infections for the previous week were 2,261, the highest recorded and equivalent to over 60,000 in terms of Japan's population.

       However, new case numbers are declining in some countries. In India, the completed two-dose vaccination rate is only 17%, and at its peak the delta strain's spread caused about 400,000 new infections daily and at times more than 7,000 deaths a day. But new case numbers have now dropped to 20,000 to 30,000 per day, less than or equal to one-tenth of the peak.

       Hamada said, "There is a view that the country has acquired herd immunity, but although infections in India are down, new case numbers are still significant and the vaccination rate is low. Just because cases decreased from the peak, it can't be said that herd immunity has been acquired."

       In Japan, daily new cases nationwide peaked at over 25,000, and began declining in late August. They have remained below 1,000 for multiple days in October. Government subcommittee experts cited a combination of factors behind the decrease, including not only vaccinations progress but also that there were more seriously ill patients during the fifth wave, which led people to avoid high-risk activities.

       Included in the committee's Sept. 3 proposal was a statement on herd immunity based on factors such as the situation overseas. It reads, "Even if everyone wishing to be vaccinated completes the process, it will be difficult to acquire it in a sense that society as a whole is protected."

       (Japanese original by Naomi Hayashi, Lifestyle and Medical News Department)

       Font Size S M L Print Timeline 0