TOKYO — Some experts say cross immunity is the reason for the low death rate among novel coronavirus patients in Japan and other parts of Asia compared to that in Europe and North America.

When viruses that an individual has never been infected with enter the human body, the body makes antibodies, or proteins that combat the foreign substances. The first antibody to appear is Immunoglobulin M (IgM), while Immunoglobulin G (IgG) is produced later. While IgM antibodies disappear fairly quickly, IgG antibodies remain for a prolonged period and continue to protect the body from the same viruses.

When a team of researchers including Tatsuhiko Kodama, leader of the cancer metabolism project at the University of Tokyo’s Research Center for Advanced Science and Technology, studied the blood samples of coronavirus patients, IgG antibodies had increased in volume faster than IgM antibodies for some.

Kodama says this proves such people have immunity, since they were infected with a virus similar to the novel coronavirus that may have spread across Asia before. “Cross immunity may have worked when those people got infected with the new coronavirus,” he commented.

A team of researchers at the La Jolla Institute for Immunology in the United States has also pointed out that cross immunity may have worked for some people. According to a paper released by the American scientific journal Cell, researchers found immune cells that reacted to the new coronavirus from about a half of the blood samples collected from 20 people in the United States between 2015 and 2018.

Meanwhile, a separate research team including Yasuhiko Kamikubo, a program-specific professor at Kyoto University, provided a different explanation. They say because Japan was slow in closing its borders, a type of novel coronavirus with weak pathogen spread, and some people were already immune when the virus with strong pathogen spread later — which was why the damage is smaller compared to other countries.

Atsuo Hamada, professor of infectious diseases at the Tokyo Medical University, said, “If we have a clear understanding of the reason why there are gaps in the number of deaths in different countries and areas, we can understand how to respond to this infectious disease. Cross immunity due to the spread of a similar virus, mutation of the novel coronavirus and other factors to do with the virus itself are worth considering.”