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Japan's mysteriously low Coronavirus death rate

Why haven't more people in Japan died from Covid-19? Its a question that has spawned dozens of theories, from Japanese manners to claims that the Japanese have superior immunity.

In fact Japan doesn't have the lowest death rate for Covid-19 - in the region, South Korea, Taiwan, Hong Kong and Vietnam can all boast lower mortality.

But in early 2020, Japan saw fewer deaths than average. This is despite the fact that in April, Tokyo saw about 1,000 "excess deaths” perhaps due to Covid. Yet, for the year as a whole, it is possible that overall deaths will be down on 2019.

So does Japan have special immunity?

Tokyo University professor Tatsuhiko Kodama - who studies how Japanese patients react to the virus - believes Japan may have had Covid before. Not Covid-19, but something similar that could have left behind "historical immunity".

This is how he explains it: When a virus enters the human body, the immune system produces antibodies that attack the invading pathogen.

There are two types of antibody - IGM and IGG. How they respond can show whether someone has been exposed to the virus before, or something similar.

"In a primary (novel) viral infection the IGM response usually comes first. Then the IGG response appears later. But in secondary cases (previous exposure) the lymphocyte already has memory, and so only the IGG response increases rapidly."

So, what happened with his patients?

"When we looked at the tests we were astonished... in all patients the IGG response came quickly, and the IGM response was later and weak. It looked like they had been previously exposed to a very similar virus."

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