Covid in Japan

Tokyo antibody seroprevalence and PCR cases (Hibino et al.)

Published: September 25, 2020
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Japan is well-known for not having imposed a lockdown and yet having by far the lowest covid death rate among G8 countries – 1400 official covid deaths in a population of 125 million; excess mortality is higher but still within the normal range – in spite of having the oldest population in the world.

Japan didn’t engage in mass PCR testing, either; for comparison, Germany is testing more people per week than Japan has tested since April. The highly questionable mass PCR testing of the general population is costing Germany and other Western countries billions of dollars.

Due to the low testing rate, it wasn’t clear how widespread the new coronavirus really was in Japan; early antibody surveys indicated a prevalence of only a few percent. However, since then there was a first infection wave in April and a second and stronger infection wave in July/August.

A new preprint study by researchers in Tokyo and Boston now shows that cumulative antibody seroprevalence (IgM and IgG) in Tokyo climbed from 5% in May to almost 50% by late August, when daily new infections were already dropping again (still without lockdown). The study included about 1900 healthy employees of a large corporation from 11 disparate locations across Tokyo.

If this result is representative, it means that Tokyo has achieved a high degree of collective immunity with a minimal death rate (about 300 deaths in 14 million people), resulting in a record low IFR of just 0.0006% (page 8 of the study), and a very low hospitalization rate.

This result moreover means that even in Japan, face masks did not work – they didn’t prevent the spread of the coronavirus. Japan’s very low death rate must then be due to genetic factors, health factors (very low rate of obesity and cardiovascular disease), or pre-existing immunity.

Indeed, Japanese researchers report that they are observing a specific immune response pattern (IgG rising faster than IgM antibodies) indicating earlier exposure to similar SARS-like coronaviruses that may be prevalent in some Southeast Asian countries.

There are some limitations to this study, however. The cohort wasn’t based on random sampling of the entire population and included mostly people of working age and few senior citizens. It is possible that antibody seroprevalence among senior citizens is still lower due to self-isolation; then again, collective immunity of the general population is their best protection, anyway.

Study (preprint): Dynamic Change of COVID-19 Seroprevalence among Asymptomatic Population in Tokyo during the Second Wave

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