Call it the hunt for a potential “X factor,” such as genetics, that may explain the trend and inform how Japan could deal with the next wave. While the new highly transmissible omicron variant has appeared in the country and experts suspect there is already some community spread, the overall transmission rate of the virus and coronavirus-related deaths in Japan have remained low.

“Honestly, we do not know the exact reason behind the sudden drop in covid deaths in Japan,” said Taro Yamamoto, professor of global health at Nagasaki University’s Institute of Tropical Medicine.

Throughout the coronavirus pandemic, Japan has had much lower rates of infection and death than in many Western countries, though there was a severe spike over the summer that overwhelmed hospitals…

In Japan, researchers have also examined factors like weather, cyclical patterns in the spread of the virus, and potential past exposure to mild coronavirus variants that may have led to the low case and death counts. Experts have identified potential genetic characteristics among the Japanese that may have led to a stronger immune system response to the coronavirus, but said there needs to be more research to draw definitive conclusions.