Researchers are pondering why North America and Western Europe appear to have more fatalities from the coronavirus than Asia. While parts of Asia acted swiftly to thwart the spread of the virus, scientists are analyzing other reasons, such as the differences in genetic and immune responses, obesity and general health in these populations.

They’ve examined several environmental factors, too, including diverse regional climates, previous exposure to similar viruses, vaccination history and the possibility that different strains of the virus in certain regions are more deadly than others. Surprisingly, one risk factor appears to stand out as a common thread among nations with elevated mortality rates: obesity.

According to the The Washington Post, deaths from COVID-19, per 100,000 people, numbered 81.7 in Belgium, 58 in Spain, 55.8 in Britain, 54.5 in Italy, and 29.8 in the U.S. In Japan, South Korea, Indonesia, China and even India, the numbers were less than one. In Vietnam, there were zero deaths per 100,000 cases of COVID-19.

Experts said that SARS-CoV-2 is just as lethal in one part of the world as another, so why the huge discrepancy in mortality?

“We’re all facing the same bug with the same general arsenal of immune responses,” said Jeffrey Shaman, an epidemiologist at Columbia University, according to the Post.

Some experts argued that Asian countries were swifter to respond to the novel coronavirus based on their experience with SARS and MERS. But Japan and India also had low death tolls, as well as Pakistan and the Philippines. According to the Post, scientists at Los Alamos National Laboratory claimed that a more highly contagious strain took hold in Europe and the U.S.

But researchers also pointed out that the obesity rates in Asian countries is significantly lower. According to the World Health Organization, 36% of Americans are obese compared to just over 4% of Japanese and less than 5% of South Koreans.

According to Medical News Today, evidence suggests that obesity is associated with a higher risk of developing severe symptoms and complications of COVID-19. An increasing number of reports have also linked obesity to coronavirus mortality and the Centers for Disease Control and Prevention lists severe obesity as a risk factor for severe COVID-19.

But experts quoted in the Post article said that we don’t have enough data to make a hard-and-fast call on why some areas are harder hit than others. According to the Post, we can’t rule out random chance. Russia was practically virus-free for a couple of months, and now it’s a hotspot.