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Researchers ponder why US and Europe are harder hit than Asia

A memorial dedicated to those who lost their lives to COVID-19 was constructed at Greenwood Cemetary in New York City.
A memorial dedicated to those who lost their lives to COVID-19 was constructed at Greenwood Cemetary in New York City.ANGELA WEISS/AFP via Getty Images

TOKYO — It is one of the many mysteries of the coronavirus pandemic: Why has the death toll from COVID-19 apparently been lower in Asia than in Western Europe and North America?

Even allowing for different testing policies and counting methods, and questions over full disclosure of cases, stark differences in mortality across the world have caught the attention of researchers trying to crack the coronavirus code.

Parts of Asia reacted quickly to the threat and started social distancing earlier. But researchers are also examining other factors, including differences in genetics and immune system responses, separate virus strains, and regional contrasts in general health.

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China, where the virus emerged late last year in Wuhan, has recorded fewer than 5,000 deaths, which translates to three deaths per million inhabitants. Japan has around seven per million, Pakistan six, South Korea and Indonesia five, India three, and Thailand fewer than one per million. Vietnam, Cambodia, and Mongolia say they have recorded zero COVID-19-related deaths.

Compare that with about 100 deaths per million in Germany, about 180 in Canada, nearly 300 in the United States, and more than 500 in Britain, Italy, and Spain.

Scientists at Japan’s Chiba University plotted the trajectory of the virus across the world and said they noticed stark regional disparities.

‘‘That means we need to take into consideration regional differences first, before analyzing what policies and other factors are affecting the spread of infection in any given country,’’ said Akihiro Hisaka of the university’s Graduate School of Pharmaceutical Sciences.

The baseline assumption, at the moment, is that the virus — officially SARS-CoV-2 — mutates the way all viruses do and is just as innately contagious and lethal in one part of the world as in another.

Part of the reason for the high number of deaths in the United States and Western Europe may lie in an initial reluctance to react to an epidemic that seemed distant and unthreatening. In Asia, meanwhile, previous experience with the SARS and MERS epidemics enabled much faster responses to the new threat.

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Taiwan, for example, has been widely praised for its speedy response to the epidemic, including early screening of air passengers from Wuhan. South Korea built a massive program of testing, tracing, and isolating patients.

But in Japan and India, two very different countries, the relatively low death toll has baffled many scientists. Similar mysteries have emerged from Pakistan to the Philippines.

Hot and humid weather could be a factor in places such as Cambodia and Singapore. Several studies have suggested that heat and humidity can slow, although not stop, the spread of the virus, just as is seen with influenza and coronaviruses that cause common colds. But some equatorial countries, like Ecuador and Brazil, have seen many cases and deaths linked to the virus.

Demographics also play a role in regional disparities. Africa’s generally younger population may have been more resistant than northern Italy’s older communities, for example.

In Japan, which has the world’s oldest population, different reasons are being explored.

There is a widespread belief in Japan that good hygiene and habits, like wearing masks and avoiding handshakes, helped slow the spread of the virus, while universal health care and the country’s emphasis on protecting the elderly may have lowered the death toll.

Research by a team at Cambridge University showed how the virus mutated as it left East Asia and traveled to Europe, noting the possibility that the initial strain may have been ‘‘immunologically or environmentally adapted to a large section of the East Asian population’’ and needed to mutate to overcome resistance outside that region.

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Other specialists have said the significance of emergent strains remains unclear.

Different immune responses could also play a role.

Tatsuhiko Kodama of the University of Tokyo said preliminary studies show that Japanese people’s immune systems tend to react to the coronavirus as though they had previous exposure, and notes that there are centuries of history of coronaviruses emerging from East Asia.

Other studies have suggested that rates of Bacille Calmette-Guerin (BCG) vaccination might have played a role, because the antituberculosis shot can potentially cause an immune-boosting response at the cell level.

‘‘Our hypothesis is that BCG, plus infection or exposure to TB, would be protective,’’ said Tsuyoshi Miyakawa of Fujita Health University.

But Japan has a record of BCG vaccination similar to that of France and yet a very different COVID-19 mortality rate.

Specialists are divided, but clinical trials are underway.