While some countries have experienced a devastating amount of illness and death due to Covid-19, the disease caused by the new coronavirus, others have experienced relatively mild outbreaks of the disease. Writing for the New York Times, Hanna Beech and colleagues outline four theories that could explain why some areas have been hit by the global pandemic harder than others.

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Some countries see thousands of cases of Covid-19—while others see less than 100

Almost every country has reported at least a few cases of Covid-19, but some areas have been hit much harder than others.

For example, researchers have noted that some large cities—including New York and Paris—have been epicenters of their respective countries' coronavirus epidemics, while other big cities—including Bangkok and New Delhi—have experienced much smaller outbreaks.

Researchers also have noted that some neighboring countries have had vastly different experiences with the new coronavirus. For instance, while the new coronavirus has been tied to thousands of deaths in Iran, Iraq has reported fewer than 100 deaths tied to the virus. Similarly, the Dominican Republic has reported almost 7,600 cases of Covid-19, while bordering country Haiti has reported about 85 Covid-19 cases so far.

Now, researchers are speculating why the new coronavirus has infected thousands in some countries, but has left other countries relatively untouched—and the answer "could have profound implications for how countries respond to the virus, for determining who is at risk, and for knowing when it's safe" to ease certain mitigation measures, Beech and colleagues write.

Why are some countries reporting more coronavirus cases than others?

While physicians and researchers have yet to establish a "full epidemiological picture" of Covid-19, researchers thus far have developed four probable theories that could explain why the new coronavirus has devastated some countries, but not others.

1. Younger populations

Researchers have noted that a lot of the countries experiencing comparatively lower rates of Covid-19 cases also have comparatively younger populations, and therefore have theorized that countries with younger residents on average are less likely to see widespread outbreaks of Covid-19.

This theory could explain why Africa, which has the youngest population on average when compared with all other continents, so far has reported about 45,000 cases of Covid-19 among about 1.3 billion people, while Italy, which has a national median age of more than 45, is among the countries that have been most affected by Covid-19, Beech and colleagues report.

According to Robert Bollinger, a professor of infectious diseases at the Johns Hopkins School of Medicine, younger patients often are less likely to have underlying health conditions like diabetes and hypertension, which can cause potentially fatal complications in Covid-19 patients.

In addition, Josip Car, a population and global health expert at Nanyang Technological University in Singapore, said younger patients usually have stronger immune systems than older patients, which can make them more likely to experience milder cases of Covid-19.

However, Beech and colleagues write that researchers' overall theory on a country's average age affecting how hard it's hit by the new coronavirus does have some exceptions—like Japan, which has the oldest average population in the world but so far has reported fewer than 520 Covid-19-related deaths.

2. Cultural distance

People in some countries tend to be more socially distant than others, which researchers theorize could help to prevent the new coronavirus from spreading, Beech and colleagues write.

For instance, in India and Thailand, people typically greet each other from a distance by putting their own palms together—a greeting that does not require people to touch others. These countries have reported relatively low numbers of Covid-19 cases, Beech and colleagues report.

Likewise, in Japan and South Korea—which also have reported comparatively low numbers of cases of Covid-19—people typically bow to greet each other and tend to wear facemasks in public when they're not feeling well, Beech and colleagues write.

"National distancing," or being isolated from other nations, also appears to have been beneficial for some countries, Beech and colleagues write. For instance, places in the South Pacific that have had comparatively fewer visitors from abroad than other countries have reported comparatively fewer cases of Covid-19.

But there are exceptions to this theory, as well, Beech and colleagues report. For example, they note, there are "many parts of the Middle East, such as Iraq and the Persian Gulf countries, [where] men often embrace or shake hands on meeting, yet most are not getting sick."

3. Environment

Researchers have noted that the new coronavirus has appeared to spread the fastest in countries with temperate environments like the United States. They noted that, when countries first started reporting cases of Covid-19, countries with warmer climates were not reporting many cases. The observations led researchers to question whether the new coronavirus struggled to survive in heat.

Further, Beech and colleagues note that a study by ecological modelers at the University of Connecticut has found that ultraviolet rays could inhibit the new virus, suggesting that "surfaces in sunny places may be less likely to remain contaminated."

But again, Beech and colleagues cite exceptions, such as

the severe outbreaks seen in tropical places like the Amazonas region of Brazil.

"The best guess is that summer conditions will help but are unlikely by themselves to lead to significant slowing of growth or to a decline in cases," said Marc Lipsitch, director of the Center for Communicable Disease Dynamics at Harvard University.

4. Governments' response

Countries that enacted strict social distancing and stay-at-home policies early in their outbreaks mostly experienced milder outbreaks overall, Beech and colleagues write.

They note that countries in Africa that previously experienced outbreaks of HIV and Ebola "reacted quickly" and were able to enact certain mitigation measures faster than other countries. For instance, employees at airports in Sierra Leone and Uganda were wearing masks and asking travelers for contact details long before some Western countries enacted similar precautions, Beech and colleagues write. Sierra Leone also repurposed disease-tracing protocols that officials had used during West Africa's Ebola outbreak in 2014. So far, the country has reported only 155 confirmed cases of Covid-19.

Countries that initiated stay-at-home and social distancing policies early also have reported fewer cases of Covid-19. For instance, countries such as Jordan and Thailand saw their rates of newly reported Covid-19 cases drop relatively quickly after they closed schools, most businesses, and their national borders. Likewise, "the widespread shuttering of mosques, shrines, and churches happened relatively early" throughout the Middle East "and probably helped stem the spread in many countries," Beech and colleagues report.

But there have been some exceptions, Beech and colleagues write. Even though Christian and Muslim residents from Lebanon were still making religious pilgrimages to Italy and Iran, Lebanon has not reported a higher number of cases of Covid-19.

"We just didn't see what we were expecting" to happen in Lebanon in terms of its Covid-19 outbreak, said Roy Nasnas, an infectious disease consultant at the University Hospital Geitaoui in Beirut. "We don't know why," Nasnas added.

Does it all come down to timing—or luck?

Researchers say these theories are the most probable explanations for the differences in countries' Covid-19 outbreaks. However, each theory comes with "considerable caveats" as well as "confounding counter-evidence," Beech and colleagues write.

According to Beech and colleagues, that's giving rise to another theory that's been difficult to prove this early into the pandemic: that the new coronavirus simply may not have reached certain areas in a widespread manner yet.

Researchers point to the 1918 Spanish outbreak as a basis for the theory, noting that areas like Alaska and the South Pacific weren't hit by the outbreak until its third wave occurred in 1919.

In terms of the new coronavirus pandemic, the world still is in the "really early" stages, Ashish Jha, director of the Harvard Global Health Research Institute, said. "If this were a baseball game, it would be the second inning and there's no reason to think that by the ninth inning the rest of the world that looks now like it hasn't been affected won't become like other places," Jha explained.

Overall, Beech and colleagues write that it's likely "some combination" of researchers' theories that ultimately explains why the global Covid-19 pandemic has hit some areas harder than others—"as well as one other mentioned by researchers: sheer luck" (Beech et al., New York Times, 5/3).