If you’re a respiratory virus, there’s no better place to be this month than Mecca, Saudi Arabia. Beginning around September 21, an estimated 2 million pilgrims will gather there for the hajj, the pilgrimage all Muslims who are physically and financially able must perform once in their lives. Over the course of about five days, the pilgrims will gather in extremely close quarters, especially as they circle the Kaaba, the black cube-shaped building in the center of the Grand Mosque and the holiest site in Islam. In conditions like that, just one sneeze or cough loaded with a sufficiently contagious microbe could spell big trouble.

Unluckily, the Kingdom of Saudi Arabia also happens to be ground zero for a germ that might have that potential: the Middle East respiratory syndrome coronavirus, the virus responsible for MERS. The disease first appeared in the Kingdom in 2012, and the way it spreads is still mysterious. Some people who catch MERS feel like they have a mild cold; others develop a high fever, severe cough, shortness of breath, and even complications like pneumonia and kidney failure. Thirty-six percent of people infected with MERS have died.

The world turned its attention to MERS earlier this year when the virus made its way to South Korea and caused an outbreak there, ultimately killing 36 people. But world health officials currently have their eyes on another cluster of cases, centered around a hospital in Riyadh, the capital of Saudi Arabia. Since August 17, the country’s Ministry of Health has reported 118 cases of MERS in Riyadh, sometimes counting as many as 10 new cases a day. In its September 3 report, the World Health Organization traced the source of the outbreak to an overcrowded emergency room where the virus was able to spread between patients.

Now, Riyadh is a long way from Mecca—about an eight hour drive—and the vast majority of pilgrims pouring into Saudi Arabia this week and next won’t ever pass through the capital (much less visit a hospital there). And Friday’s crane collapse at the Grand Mosque, which killed over 100 people and injured more than 200 others, is a stark demonstration that disease isn’t the only health risk when you’ve got millions of people gathered in cramped quarters. But mass gatherings like the hajj don’t necessarily play by the rules that govern public health in crowded but more settled places, like cities.

“There’s really no discipline that’s concerned with dealing with the preparation and preventative measures to prevent major public health disasters during these events,” says Ziad Memish, a doctor specializing infectious diseases and the former Deputy Minister of Health for Public Health in Saudi Arabia. He’s called for the creation of a field called “mass gatherings health” that focuses on understanding the unique risks swirling around these kinds of events, from the World Cup to the hajj.

When it comes to disease risk, there’s one key difference between mass gatherings and settled cities, says Kamran Khan, an infectious disease physician at St. Michael’s Hospital in Toronto and the founder of a disease tracking company called BlueDot: “It’s the nature of the interaction between individuals. How densely crowded are they?”

At the World Cup or the Olympics, thousands of people may pack into stadiums, but the clusters are spread throughout a city or country. When it comes to sheer density, even the biggest sporting event can’t compete with religious mass gatherings like the hajj or the Kumbh Mela in India, in which up to 100 million people gather to bathe in a sacred river. “When you’re dealing with communicable disease, and people are that close, the ability for one case of disease to amplify is more significant,” Khan says.

Another complicating factor presented by the hajj and other religious mass gatherings is that participants tend to be on the older side. Often, they’ve saved and planned for years or even decades to be there, pushing up the average age. “Religious mass gathering bring a huge number of people who are usually elderly, who have many comorbidities,” or preexisting chronic conditions like diabetes or heart disease, says Memish. “They’re prone to have complications from their diseases and tend to cause higher mortality and higher morbidity” at the mass gathering.

Millions of people with weakened immune systems jammed into a relatively small space like the Grand Mosque could give a virus like MERS the opportunity to really take off. That’s why Saudi Arabia is recommending that elderly and chronically ill people, along with those who are pregnant or have compromised immune systems, not perform the hajj this year. But that’s just a guideline, and the demographics of the hajj are always likely to be riskier than those of the World Cup.

The hajj also draws a more diverse crowd than big sporting events when it comes to socioeconomic background. Pilgrims, who travel to Mecca from over 180 countries, are rich, poor, and everything in between—and their countries of origin are too. That means that some pilgrims have access to the best healthcare in the world, while others have probably only seen a doctor a few times in their lives. That disparity is particularly worrisome when it comes to MERS, which has its epicenter in Saudi Arabia, Khan says. “The concern would be, could the pilgrims get infected there and then go back.” The countries they return to may not have the public health resources to contain an outbreak.

The good news is that both Khan and Memish think it’s unlikely that MERS will cause a problem at this year’s hajj, despite the bad timing of the Riyadh hospital outbreak. “It’s important to be aware of [those cases], but it’s definitely not as worrisome as a community based outbreak” happening outside a hospital would be, Khan says. Memish was part of the team that kept H1N1 influenza cases to a minimum at the 2009 hajj—the year when swine flu swept the globe—and he says that Saudi Arabia’s successful handling of potential disaster “gives the world an idea of how sophisticated the preparation is in the Kingdom.” (I mean, they’ve only been hosting the hajj for 1,500 years.)

Similarly, MERS prevention during the event “has been very successful over the last two years,” Memish says. Despite all the risk factors among hajj attendees and the lingering mystery of exactly how MERS spreads, we’ve yet to see a global MERS pandemic explode after the pilgrimage. At mass gatherings, “the conditions may be there for epidemics, but we generally don’t see them as often as they could occur,” Khan points out. Let’s hope this year’s hajj is no exception.

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