Siddhartha Mukherjee writes:

On December 2nd, Mukul Ganguly, an eighty-three-year-old retired civil engineer in Kolkata, India, went to the Salt Lake Market to buy fish. The pandemic was surging around much of the world, and he wasn’t oblivious of the risks of spending time at a wet market. His wife, a former forensic analyst, protested vehemently. But Mr. Ganguly wouldn’t be deterred. He picked up his fabric shopping bag, tucked a doubled-up handkerchief in his pocket, and stepped out.

Mr. Ganguly lives in a modest, two-story, book-filled house a few blocks from the market. He tied his folded handkerchief into a makeshift mask, and spent about two hours buying groceries, choosing vegetables and sweets, and bargaining with the venders. (Give a man a fish and you feed him for a day; teach a man to haggle with a fishmonger and you’ll feed him for a lifetime.) Two days later, he came down with a fever and a dry, incessant cough; he was barely able to walk to the bathroom. His daughter-in-law, in New Jersey—a cousin of mine—called me in a panic: he had tested positive for COVID-19.

We worked up a plan. He was to be isolated in a room with a pulse oximeter. His vitals were monitored twice daily. We arranged for a supplemental oxygen tank to be brought home in case his O₂ levels dipped too low. I called my doctor friends in Kolkata and asked them to stand by. For two days, Mr. Ganguly had a fever—100 degrees, 101 degrees—and then it subsided. By Christmas, he was pretty much back to normal. When I spoke to him in late December, he told me, in Bengali, that his experience had been typical. Various friends, all in their seventies and eighties, had contracted COVID-19. All had bounced back.

I called a friend in Mumbai, Shashank Joshi, who is a member of his state’s COVID-19 task force. “Our I.C.U.s are nearly empty,” he told me. Joshi is a doctor with seemingly infinite reserves of energy: a stethoscope perpetually dangling across his chest, he has spent the past several months carrouselling among slums, hospitals, and government offices, coördinating the state’s response. Early last spring, when the first serious spread of COVID-19 was reported in India, Joshi jumped into action. Dharavi, in Mumbai, is Asia’s largest slum: a million residents live in shanties, some packed so closely together that they can hear their neighbors’ snores at night. When I visited it a few years ago, open drains were spilling water onto crowded lanes. (The next monsoon season, three young boys fell into the drains and died.) The tin roofs of the houses overlapped one another like fish scales; a roadside tap dripped a brown fluid that passed for potable water. When a toddler ran out from an open door onto the street, a neighbor caught him and lifted him up. Someone in the family—I counted six people in a single room, including an elderly couple—sent another child to retrieve him. In that episode alone, I later realized, I had witnessed at least nine one-on-one contacts.

After the pandemic was declared, last March, epidemiologists expected carnage in such areas. If the fatality rate from the “New York wave” of the pandemic were extrapolated, between three thousand and five thousand people would be expected to die in Dharavi. With Joshi’s help, Mumbai’s municipal government set up a field hospital with a couple of hundred beds, and doctors steeled themselves to working in shifts. Yet by mid-fall Dharavi had only a few hundred reported deaths—a tenth of what was expected—and the municipal government announced plans to pack up the field hospital there. By late December, reports of new deaths were infrequent.

I was struck by the contrast with my own hospital, in New York, where nurses and doctors were prepping I.C.U.s for a second wave of the pandemic. In Los Angeles, emergency rooms were filled with stretchers, the corridors crammed with patients straining to breathe, while ambulances carrying patients circled outside hospitals.

And there lies an epidemiological mystery. The usual trend of death from infectious diseases—malaria, typhoid, diphtheria, H.I.V.—follows a dismal pattern. Lower-income countries are hardest hit, with high-income countries the least affected. But if you look at the pattern of COVID-19 deaths reported per capita—deaths, not infections—Belgium, Italy, Spain, the United States, and the United Kingdom are among the worst off. The reported death rate in India, which has 1.3 billion people and a rickety, ad-hoc public-health infrastructure, is roughly a tenth of what it is in the United States. In Nigeria, with a population of some two hundred million, the reported death rate is less than a hundredth of the U.S. rate. Rich countries, with sophisticated health-care systems, seem to have suffered the worst ravages of the infection. Death rates in poorer countries—particularly in South Asia and large swaths of sub-Saharan Africa—appear curiously low. (South Africa, which accounts for most of sub-Saharan Africa’s reported COVID-19 deaths, is an important exception.)

As the pandemic engulfed the world during the past several months, I kept returning to the question of what might explain these discrepancies. It was an epidemiological whodunnit. Was the “demographic structure” of a population the real factor? Were the disparities exaggerated by undercounting, with shoddy reporting systems hiding the real toll from public-health analysts? Was government response a critical variable? Or were other, less obvious factors at play? Perhaps any analysis would prove premature. If new viral strains, such as the South African variant of the virus, known as 501Y.V2, were to sweep through Africa, every prediction of mortality might be overturned. But as I started speaking with colleagues from around the world I found that my puzzlement was widely shared. For many statisticians, virologists, and public-health experts, the regional disparities in COVID-19 mortality represent the greatest conundrum of the pandemic. [Continue reading…]