In the spring of 1846, a Dutch physician named Peter Ludwig Panum arrived on the Faroe Islands, a volcanic chain about two hundred miles northwest of Scotland. He found the Faroes to be a harsh and unforgiving place. The islands’ eight thousand inhabitants, who were Danish subjects at that time, spent their days outdoors, buffeted by sea winds, fishing and tending sheep. The conditions, Panum wrote, were unlikely “to prolong the lives of the inhabitants.” And yet, despite the scarcity of medical care and a diet of wind-dried, sometimes rancid meat, the average Faroese life span was forty-five years, which matched or exceeded that in mainland Denmark. The islanders benefitted from a near-complete lack of infectious disease; many illnesses, including smallpox and scarlet fever, rarely reached them. Panum had arrived to study a measles epidemic—the first outbreak of that virus in the Faroe Islands in sixty-five years.
For the most part, the course of the outbreak was devastating and predictable. In six months’ time, more than three-quarters of the islands’ inhabitants were infected, and about a hundred people died. But the outbreak was also unusual in many ways. In mainland Europe, measles was typically a childhood infection. Few Faroese children died in the outbreak; instead, adults bore the brunt. Their mortality rates increased with every decade of life until about the age of sixty-five, and then dropped off. It turned out that those who’d been infected during the islands’ last measles epidemic, in 1781, were still protected by the immunity that they’d acquired decades before. Of these “aged people,” Panum wrote, “not one, as far as I could find out by careful inquiry, was attacked the second time.”
Panum’s study remains a striking demonstration of a remarkable fact: the body remembers. It learns to recognize the pathogens it encounters, and, in some cases, it can hold on to those memories for decades, even a lifetime. Ancient civilizations knew about immune memory long before they understood it; Thucydides, in his account of the plague of Athens, wrote that “the same man was never attacked twice—never at least fatally.” Many of us draw our ideas about the immune system from stories like these. We think of immunity as a binary state: without it, we’re vulnerable; with it, we’re safe.
For many pathogens, however, including coronaviruses, immunity is less clear-cut. The coronavirus family includes SARS-CoV-2, the virus responsible for COVID-19, along with four seasonal coronaviruses—HCoV-229E, HCoV-OC43, HCoV-HKU1, and HCoV-NL63—which together cause an estimated ten to thirty per cent of common colds. Today, these seasonal coronaviruses are the cause of common childhood infections, as measles was in Panum’s time. In sharp contrast to measles, though, adults are reinfected by seasonal coronaviruses every few years. [Continue reading…]