We’re Asking the Impossible of Vaccines

In 1846, the Danish physician Peter Ludvig Panum traveled to the Faroe Islands in search of measles. The rocky archipelago, which sits some 200 miles north of Scotland, had been slammed with an outbreak, and Panum was dispatched by his government to investigate. The trip predated the formal discovery of viruses and antibodies by several decades, but Panum still stumbled upon a beguiling immunological trend: Dozens of the islands’ eldest residents, who had survived another measles epidemic in 1781—65 years earlier—weren’t getting sick this time around. “Not one, as far as I could find out by careful inquiry,” he wrote in a treatise, “was attacked the second time.”

Panum probably didn’t realize it then, but his observations helped spark the inklings of a notion that would survive his century, into the next, and the next: the promise of perfect immunity, a protection so comprehensive and absolute that it might even stave off measles for a lifetime. After measles vaccines were licensed in the 1960s, that expectation ballooned even further. Experts eventually came to describe the shot’s defenses of measles, but the very possibility of the pathogen’s proliferation at all. To modern immunologists, the phenomenon is known as , the ability to “totally prevent infection,” Taia Wang, an immunologist at Stanford University, told me. The is still as its .