| Ever since the pandemic began, people have been wondering when it will be over. First, authorities pinned their hopes on brief shutdowns. When that failed, everyone from TV commentators to armchair epidemiologists on Twitter latched onto another alluring concept: herd immunity. The idea, first described in herds of sick cattle, is simple on paper. But in the real world, it can get complicated fast. Unlike cows, people have political and cultural differences hampering vaccinations. Meanwhile, the constantly mutating coronavirus is proving to be an elusive target. In light of that, epidemiologists say it may be time for the public to stop fixating on a single hard-to-pin-down herd immunity number that could signal a conclusion to the pandemic. A more realistic way to imagine the end of the pandemic is a slow, gradual improvement, with many bumps on the way. Even U.S. government officials like Anthony Fauci have shied away from citing specific herd immunity figures in recent weeks. "It's a little bit misleading to think we are just approaching this magic number, and that when we hit it the threat is going to go away," says Lauren Ancel Meyers, director of the Covid-19 Modeling Consortium at University of Texas at Austin. "As long as the virus is spreading in pockets of the population, it will remain a threat to all of us."  Young Israelis gather in Tel Aviv on April 19. For one thing, she notes, vaccine coverage varies widely from place to place even within the U.S. Meanwhile, in many countries outside the U.S., campaigns have barely begun. Herd immunity is misunderstood by the public as a destination, a point at which the virus stops spreading entirely, says Justin Lessler, an associate professor of infectious disease epidemiology at Johns Hopkins University. He encourages people to think of the concept as a spectrum instead, because as more people get vaccinated, the population is increasingly protected. Even if some version of herd immunity is reached in the U.S. and other wealthy countries, it may not be a permanent state of affairs, so long as the virus continues spreading—and mutating—in other parts of the world, he said. "It seems like the virus is going to continue to evolve fast enough that we will probably never eradicate it," Lessler says. "I hope I'm wrong. But my guess is it's never going to be eradicated."—Robert Langreth and Emma Court | 
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