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Are we at the beginning of the end of COVID-19? The tricky road to herd immunity, explained

Tom Avril, The Philadelphia Inquirer on

Published in Health & Fitness

One year into a changed world, the numbers defy comprehension. More than 21 million confirmed cases of COVID-19 in the United States — a rate approaching one in 15 people — plus untold millions who had mild or no symptoms and were never identified. Add the millions who have now received their first doses of vaccine, and it is fair to wonder:

Can we start to look forward to when life returns to normal?

The answer hinges on that often-misunderstood concept of herd immunity, made all the more complicated by the emergence of two variants of the coronavirus that seem to spread more rapidly, one of which is already in the U.S. Evidence so far suggests that the new mutations will have little, if any, impact on how well the vaccines work, and they do not seem to result in more severe illness.

Yet given the wintertime surge in cases — possibly aggravated by the new mutations — and the question of how long immunity lasts, public health experts say this is no time to relax precautions.

For a reality check on how far we’ve come, we spoke to three researchers who study the spread of infectious disease: Temple University epidemiologist Abby Rudolph, Drew University biologist Brianne Barker, and Pennsylvania State University biologist Maciej Boni.

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The term herd immunity was coined a century ago by veterinarians studying the spread of disease in livestock. Simply put, they meant that if enough cows developed immunity to an illness, the herd would be protected. Even if an animal or two were somehow to become infected on occasion, they would be unable to pass it on.

Boni, who has served on World Health Organization committees for malaria and other disease epidemics, likens the concept to a forest fire.

“If a lot of the trees have been burned down,” he said, “you have a forest that’s sparse enough that no fire can take off in the forest.”

But in practice, achieving the threshold necessary for mass protection from disease is complicated. For one thing, the number varies depending on how easily the microbe in question spreads.

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