Health Advice



It's too soon to declare herd immunity against COVID-19. My blood helps explain why

Tom Avril, The Philadelphia Inquirer on

Published in Health & Fitness

Texas Gov. Greg Abbott caused a stir recently when he said his state was "very close" to achieving herd immunity against COVID-19, that much-discussed but often-misunderstood goal that we've all been anticipating for months. If so, other states would be even closer, to judge by the numbers who have been infected, vaccinated, or both.

Could Abbott be right?

The short answer, in most places: probably not yet. And it might not even be the correct question to ask.

In a very small way, I helped researchers start to solve this puzzle last year, pricking my finger and sending in a blood sample for an antibody study at the National Institutes of Health. Thousands of other volunteers across the country did so, too, and the results are in, providing a preliminary estimate of how many have been infected with the coronavirus.

In theory, you could add that figure to the number of people who have been vaccinated, and get an idea of whether enough of the population is immune that the virus can no longer spread — the definition of herd immunity. For this virus, epidemiologists estimate that figure needs to be above 80%, and we're not there yet.

More on the numbers below, but for a variety of reasons the answer is not simple, said Matthew J. Memoli, one of the NIH study leaders.


"I'm not a big fan of the term herd immunity," he said. "The real question is: When can we stop worrying about this?"

Among the key remaining questions: How is natural immunity different from the level of protection that comes with a vaccine? What about people who underwent both things, a case of COVID followed by a shot (or two) in the arm? And how long does each type of immunity last?

Finally, people should not expect that when we reach 80%, the pandemic disappears overnight, he said. COVID-19 could remain a serious seasonal problem like the flu, blamed for thousands of deaths each year. Or perhaps it fades into the background.

Still, much of the country could be in good shape by the start of summer, he and other infectious-disease experts say.


swipe to next page
©2021 The Philadelphia Inquirer, LLC. Visit at Distributed by Tribune Content Agency, LLC.