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The only way this ends: herd immunity

It has a bad rap, but in the long run it’s our best hope.

People filled the Esplanade last summer for the Boston Pops Fireworks Spectacular. Large gatherings like these may seem unimaginable now, but they should come back once most people have developed immunity to the coronavirus, either by vaccine or carefully controlled exposure.Erin Clark for The Boston Globe

Last week Governor Charlie Baker released projections of how many Massachusetts residents were likely to contract the coronavirus. By this reckoning, Baker said, the state would experience “somewhere between 47,000 and 172,000 cases during the course of the pandemic.” This represents between 0.7 and 2.5 percent of the state’s population.

These are daunting numbers. Unfortunately, they are not nearly daunting enough. Because while there is still a lot we don’t know about COVID-19, including exactly how many people are or have been infected, epidemiologists believe that this virus won’t begin to disappear until a far higher percentage of the population — at least 60 percent — develops immunity. If that doesn’t happen with a vaccine, it has to happen through exposure.


For weeks, the most pressing policy challenge has been relieving the life-and-death pressure on our hospitals. But all that justifiable emphasis on flattening the curve may have created a dangerous illusion that we can get away with relatively small infection rates.

It’s easy to forget that if a disease can’t be contained — and it’s too late for that in the COVID-19 pandemic — then there’s only one possible ending to the story: We must collectively develop immunity to the disease. In lieu of a vaccine, that means most of us will need to be exposed to the virus, and some unknowably large number of us will die in the process.

This is the simple, scary math that Harvard epidemiologists Marc Lipsitch and his colleague Yonatan Grad have tried to convey in a series of recently published papers: If each person infected with COVID-19 disease in turn infects three more, as we now think, then in order to bring the disease to heel, Grad says, two of those people must already be immune. “If one person can only spread the disease to one other person, the virus is no longer an epidemic,” he says. Two-thirds of the population of Massachusetts, by the way, is 4.5 million people.


When asked why state officials would suggest that the outbreak might infect far fewer people in Massachusetts, Lipsitch said: “It doesn’t make any sense to me to project that.” Indeed, when asked about the figures, Brooke Karanovich, a spokeswoman for the Executive Office of Health and Human Services, said they came from a model the state produced to “inform an analysis about building hospital surge capacity, not predict every detail about this fast-developing outbreak.”

Herd immunity got a bad rap last month, when Boris Johnson’s government floated the strategy of allowing COVID-19 to rip through the British population quickly, letting the chips fall where they may. The idea was rightly dismissed as a Wall Street fever dream, a bargain in which we might have tried to trade the most vulnerable members of our society in order to save the stock portfolios of the most prosperous.

But the fact remains that herd immunity isn’t merely a possible strategy. In the long run it’s the only strategy. The question, then, is how to get there responsibly.

“The objective is to get the trajectory right,” said Nadia Abuelezam, a Boston College epidemiologist. That trajectory is what guides public policy. And right now public policy needs to bake in the understanding that unless we plan on spending the year or more it’ll take to widely distribute a vaccine sequestered in our homes without respite, we will need to immunize the state’s population the hard way.


For instance, one of Lipsitch and Grad’s findings is that the better we are at social distancing this spring, the worse the subsequent spikes become. In fact, a loose version of isolation that is less immediately effective might actually be preferable. Instead of trying to flatten the curve as much as possible now, Lipsitch and Grad’s findings indicate that it would be preferable to have periods in which some of the population resumes normal social interactions followed by renewed suppression.

Case in point, Grad said, is Singapore. Widely portrayed as a success story these past few months, the city-state has now issued a month-long lockdown. “They did a good job containing the disease initially,” said Grad. “But what that did was ensure that most of the population was still susceptible, so now they’re seeing a spike in infections.” The lesson here is that without a vaccine, you can delay the pain, but you can’t prevent it.

Controlled exposure is at odds with policies currently being followed around the world. That makes sense in the short term, because it should be done in a way that minimizes harm. But once more widespread testing is in place and hospitals have the resources they need to treat COVID-19 patients, then we could switch gears and allow for more exposure than we are allowing now. “We can do this carefully and deliberately,” Abuelezam said.


There are several reasons to believe we can minimize harm while building immunity throughout the population. “An advance in therapeutics might change our approach,” Grad said. There are 40 drugs already approved by the Food and Drug Administration that are currently in trials for use against COVID-19. Success with any one of these could mean shorter hospital stays, which would dramatically increase critical care capacity and thus, the number of people allowed back to work and play.

Critical to any future policy, though, will be vastly improved surveillance. It’s a word with distinctly negative political connotations in America, but that may need to change: For an epidemiologist, it simply means being able to gather the kinds of data that are currently so desperately lacking. The common consensus is that because COVID-19 presents with mild or no symptoms at all in some large, but as yet unknown, percentage of the population, our current case counts are low by an entire degree of magnitude, or maybe more.

“No one knows for sure,” said Samuel Scarpino, a mathematical epidemiologist at Northeastern University. “We just know they’re wrong by a lot.”

Jeff Howe is an associate professor of journalism at Northeastern. Follow him on Twitter: @crowdsourcing.