Coronavirus and herd immunity


I’m as eager to see the economy start to reopen as the next guy.  The Saint Louis Federal Reserve president estimates the United States is losing $25 billion in lost output every day. Unemployment could hit 15 percent or higher, and Goldman Sachs calculates the global damage as being four times worse than the Great Recession.

Because we’re still probably a year to 18 months away from a vaccine for SARS-CoV-2, a lot of people are trying to figure out how the United States and other countries can reach “herd immunity” — which is what happens when enough members of a population have antibodies to a contagious disease that it can no longer easily spread from one person to another.

But there are some giant life-and-death questions about herd immunity that need to be answered.

(1) What percentage of the population will we need to get there? The good news is that for some diseases, it can be as low as 40 percent.  But for some diseases, such as measles, we need 95 percent of the population to be immune.

(2) How long does a person’s body retain the antibodies that it needs to fight off the virus? For two other coronaviruses, it’s about 45 weeks. If that is the case for SARS-CoV-2, it means that, in about 10 months, you can get reinfected again. If we have a vaccine in a year, we will be close to okay. If it takes longer than a year . . . a lot of the people who were infected and recovered will be in danger of getting sick again.

(3) If opening up the economy, even partially, means that inevitably more people will get infected — herd immunity requires more infections! — how many more people will die along the way?

As many people like to note, we’re making educated guesses as to the fatality rate, because we’re undercounting cases — both asymptomatic individuals and those whose symptoms are so mild, they’re not going to a doctor to get diagnosed — and we’re probably undercounting deaths, too. (Then again, as Andy lays out, New York City is now counting cases where a discovered dead body is not tested, but the coronavirus is “probable” or may be “presumed.” That sounds like a way to compensate a likely under-count in virus deaths by potential over-count of virus deaths.)

The WHO last put the virus death rate at 2 percent in early March. As of this writing, the world has 141,195 deaths and 2,115,624 confirmed cases, which comes out to 6 percent. As of this writing, the United States has 32,707 deaths and 650,833 cases, which comes out to 5 percent!*

Let’s assume achieving herd immunity requires that minimal threshold for contagious diseases, 40 percent. The U.S. population is 330 million people. That means the minimum threshold for herd immunity is for 132 million people to catch the virus. Let’s also assume the current death rate in the U.S. overstates this by a factor of ten, and that the death rate would be one half of one percent.

One half of one percent of 132 million people would be . . . 660,000 people dying along the way to herd immunity. Now let’s look at the worst-case scenario. If herd immunity for SARS-CoV-2 needs 95 percent of the population to have antibodies or be immune, like with the measles, that means about 313 million Americans would need to catch the virus. One half of one percent of 313 million people would be . . . 1.5 million Americans dying along the way to herd immunity.

And remember, each person’s immunity could only last ten months or so.

This doesn’t mean, “Don’t open up the economy until we have a vaccine.” It means, if we decide to go down this path, we should go in understanding that the road to herd immunity will probably be long, slow, and painful.

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