What is herd immunity?

Because the COVID-19 virus is a brand new virus to humans, the baseline immunity to it prior to 2020 was zero.

Herd immunity happens when enough of the population is immune to a contagious disease so that uninfected people have a much lower chance of catching it. Immunity happens in one of two ways: either you have the disease and develop antibodies to it naturally or you are vaccinated against it and develop antibodies from the vaccine. The goal is to create immunity in as close to 100% of the population (natural infections + vaccine) so that previously uninfected people can’t catch it. If 50% of the population is immune, that would mean that your chances of catching the disease are half of what they would be when no one has immunity. If 100% of the population is immune, you can’t catch the disease at all (with rare exceptions).

So, what percentage of the population needs to be immune in order to confer herd immunity? It’s at least 70%, but 80-90% is much more likely to be successful. Once the immunity of the population is greater than 90%, herd immunity is effective in preventing widespread disease, and the epidemic (or pandemic) will come to an end.

Let’s take a look at the current situation in the U.S. with COVID-19. As of today, there are about 825,000 cases of COVID-19 reported in the U.S., which is 0.25% of the country’s population of 328 million. We know that many more people have had the disease and have recovered than have been tested. Let’s assume that 50 times as many people have had the disease than we know about (current estimates are anywhere from 5 times to 100 times). That would mean that about 36 million Americans would be considered immune to COVID-19, which is still only 11% of the population. Therefore, we still have an enormous way to go to achieve a minimum of 70% to even consider herd immunity. Of course, this model assumes that people who have recovered from COVID-19 are in fact immune, which we think is the case but are not certain.

There are two ways to “speed up” herd immunity. The first is to have no public health interventions and allow infected people to freely mingle with uninfected people. There would be no social distancing, no masking, and sporting events and concerts would go on as planned. In this scenario, herd immunity might be achieved in just several weeks, but at what cost? In order to achieve 70% immunity, almost 230 million Americans would need to become infected. At a mortality rate of 1%, that translates to 2.3 million deaths. And that scenario would overwhelm the U.S. health care system in a matter of days. Little effective care could be provided to anyone (with or without COVID-19). Effective drug therapy (of which none has been scientifically proved) could reduce the mortality of COVID-19 but won’t have an effect on creating herd immunity.

The second way to “speed up” herd immunity is vaccination. In fact, this is the only practical and safe solution. After vaccination for most diseases, relevant immunity begins to develop in just 2-4 weeks. Herd immunity could therefore be rapidly achieved assuming a safe, effective vaccine is able to be developed. But there are several assumptions built into this theory, some of which may not turn out to be true. While numerous vaccines are in development and are even in human trials, we do not yet know if the vaccines will be safe (not have serious side effects) or be efficacious (produce antibodies that will actually prevent disease in humans). It typically takes several years for a vaccine to be developed and put into widespread use, and the vast majority of vaccines that are developed fail. But with the global concern for COVID-19, it is conceivable that a vaccine could be ready for use in 12-18 months. If we are extremely lucky one could come sooner, but most medical experts advise not to count on that.

So, what do we do for the time being? Exactly what we have been doing – keeping our distance from others, especially those who are most vulnerable. Now that we have made substantial progress in flattening the curve, we need to work that much harder to bend it down.

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