What is herd immunity?

What is herd immunity?

By now you have likely heard the term “herd immunity” on the news and on social media. It’s important to understand what it is and why it’s essential to eradicate the COVID-19 pandemic.

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).

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

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.

Dr. Robert Citronberg is director of infectious disease at Advocate Lutheran General Hospital in Park Ridge, IL. This article was originally posted on his Facebook page.

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  1. I agree to “exactly what we’ve been doing”. As it has been said numerous times that we are not even in the peek season of COVID 19 and people are already getting too relax to eliminate social distancing. We need to stand still and let this virus pass us by, by staying safe, protecting ourselves/loved ones, and following the “SAFER AT HOME LAW”.

  2. George Fornaris April 23, 2020 at 3:56 pm · Reply

    Not to diminish the seriousness of Covid-19 but based on your assumptions in the article that infection rate was 50 times higher you have overstated the death rate at 1% when it should be 0.1% which based on all your numbers would mean 230,000 total Covid-19 deaths still a lot but much less than 2.3 million

  3. Patti Greenberg April 23, 2020 at 4:25 pm · Reply

    As I have been tracking cases vs. deaths, it has be 3-5% throughout. Italy had 8%!

  4. So is it fair to assume we will need to continue social distancing for another 12 months? Do we know when our children will return to school? And are there any suggestions on how to get our economy up and running while maintaining social distancing?

  5. Kristen Taninah April 24, 2020 at 7:04 am · Reply

    Can you please explain in detail the data you are using to extrapolate a 1% death rate? And are these deaths including those who died of comorbidities and happened to test positive for COVID-19?
    According to the data the flu vaccine has not been safe or effective. Why would the COVID-19 vaccine be different? Can you share more about how natural immunity and cross immunity happens and what the differences between that and “vaccine herd immunity” are? Thank you for your very thought-provoking articles.

  6. This article doesn’t really tell us anything new. There is no reasonable way to keep an entire country isolated for the next year, with no guarantee that the vaccine will actually be approved and made to administer to everyone by then. People are already protesting and others are already being overtaken by depression and other ailments from being isolated this long. Not to mention the economic effects that would have on people and businesses. This article is just spreading fear, not providing information.

  7. In the article, it states there are 825,000 cases of Covid19 in the U.S. Then apply the assumption of 50 times as many infections as are now being reported would equal 41,250,000 not 36,000,000 people. Then if we divide 50,000 deaths by 41,250,000 we get a mortality rate of .10%. That’s 230,000; not 2.3 million. I agree with the George; the math is wrong. Being off by 2 million people is way off.

  8. Dr. Citronberg—-What if the Swedish way is correct???

About the Author

Dr. Robert Citronberg
Dr. Robert Citronberg

Dr. Robert Citronberg is Executive Medical Director of Infectious Disease and Prevention for Advocate Aurora Health.