Understanding Herd Immunity

As COVID-19 continues to spread through the United States, I have often heard people saying that "it just needs to run its course.” This, coupled with reports, disputed by some, of people attending so-called “COVID parties,” contribute to harmful misperceptions on the concept of herd immunity. 

Herd immunity is achieved when large percentages of a population become immune to a disease and therefore indirectly protect those who do not have immunity. If, for example, four out of five of the people who are exposed to someone with an infectious disease are immune to it, the disease is much less likely to spread. However, the percentage of people who must be immune in order to achieve herd immunity varies based on multiple factors including the mode of transmission, and how easily and quickly a given disease spreads. In most cases, herd immunity is not achieved without an effective vaccine. For COVID-19, the percentage of the population that needs to be infected to achieve herd immunity is estimated to be between 70% and 90%, and this is assuming lasting immunity is possible.

Why We Need Vaccines & How Chickenpox is Different from COVID-19

Vaccines are typically required to achieve herd immunity. Childhood illnesses such as measles, mumps, diphtheria, polio, chickenpox, etc., all reached herd immunity through this approach. We have seen measles outbreaks in communities who have lost herd immunity due to anti-vaccine movements. 

Prior to vaccines those diseases had a level of herd immunity in adult populations, but outbreaks regularly occurred in children (hence "childhood illness") and the immunocompromised. 

When I was a child, some parents would hold chickenpox parties to achieve immunity. While chicken pox (varicella virus) can cause severe disease, the rate of severe illness is much lower than with COVID-19. About 150 people die from chicken pox each year and the virus has also been linked to birth defects if pregnant women get infected. These statistics do not include shingles, which is a reactivation of the chickenpox virus later in life that may occur in adults with compromised immune systems. Luckily, shingles is normally caught and treated early, so it accounts for less than 100 deaths a year. Even with these low rates, the CDC recommends vaccination rather than natural infection.

Sars-CoV2, the virus that causes COVID-19, presents more complicated issues. First, we are unsure how long immunity lasts. Based on other severe coronavirus infections, it probably lasts months to years, but not lifelong. Immunity may also be less protective in patients who had mild or asymptomatic disease. If immunity is not long lasting, or if people with mild infection do not develop immunity, herd immunity without a vaccine cannot be achieved. COVID-19 also has much higher rates of severe illness than many other viral infections with up to 15% of infected people hospitalized and mortality rates estimated at 0.5 to 1%. These rates are 10 times higher than seasonal influenza. 

Achieving Herd Immunity

Assuming immunity is long lasting, to achieve herd immunity in the U.S. without a vaccine would likely require more than 230 million Americans become infected. Even if mortality rates are at the low end of the estimates, there would be more than a million deaths.

Mortality is the tip of the iceberg when it comes to this disease. For every death, there are many more hospitalizations and ICU admissions. Patients who are not hospitalized can still have severe illness and be debilitated for weeks. Long term effects of COVID- 19 are just being defined but are likely to be significant in some infected patients. Add to this the risk of strokes and other clotting events in otherwise healthy patients as well as multi-system inflammatory syndrome in children, and the idea of pursuing herd immunity through infection becomes painfully ill-conceived.  

Those who believe “COVID-19 parties” are examples of how herd immunity can be achieved are misguidedly thinking that the virus just needs to run its course and it will be gone. One glaring problem with that idea is we don’t know that people are immune once they’ve been infected. Ironically, these may be the same people who oppose or down-play the need to wear masks or social distance. And wearing a mask and physical distancing actually follows the same mathematical concept as herd immunity.  If we reduce the spread of the virus, we stop COVID-19 without sickness or death.  

Until there is a vaccine, stay home, stay safe, wear a mask and save lives.

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