Can Herd Immunity Combat Viral Epidemics like COVID-19?

Abstract

One strategy you may have heard suggested for dealing with the COVID-19 epidemic is to let nature take its course and let COVID-19 infect enough of the population for us to reach the herd immunity threshold. Does this make sense as a public health strategy? You can explore this question scientifically using SimPandemic, a free online tool for modeling infectious disease outbreaks.

Before you begin, you will need to know a bit about herd immunity. Herd immunity, sometimes called community immunity, is the concept that that if there are enough people in a population immune to a disease, then they protect others, who are not immune, from contracting the disease. Watch the video and you will see that one way to gain community immunity is to vaccinate individuals. In fact, public health officials often calculate the herd immunity threshold before they start a vaccination campaign. The herd immunity threshold is the minimum number of people who need to be immune to a disease in order to prevent an outbreak from occurring.

If there is no vaccine available, can the herd immunity threshold still be met as the infection naturally spreads? Start exploring the question by working your way through the Preventing Outbreaks with Herd Immunity notebook at SimPandemic. The Sandbox section of the notebook allows you to set your own outbreak scenarios and model what happens. To determine if it makes sense to reach the herd immunity threshold by letting COVID-19 run its course without intervention, you will need to first use the Sandbox to experimentally determine the herd immunity threshold. Then, compare the outcomes (number of infected, deaths, burden on hospital capacity, economic toll, etc.) of reaching the COVID-19 herd immunity threshold naturally versus with a vaccine.

SimPandemic can also be used to explore a number of other herd immunity-related questions. For example:

• Is the herd immunity threshold the same for every virus? If not, what viral factor(s) cause the threshold to increase or decrease?
• Choose a virus that interests you, like measles, smallpox, or polio. Knowing that it is impossible to vaccinate everyone, what would your minimum goals be for a successful vaccination campaign to ensure that an outbreak of that virus does not occur?
• What would happen if a cruise ship full of infected COVID-19 individuals disembarked in a city that had already met the herd immunity threshold for COVID-19? Is it possible to overwhelm the herd immunity threshold with a large influx of infected individuals?

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