Estimating the cost of reaching herd immunity

When enough people have been exposed to a virus and have developed immunity, the virus has trouble spreading because each newly-infected person is less likely to come in contact with other vulnerable people.[1] For example, measles, mumps, polio, and chickenpox were all common diseases in the United States. But now those diseases are all rare in the US, thanks to high levels of vaccinations that have achieved herd immunity.

Resistance to vaccination in the US has recently led to lapses in herd immunity protection from some of those diseases, which has led to new outbreaks of formerly-rare diseases. For example, the 2019 outbreak of measles at Disneyland in California,[3]

What’s the threshold?

Different viruses are different. We can't stop every virus through herd immunity. It doesn’t seem to be possible with the strain of HIV that causes AIDS.[4] And we still don’t know enough about the SARS-CoV-2 virus that causes the COVID-19 disease to be certain that herd immunity will even stop it.

For a really contagious disease like measles, 90 to 93 percent of the population needs to develop immunity to stop the spread of infection.[5] Which explains why it’s so easy for herd immunity to lapse for measles, and why it’s so easy for new outbreaks to start when enough people reject vaccination.

For SARS-CoV-2, epidemiologists think that about 70% to 90% of the population may need to develop immunity to have herd protection.[1] But there is a caveat:

Would it even stop at that point?

Herd immunity protects a population against new infections. But if there is already an outbreak in progress, then it won't immediately stop when the proportion of infected people reaches the herd immunity threshold.[6] An infection that’s already underway will continue to spread.

A runaway train doesn’t stop when the track begins to slope uphill, and a rapidly spreading virus doesn’t stop right when herd immunity is attained.

Depending on the pace of the outbreak, the COVID-19 outbreak could continue to spread through the United States for months after we reach the herd immunity threshold, resulting in many more infections and many more deaths. This concept is called “overshoot.”

How many more infections? We can’t know. It depends on too many factors, and there are too many unknowns. But we do know that allowing the virus to spread uncontrolled would make the spread faster and would result in going farther over the threshold.[7]

How many people would die getting to that point?

To estimate the cost of reaching herd immunity in the United States, I’m using the estimate that the Imperial College of London used to estimate what would happen if the virus were to spread uncontrolled through our country. Because that's what people are suggesting when they propose surrendering to the virus and trying to reach herd immunity by letting everyone get exposed. And I’m using the estimated infection prevalence rates for the US and Florida from their report this week.[8]

Those researchers estimated that 81% of Americans would be infected if we were to do that.[9] I used that same percentage to illustrate the cost in human lives of reaching that point, based on the amount of death that we have already seen.

David Dowdy, MD, PhD, infectious disease epidemiologist from Johns Hopkins University, inspired this visualization with a powerful illustration.

This resulting chart illustrates that millions of people in the US could die if we were to surrender to the virus and allow it to spread freely:

The Imperial College report also includes an infection prevalence estimate for US states.[8] I used that estimate and current public data from the Florida Department of Health to provide the same visualization for the state of Florida:

Infection prevalence estimates are not available at the county level, but an ongoing study that attempts to estimate the amount of underreporting of SARS-CoV-2 infections in the United States.[10] I have been using their estimate of how many infections are reported as cases to estimate the number of infected people at the county and city levels, and those estimates have proven to be in-line with infection prevalence estimates from many different researchers. I used the same estimation method to provide visualization of the cost of reaching herd immunity for Miami-Dade and Broward Counties.

Okay, so what does this mean?

Intentionally aiming at herd immunity is not the answer. If we allow the virus to spread uncontrolled, or if we go so far as to encourage the spread, then the outbreak will rocket past the herd immunity threshold, and people will continue to be infected. And we will have more deaths for months after we reach that point.

We must reduce the final death count. The only way to do that is by lowering the fatality rate through new therapies, or by stopping the spread with a vaccine. We must buy time for those things by limiting the spread now. The biggest factor that affects the spread is under our control: social distancing. Weather, overall health, and other things are all factors, but social distancing dwarfs those factors.[11,12] We’re at a point right now where we risk a second outbreak that we might not be able to control, and the best way that we have to stop it is to maintain social distancing.

This is the challenge of our lifetime. We can't simply surrender to the virus. Too many people who we love and who we need would die. We must do this. I’m aiming my visualizations here at reminding us what we're fighting for.

So then what IS the answer?

There is no way to avoid an enormous cost from this. The only control that we have is over where the cost is. We can either

1. Surrender to the virus, and the cost will be hundreds of millions of human lives worldwide,
2. Shut down forever, and the cost will be the irreparable destruction of the global economy, and mass suffering from that, or
3. Mobilize and invest in testing and tracing and isolating so that we can quarantine the correct people instead of quarantining everyone.

If you choose not to decide, you still have made a choice.[13]

Your community

If you’re reading this from somewhere outside of South Florida, and you’re interested in seeing what the cost of reaching herd immunity might look like for your community, please take a look at the Excel file that I used to generate these charts. I designed it so that it’s easy for you to plug in data for your area. You’ll need a population count, and if you want to go more fine-grained than the state level, then you’ll currently need to use a confirmed case count to estimate your true infection prevalence in your area. Please be sure to check the latest underreporting estimate from the Center for Mathematical Modeling of Infectious Diseases at the London School of Hygiene and Tropical Medicine[10], because they update their estimates frequently.

References

1. 1. What is Herd Immunity and How Can We Achieve It With COVID-19? Johns Hopkins Bloomberg School of Public Health; 19-April-2020 link (Accessed 2020-05-23 12:00)
2. 2. Gold JE. MMR Vaccine Appears to Confer Strong Protection from COVID-19: Few Deaths from SARS-CoV-2 in Highly Vaccinated Populations. ResearchGate; May-2020 link (Accessed 2020-05-23 18:20)
3. 3. A tourist infected with measles visited Disneyland and other Southern California hot spots in mid-August. The Washington Post; 24-August-2019 link (Accessed 2020-05-23 18:00)
4. 4. Pandemic HIV-1 Vpu overcomes intrinsic herd immunity mediated by tetherin. Springer Nature Limited; 17-July-2015 link (Accessed 2020-05-23 18:00)
5. 5. Funk S. Critical immunity thresholds
 for measles elimination. Centre for the Mathematical Modelling of Infectious Diseases London School of Hygiene & Tropical Medicine; 9-October-2017 link (Accessed 2020-05-23 18:30)
6. 6. Bergstrom CT, Dean N. What the Proponents of ‘Natural’ Herd Immunity Don’t Say. The New York Times; 1-May-2020 link (Accessed 2020-05-23 18:40)
7. 7. Rogers K, Wolfe J, Bronner L. Without A Vaccine, Herd Immunity Won’t Save Us. 538; 13-May-2020 link (Accessed 2020-05-23 22:00)
8. 8. Unwin HJT, Mishra∗2 S, Bradley VC, et al. Report 23: State-level tracking of COVID-19 in the United States. Imperial College COVID-19 Response Team; 21-May-2020 link; DOI: 10.25561/79231
9. 9. Report 9: Impact of non-pharmaceutical interventions (NPIs) to reduce COVID-19 mortality and healthcare demand. Imperial College London - MRC Centre for Global Infectious Disease Analysis; 16-April-2020 link; DOI: 10.25561/77735
10. 10. Russell TW, Hellewell J, Abbott S, et al. Using a delay-adjusted case fatality ratio to estimate under-reporting. Centre for the Mathematical Modelling of Infectious Diseases at the London School of Hygiene & Tropical Medicine; 17-April-2020 link (Accessed 2020-04-17 19:30)
11. 11. Forecasting the Impacts of Weather and Social Distancing on COVID-19 Transmission Across the U.S. PolicyLab at the Children’s Hospital of Philadelphia Research Institute; link (Accessed 2020-05-21 15:20)
12. 12. PolicyLab Studying Effect of Weather, Social Distancing on Local COVID-19 Spread. PolicyLab at the Children’s Hospital of Philadelphia Research Institute; 21-April-2020 link (Accessed 2020-05-21 15:15)
13. 13. Peart N. Free Will. Mercury Records; 14-January-1980 link