Herd immunity

The process of reaching herd immunity against COVID-19 would be an unprecedented humanitarian disaster, leading to tens of millions of preventable deaths. This is not an acceptable control strategy for the global pandemic. For all mankind, we must take every possible measure to prevent this from happening.

Herd immunity is a key concept in epidemiological modeling. This is the state at which enough people are immune that a new outbreak cannot take off—because there are no vulnerable people around for an infected person to infect, even without mitigation such as social distancing. Eventually, each new infected person infects fewer than one new people. Experts estimate that the threshold for reaching herd immunity from COVID-19 would be when about 50% to 66% of people have been infected.

Without a vaccine or sustained mitigation measures, herd immunity will be the default control strategy for COVID-19. But this would be a disaster.

Unacceptable as a control strategy

One estimate for the herd immunity threshold, based on an R₀ value of 2.5 for SARS-CoV-2, is 60%. Estimates for the infection fatality rate are currently in the range of 0.5% to 1%. If the population of the Earth is currently about 7.6 billion people, then 46 million people could die worldwide based simply on 1% of 60%. The cost in human lives makes this approach unacceptable as an intentional strategy. But you can’t just let the infection spread unchecked and expect to see “only” 60% infected, or “only” 46 million deaths.

One reason is that the infection fatality rate rises when health care systems are overrun. The eye-opening conclusion of the Imperial College report in March was that many more people would die without mitigation and suppression measures.[1]

Overshoot

Another reason is “overshoot”, a key concept about the timing of the epidemic.[2] When the epidemic reaches the herd immunity threshold, it doesn't immediately stop spreading. It has a sort of momentum.[3] You reach herd immunity at the point when each infected person infects less than one other person, but people are still getting infected at that point. That point where you reach herd immunity, perhaps 60%, is the _peak_ of the epidemic, not the _end_ of the epidemic.

17. In fact, for an SIR or SEIR model, you reach herd immunity not when the epidemic is nearly over, but rather *precisely* at the epidemic peak. This makes sense. Before the herd immunity threshold, each case creates >1 new cases, and the epidemic curve keeps increasing. pic.twitter.com/yAFteJrTSt

— Carl T. Bergstrom (@CT_Bergstrom) April 19, 2020

How many more infections does this overshoot represent? It could be a lot. For the model shown in Carl T. Bergstrom’s tweet, above, the overshoot represents nearly 30% of the population. So, in this scenario, 90% of the people on the planet will get infected and maybe as many as 70 million people will die. Or more, depending on the effectiveness of the world’s healthcare systems.

Second-wave overshoot

Overshoot is a particularly relevant concept when exploring the prospect of a second wave, or multiple outbreaks. In a scenario where strong intervention measures have been applied, a second wave is possible if the infection is reintroduced into the population.[4]

If control of the first outbreak depleted resources—such as drug stockpiles or ‘goodwill’ among the population to follow quarantine measures—the second outbreak will be largely uncontrolled, producing a significant overshoot

Does that sound familiar? Depletion of goodwill among the population to follow quarantine measures? The result could be many more deaths in the long term.

Implications

Assuming 90% of the world’s population infected during the entire pandemic due to overshoot, a 1% infection fatality rate would result in about 69 million deaths worldwide.

A 2% infection fatality rate would lead to around 138 million deaths.

A 3% rate would lead to 207 million deaths worldwide. And the true worldwide average rate could be higher.

Conclusion

Herd immunity is the default control strategy if we can’t mobilize a better approach. But this would be unacceptable as a conscious strategy because it would result in tens of millions of preventable deaths worldwide. Reaching herd immunity would require huge numbers of infections, and even more would be infected because of overshoot. Preventing this unprecedented humanitarian disaster is the challenge of our generation.

References

1. 1. Walker PGT, Whittaker C, Watson O, et al. Report 12: The Global Impact of COVID-19 and Strategies for Mitigation and Suppression. Imperial College London - MRC Centre for Global Infectious Disease Analysis; 26-April-2020 link; DOI: 10.25561/77735 (Accessed 2020-04-19 11:30)
2. 2. Bergstrom CT. Twitter thread on overshoot. link (Accessed 2020-04-20 11:00)
3. 3. Tanaka MM, Althouse BM, Bergstrom CT. Timing of antimicrobial use influences the evolution of antimicrobial resistance during disease epidemics. Evolution, Medicine, and Public Health; link
4. 4. Handel A, Ira M Longini J, Antia R. What is the best control strategy for multiple infectious disease outbreaks? The Royal Society; link

Related