Written by: Isabella Gamez
Herd immunity has been a hot buzzword going around recently, but it is likely that many don’t fully understand the concept. For herd immunity to work, a large enough portion of a population has to be immune to a disease to protect the few who aren’t. For COVID-19, this cannot be accomplished through natural infection and it is important to continue preventative measures until a vaccine is produced.
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What is herd immunity?
The term herd immunity stems from the 1910’s when veterinarian George Potter was trying to find a solution to a contagion causing an “epidemic of spontaneous miscarriages in cattle and sheep” (Jones & Helmreich, 2020). Potter likened the disease to fire, which would die out if not given constant fuel; similar to how herd immunity could be developed increasing immunity to slow the spread of disease. The idea was quickly picked up throughout the scientific community and in the 1920’s, William Whiteman Carlton Topley and Graham Selby Wilson took to testing the concept in mice populations. They found that when introducing a vaccine alongside a pathogen, the spread of disease would be slowed and susceptible individuals were protected (Topley & Wilson, 1923). Now, the CDC defines herd immunity as “a situation in which a sufficient proportion of a population is immune to an infectious disease (through vaccination and/or prior illness) to make its spread from person to person unlikely” (CDC, 2020).
What would herd immunity look like now?
What many don’t realize now is that herd immunity is much harder to put in practice both functionally and ethically. Depending on the disease, there is a threshold for how many people need to be immune for herd immunity to function properly. For COVID-19, that number has been estimated to fall around 67% of the population (Fontanet & Cauchemez, 2020). This can generally be achieved two different ways: natural infection or vaccination.
If we were to go along with the first option in the United States, this would mean that 67% of the 328.2 million people living here would have to be infected, which would be around 219.9 million cases (we have 7.71 million cases right now). Unfortunately, COVID-19 is estimated to have an infection fatality ratio between 0.3% to 1.3% (Salje et al., 2020) so in a worst case scenario there would be around 2.86 million deaths and best case, around 660,000 deaths. Furthermore, here have been an increasing number of reports of possible long-term complications related to COVID that would be detrimental to the future overall health of the population Not only would this plan be ethically questionable, minority groups would be disproportionately affected and it would also put a terrible strain on our healthcare system.
It is also important to note that the first option is based on the idea that once someone is infected and has recovered, they will gain immunity to being reinfected. However, this hasn’t been holding true for COVID-19. A study performed in Spain and Italy, two countries that were hard hit early in the pandemic, found that the nationwide prevalence of antibodies was between 1% and 10%, and only peaked from 10% to 15% in more heavily infected areas (Byambasuren et al., 2020). This is an incredibly concerning fact being that such hard hit nations would be expected to have a much higher prevalence. Even more concerning, we currently don’t know how long naturally acquired immunity for SARS-CoV-2 lasts and it could wear off over a short period of time.
Ultimately, a vaccine is a much more effective method to achieve herd immunity. With several different potential vaccines in phase III trials right now, there is a possibility that they will be available by 2021. When an effective vaccine is created, it will be important to be ready with enough supplies and a plan for distribution. It is critical that we target the most vulnerable groups first as well as groups that are more likely to spread the virus like essential and healthcare workers. This way, we can effectively protect our population and slow the spread of the virus and possibly reach herd immunity.
What can we do until then?
Since a vaccine is still not in our immediate future, it is important to continue practicing the preventative measures that have been laid out before. Social distancing, face masks, and hand hygiene have been proven to be effective measures in slowing the spread of the virus and we can’t stop now. Additionally, now that we are heading into flu season, it is especially important to continue these practices as well as get a flu shot so that we don’t have to differ between two different viral, respiratory diseases this winter.
References
Byambasuren, O., Dobler, C. C., Bell, K., Rojas, D. P., Clark, J., Mclaws, M., & Glasziou, P. (2020). Estimating the seroprevalence of SARS-CoV-2 infections: Systematic review. Preprint at MedRxiv. doi:10.1101/2020.07.13.20153163
Fontanet, A., & Cauchemez, S. (2020). COVID-19 herd immunity: Where are we? Nature Reviews Immunology, 20(10), 583-584. doi:10.1038/s41577-020-00451-5
Jones, D., & Helmreich, S. (2020). A history of herd immunity. The Lancet, 396(10254), 810-811. doi:10.1016/s0140-6736(20)31924-3
Vaccine Glossary of Terms. (2020, July 30). Retrieved October 12, 2020, from https://www.cdc.gov/vaccines/terms/glossary.html
Salje, H., Kiem, C. T., Lefrancq, N., Courtejoie, N., Bosetti, P., Paireau, J., . . . Cauchemez, S. (2020). Estimating the burden of SARS-CoV-2 in France. Science, 369(6500), 208-211. doi:10.1126/science.abc3517
Topley, W. W., & Wilson, G. S. (1923). The Spread of Bacterial Infection. The Problem of Herd-Immunity. Journal of Hygiene,21(3), 243-249. doi:10.1017/s0022172400031478
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