Herd immunity is humanity’s perceived salvation from COVID. The major questions are: what is herd immunity and what percentage of immune individuals in a population will achieve this. These questions are easier to ask than to answer. The reasons why are many.
What is Herd Immunity?
Herd immunity is the indirect protection from an infectious disease that can occur when a sufficient percentage of a population has become immune to the infection, through vaccination or prior infection. This, then, reduces the risk of infection for persons without immunity but who cannot be vaccinated.
How Much of the Population Must be Immune to Confer Herd Immunity?
The question naturally arises, what percentage of a population must be immune to COVID, either by vaccination or by having survived a prior infection, to establish herd immunity?
This depends on many factors. These include Ro value, vaccine efficacy, duration of immunity, variability of immunity with time and acquisition mode, mutation of the virus, types of immunity, and parallel measures.
Ro Value
The R0 or basic reproductive number of a pathogen in a given population is the average number of people in that population that one infected person is expected to infect if no other measures are taken.
The R0 of COVID in a naïve population is generally considered to be 2.5 to 3.5. Based on this alone, the herd immunity would occur when 60-71% of the population is immune, as given by the basic formula Im = (1 – 1/R0).1
Putting this in perspective, the R0 of measles is usually cited as 12 to 18. This puts the immune percentage of the population required to establish herd immunity to measles at 92% to 95%.
Vaccine Efficacy
But this presupposes that everyone in the population is susceptible and that all those vaccinated become immune. However, the effectiveness of the vaccines at this time appear to be Pfizer 95%, Moderna 94%, and Johnson & Johnson 72-85%.2
The imperfect vaccine effectiveness must be added into the formula, and, when it is the formula becomes Im = (1 – 1/R0) / e. If e is 80%, the percentage of immune population required for herd immunity increases to 75% to 89%.
Duration of Immunity
This also assumes that immunity to COVID will last indefinitely. This may not be the case. Immunity to many infectious diseases decreases with time and is the reason that some vaccines require booster doses. As immunity decreases, individuals tend to reduce the immune population and replenish the susceptible population.
Duration of immunity is difficult to estimate at this early point. It reveals itself often in Phase 4, post-marketing, studies that use the required long periods of observation of large populations.
Variability of Immunity … with Time
Additional complications involve the variability of immunity. How many individuals will achieve immunity only to severe disease, how many to milder disease, how many will have no symptoms but be infectious, and how many will be unable to transmit the disease.
Ultimately of interest is if, or how, this immunity will vary with time from vaccination or natural infection. Will immunity diminish with time from inability to transmit infection to immunity only from severe disease?
Variability of Immunity between Vaccination and Natural Infection
Vaccination and natural infection are different.
In vaccination, inoculation is with a constituent protein of the virus, not with live or dead virus. Additionally, all recipients get the same carefully controlled quantity of inoculum in vaccination. Only their physiologic response may be different.
In natural infection, inoculation is with live virus and the quantity of inoculum varies from minimal to massive. The physiologic response again varies.. Speculation exists that individuals who have survived more severe illness may have more intense or more enduring immunity.
Mutation of the Virus
Of course, evolutionary pressure, while occasionally mitigating a highly virulent infection, more frequently makes the virus more resistant to countermeasures. As a virus becomes more resistant, the effectiveness of the existing vaccines decreases, increasing the percentage of immune persons needed to achieve herd immunity.
Certainly, this is another reason to vaccinate as many people as quickly as possible, to allow the virus less time to mutate. As stated by Dr. Joshua Barocas, "The sooner we can get vaccines into arms, the less important the variants become."2
Parallel Measures
Parallel measures also influence herd immunity. Lockdown measures, mask wearing, social distancing, anything that reduces the number of people a patient can infect, that is, which reduces the R0, reduces the percentage of immune individuals in the population needed to achieve herd immunity. Of course, this means that the needed percentage varies with external conditions.
Natural Infection and Herd Immunity
Certainly, some of the immune individuals in the herd immune population have acquired their immunity by surviving natural infection. This has led some to suggest that this might be sufficient to achieve herd immunity. This thinking is wrong.
First, the cost of acquiring immunity from natural infection is too high. The fatality rate from COVID infection varies from 1% to 3.4%. The cost to immunize 96 to 99 people is 1 to 3.4 deaths. Second, there are those who survive but have long-term sequelae of their illness. Third, is the difficulty of patients with non-COVID illnesses accessing a health system overwhelmed with COVID patients. Fourth, this is a slow way to achieve herd immunity and it allows more time for the virus to mutate. A large bank of mutations favors survival of the virus.
Conclusion
The beginning question of what percentage of a population must be immune to COVID, either by vaccination or by having survived a prior infection, to establish herd immunity? A widely accepted estimate is about 80%. The estimate, however, is subject to many variables, including vaccine efficacy, duration of immunity, variability of immunity with time and mode of acquisition, mutation of the virus, types of immunity, and parallel measures. The aforementioned considerations provide an insight into why establishing the percentage is so challenging.
Thomas Falasca, DO FACA FACPM
References
1Anderson, R., Vegvari, C., Truscott, J., & Collyer, B. (2020, November 4). Challenges in creating herd immunity to SARS-CoV-2 ... Retrieved February 16, 2021, from https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)32318-7/fulltext
2 Fiore, K. (2021, March 05). How Do COVID-19 Vaccines Compare? Retrieved March 10, 2021, from https://www.medpagetoday.com/special-reports/exclusives/91489
3 McNamara, D., & Franki, R. (2021, February 13). At Current Vaccination Pace, When Can US Expect Herd Immunity? Retrieved February 17, 2021, from https://www.medscape.com/viewarticle/945699