To the editor:

Epidemiologists refer to "herd immunity" as a concept describing protection from an infectious disease that occurs when a sufficient proportion of a population can no longer acquire or transmit infection, either through vaccination or immunity resulting from previous infection. The percent of persons in a population immune to an infection that generate herd immunity is based upon a measure of contagiousness for the infective agent (usually a virus). Populations may be defined by geographic, ethnic and social boundaries.

Angola could be considered a population but interaction among other adjacent populations affect our specific boundary. Travel and lack of social distancing are the prime modifiers affecting any defined population.

Calculation of the infective agent’s basic reproduction number (R0) represents an estimate of the contagiousness and the number of secondary infections that will occur when a susceptible population is exposed to one initial infection. R0 is not a measure of disease severity. This information is largely obtained through contact tracing. An R0 greater than 1 means that infections will increase after exposure while an R0 of less than 1 indicates that infections will decrease. For any virus, the R0 is variable and can change over time especially as mutations arise.

The higher the R0 the larger percentage of persons immune to the infection are required to stop transmission. As an example, measles R0 is 12-18 and indicates that one infected person will infect 12-18 others and that approximately 95% of the population must be immune to prevent infection spread. For COVID-19 the R0 varies from 1.5 to 6.68 based on several published studies. Of 20 estimates, 13 studies were in the range of 2.2–3.6. Assuming these calculations are correct, then 70% of the at risk population must become immune to produce herd immunity. (Pi = >1−1/R0) The calculations do not take into account persons in a population that are immune from recovery of the virus or that a vaccine is less than 100% effective.

According to the CDC, 41% of Indiana residents have received at least partial COVID-19 vaccination and about 11% have recovered from the virus. If we assume that 25% of residents have achieved immunity from natural infection but were not symptomatic and were not identified, we can assume that 77% of the Indiana population is now immune and we will have exceeded the R0 for SARS-CoV-2. For Steuben county, with 37% vaccinated and 11% recovered, the immune population would be 73%.

The best test for assessing herd immune status is contact tracing and serial assessment of virus infectivity. Sequential decreasing active cases within our district would suggest that effective immunity is present. Continuing masking for those susceptible to COVID-19 would offer additional protection.

For supporting information please see:

• Kiwan A. Path. Understanding The Journey To Herd Immunity. February 2021. https://www.path.org/articles/understanding-journey-herd-immunity/. Accessed June 2021.

• Achaiah NC. et.al. NLM.NIH. R0 and Re of COVID-19: Can We Predict When the Pandemic Outbreak will be Contained? November 2020.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7751056/. Accessed June 2021.

• Park M. et.al. A Systematic Review Of Covid19 Epidemiology Based On Current Evidence. J Clin Med. 2020;9(4):967. doi: 10.3390/jcm9040967.

• Covid-19: Do many people have pre-existing immunity?. BMJ. September 2020. https://www.bmj.com/content/370/bmj.m3563. Accessed June 2021.

Lauren Vogel

Jimmerson Lake

Angola

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