Herd Immunity for COVID

After 6-months of COVID-19, we are starting to feel this is a long-drawn-out affair. Naturally, thoughts turn to how we can adapt to COVID as a way of life. We should do that, and look at ways to keep everyone safe while vaccines are developed and distributed to sufficient volumes to allow for “herd immunity.”

Herd immunity occurs when a sufficient number of people in the population have antibodies (either through the disease itself or vaccines) to stem the reinfection rate. [1] The percentage of the population that needs to be immunized or have COVID antibodies varies based on the R0 number for a disease.

“R0 represents the average number of people infected by one infectious individual. If R0 is larger than 1, the number of infected people will likely increase exponentially, and an epidemic could ensue. If R0 is less than 1, the outbreak is likely to peter out on its own.” [2]

An R0 of 2 means one infectious person typically infects two additional individuals. An R0 of 15 means, 15 people are usually infected by a contagious person. The higher the R0 number, the more aggressively the population needs to be immunized to stem to spread and build up herd immunity. For example, Measles has R0 of 12-18, which requires 92-94% of the population to be immune to Measles to stop the spread.[1] That is why it is critical to get babies vaccinated for measles. Similarly, Polio has an R0 of 2-15 and can require 50% – 90% immunization rates.

The R0 for COVID-19 when it struck Wuhan ranged from 1.4 – 5.7.[2] When COVID-19 hit Florida around March 2020, the R0 was around 3.2. We all responded by taking precautions – social distancing, masks, closing bars, limiting numbers of people in restaurants, shifting work and school to be virtual. As a result, the Rt (Reinfection rate now in September 2020) is hovering around 0.94. [3] The drop in reinfection rate down to 0.94 is due to human behavior changes rather than a fundamental shift in the virus’ ability to infect people. As Florida opens loosen up the restrictions on bars, restaurants, and bars, it will increase reinfections.

Let us play out a simple scenario of what it would be like to relax all restrictions and go back to pre-COVID behaviors. R0 for Florida was around 3.2. We can approximate that this would need about 65% of the population to have COVID antibodies. [1] Now vaccines are being developed, tested or produced, so currently, the only way someone can have antibodies for COVID is by actually catching it.

Since we have had 695,879 cases of COVID reported so far in Florida, are we close to attaining herd immunity? [4]


A 2019 estimate for Florida’s population puts it around 21.48 million.[5] So 65% of the population means we need 19.96 million people infected with COVID in Florida. With 695,879 cases so far, we would need 28.68x more people infected with COVID to get close to start establishing herd immunity. In allowing for the virus’s natural spread, we would have three responses at a minimum for people. First, some people will not be affected and not get sick. Secondly, some people will get sick and recover. Finally, some people will get sick and die. (I’m ignoring others who may die because of side-effects of not having hospital beds through the explosion in the number of cases).

So far, with 695,879 cases, we have had 13,914 deaths in Florida. Now, suppose we assume (very simply) that we would have a similar death ratio, as we scale up to a higher number of infections to reach 19.96 million people infected. I suspect the death rates will be higher, as there will be dramatic flow-on effects because the health care in Florida will struggle to cope with an increase in volume, but let us assume that it would scale linearly for the sake of argument. We would end up with 399,097 deaths just in the state of Florida. So far, the USA has had 203,479 deaths across the country. If we scale this for the country, we would have 9.5 million deaths in the USA, which is ten times the total world deaths attributed to COVID so far. To put it in perspective, heart disease, which is the biggest killer in the USA, kills 655,381 people per year. [6] We are talking about 14 years of heart disease-related deaths in the USA just with COVID.

If we assume that official figures capture only 1 in 10 cases, we would expect the results to scale by 2.9 times, leading to about 40,000 deaths in Florida and close to 600,000 deaths in the USA. Even on the lower-end, we would have 3x more deaths in the USA than the deaths so far and the same number of total annual deaths attributed to heart disease.

There is a broad range here between 600,000 and 9.5 million deaths. I concede these are very rough calculations, and I’m sure there are much more precise by controlling for different age-groups and factors. The bottom line, however, is to ask – how many unavoidable deaths are acceptable through COVID?

COVID is here to stay for a while. We do have some very knowledgable, intelligent and hard-working people working on developing vaccines and improving treatments. Even when we have successful vaccine trials, manufacturing, distribution and administration of vaccines will take time. The largest vaccine producer in the world estimated that it would take 4-5 years to vaccinate the world. [7] Until then, we really must rely on human behavior to reduce the spread of COVID and, consequentially, the deaths. Face-masks and social distancing have been vital in reducing the reinfection rate down from 3.2 to below 1. We must continue doing this for a while yet.


[1] https://theconversation.com/amp/what-is-herd-immunity-and-how-many-people-need-to-be-vaccinated-to-protect-a-community-116355

[2] https://www.the-scientist.com/features/why-r0-is-problematic-for-predicting-covid-19-spread-67690

[3] https://rt.live/us/FL

[4] https://www.nytimes.com/interactive/2020/us/florida-coronavirus-cases.html

[5] https://www.census.gov/quickfacts/FL

[6] https://www.cdc.gov/heartdisease/facts.htm#:~:text=About%20655%2C000%20Americans%20die%20from,1%20in%20every%204%20deaths.&text=Heart%20disease%20costs%20the%20United,year%20from%202014%20to%202015.&text=This%20includes%20the%20cost%20of,lost%20productivity%20due%20to%20death

[7] https://www.businessinsider.com/covid-vaccine-wont-reach-everyone-for-four-years-serum-institute-2020-9