Herd Immunity and COVID-19

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This morning I was watching TV, the Today show. as Karl Stefanovic and Allison Langdon were discussing the concept of “herd immunity” which had been considered by the UK Government early in the COVID-19 pandemic. Their casual conversion considered whether it might be a good idea to deliberately infect younger Australians with the coronavirus (SARS-CoV-2) [1] whilst isolating and protecting older and more susceptible members of the community. This apparently casual conversation raised alarms for me as I’ll explain below (see after the jump). The infographic video that Allie used to explain the concept of “herd immunity,” seemingly made a compelling argument for it. Fortunately, they brought in sensible expert advice in the form of Dr Sanjaya Senanayake an Infectious Diseases physician and Associate Professor from the Medical School at the Australian National University (ANU) (see the YouTube video below).

I almost started clapping when Dr Senanayake decisively skewered the argument for herd immunity put by Karl and Allie. As he pointed out, even for a case fatality rate of 1% then 160,000 Australians would die and many more would become seriously ill. This would overwhelm our health systems, bringing them to a point of collapse. Many of us are now familiar with the argument of “flattening the curve” by slowing the rate of infection, thereby preserving our healthcare services [2].   

This argument, originally derived from modelling studies at Imperial College, persuaded the UK Government to abandon its ill-conceived plans for herd immunity. Sir Patrick Vallance, the UK government’s chief scientific adviser who is reported as saying: “enough of us who are going to get mild illness to become immune [2].” This thinking seems to have been based upon policies for an infection like measles which is far more contagious than SARS-CoV-2 but has a lower fatality rate [2].

The basic reproduction rate, denoted by R0, is a measure of the number of people a carrier of the virus is able to infect in the absence of health control measures. The basic R0 can be very difficult to determine for a novel virus such as SARS-CoV-2. In practice, the effective reproduction number, Re, is used. This is determined from reported infection for a particular social setting and might vary from region to region. The most contagious viral infections (i.e., most likely to be communicated person-to-person) in order of R0 value [3] are:

  1. Measles (most contagious)
  2. Whooping Cough
  3. Mumps
  4. Chicken Pox
  5.  Smallpox (eradicated)
  6. =====>  COVID-19 fits in here (for a R0 value of around 3.0 [4])
  7. Seasonal Flu (least contagious)

To highlight the data behind the argument given by Dr Senanayake in the video above. Measles is one of the most contagious diseases known with an  R0 of about 18. In the absence of a vaccine, a disease as infectious as measles would be exceedingly difficult to contain: herd immunity might be the only available option. SARS-CoV-2 is not nearly as contagious (R0 value of around 3.0 [4]) as measles but has a higher fatality rate: so it makes far more sense to implement health control measures such as physical distancing or quarantine or personal-protective-equipment (PPE) to contain the spread.

The basic reproduction value R0 also determines the percentage of the population required to achieve herd immunity 100(1-\frac{1}{{R}_{0}}). For highly infectious measles we need 94% of the population to be immune. For herd immunity with less infectious SARS-CoV-2, where  R0 = 3.0, then 67% of the population needs to be immune.

The goal of health control measures is to reduce the effective Re-value to less than 1.0 which will decrease the number of infections over time; eventually bringing the disease to levels that can be more easily managed.

This article started out as an email I sent to the Channel 9 Today Show pointing out that they have a moral and social responsibility to the Australian public to take an unmistakable stance: herd immunity by deliberate infection with SARS-CoV-2 is not to be countenanced.

Channel 9 should avoid even the slightest impression that it might be a good idea for younger people to infect themselves with SARS-CoV-2 in the same way that might have been done in the past with so-called Chickenpox parties.

In addition to all that has been written above, it is common for a percentage of viral infections to have serious complications, some of which don’t manifest themselves for years afterwards.  WIth novel SARS-CoV-2 we know almost nothing about its long term effects. Consider the following points:

  • we don’t know how much immunity is conferred from recovering from COVID-19, there are conflicting data and no one knows for sure just how long immunity will last;
  • there is very little data on repeated exposure, with Dengue Fever the second infection is almost always much worse than the first and often leads to death;
  • some viruses are able to shield themselves inside the body and show up later n life, e.g.,  Chickenpox can return later in life as shingles and Hepatitis B can lead to liver cancer in later life.

With these considerations in mind, a significant proportion of younger healthier people, deliberately infecting themselves, will suffer adverse complications and will clog up the hospital system which is what our Health Authorities are engaged in extraordinary efforts to avoid.

The best way to gain herd immunity is by living the best life we can under the “new normal” of coronavirus until an effective vaccine, or another treatment becomes available (likely sometime in 2021). As well as doing everything we can by physical distancing and hand washing to avoid infection until then. Herd immunity will then be conferred more safely and effectively by the vaccine,

I’d be interested in what you think in the comments.


[1] World Health Organisation, “Naming the coronavirus disease (COVID-19) and the virus that causes it,” available online, accessed: 9th April 2020:

[2]  Cathleen O’Grady, National Geographic, Science, “The U.K. backed off on herd immunity. To beat COVID-19, we’ll ultimately need it.,” published: 20 March 2020, accessed: 9 April 2020., available online:

[4] Ana Sandoiu, Medical News Today, “Coronavirus may spread faster than WHO estimate,” published: 18th February 2020, Accessed: 9th April 2020, available online:

Update: 09 August 2020.
As I mentioned in the original article, it is difficult to get a reliable value for R0 for a novel virus. A more up-to-date R0 for SARS-CoV-2 is 5.7 [5] instead of about 3.0. Which means that SARS-CoV-2 is much more contagious than previously thought. However, this doesn’t affect any of the conclusions of the article.

[5] Vanessa Bates Ramirez, Healthline, “What Is R0? Gauging Contagious Infections,” updated: 20 April 2020, accessed: 08 August 2020, available online:

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