Putting the vaccinated at risk

Written on 2021-09-24

Background: A friend of mine shared a meme about Covid vaccinations with the caption: “The protected need to be protected from the unprotected by forcing the unprotected to use the protection that didn't protect the unprotected.” Here is my response:

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Put like that, it is a bit of a tongue-twister :D But quite honestly, this is no joking matter - and not as difficult as it sounds. The first thing you'll be taught in any epidemiology class (and yes, I have taken a couple) is that during an epidemic, there are three basic classes that people fit into: Susceptible, Infectious, or Recovered. (This SIR-model is simplified, of course, but works quite well nonetheless.)

The idea is that infectious people pass on the disease to susceptible people around them, but, once they have recovered, are immune to being infected a second time. This means that over time, there are more and more recovered/immune people in the population, so that the people currently infected are less and less likely to meet susceptible people and eventually the disease dies out.

The problem is of course that the process of you being sick may lead to you recovering and becoming immune, but it may also lead to you being very seriously sick or even dying. So the question is: how do we break the cycle of infection? And there are two ways of doing that.

The first is what we were doing all of last year: social distancing, masking, quarantining, etc. Not a nice route to go, but it helps to prevent infectious people from meeting susceptible people, thus slowing the rate of infection and saving lives.

The second way is vaccinating. Vaccines simulate the sickness process, but with two crucial differences: (1) you are much less likely to suffer severe health consequences, and (2) you are not completely guaranteed to become immune.

Basically, getting the jab “jumps” you from the “susceptible” class into the “recovered” class, without having to be properly sick along the way. Unfortunately, this only works with a probability of 60-95% (depending on the vaccine [1]). So while this is a very good deal compared with the risks of actually contracting Covid, it isn't bullet-proof.

But it doesn't have to be. Because a disease will die out if the percentage of susceptible people in the population is low enough, you don't actually need full protection for every individual to eradicate an epidemic.

Imagine a town with 10 000 inhabitants. If they were all vaccinated with a vaccine as effective as Pfizer or Moderna, about 9500 citizens would become immune. And although a random sample of about 500 would still be susceptible, that wouldn't matter much, because there are too few of them to keep the disease spreading.

But now imagine that only half the citizens were vaccinated. The unvaccinated people would keep getting sick, keeping up a high rate of infection and so quite probably exposing the 250 people who were vaccinated, but for whom the vaccine did not work as intended, to the disease.

It is exactly this effect that is the problem. The Covid vaccines we have work remarkably well, and have been shown to do so in every country that is using them [2]. But they are still not perfect. And because of that, it does make a difference how many people are vaccinated overall.

In short: if you haven't been vaccinated yet, you are raising the risk of others around you - including some who have already been vaccinated. So get your vaccine and help to save a life ;-)

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Addendum 1: But vaccinated people still get sick, and then infect a lot more people, because they don't have symptoms and don't realise they are infected!

Vaccinated people are much less likely to become infected at all [2], and if they're not infected, they can't pass it on. It is true that there are cases of unsymptomatic infections in vaccinated persons, but these are (a) comparatively rare, and (b) current evidence indicates that unsymptomatic cases are probably not as infectious as symptomatic cases [3]. Also, it's not that easy to quantify how much somebody is at risk - although there are some clear risk indicators, that doesn't mean that healthy young adults cannot get sick, and badly so [4].

Addendum 2: The government shouldn't force you to get vaccinated, it's the individual's choice!

Whether or not it should be mandatory is a whole other question. (Although note that mandatory vaccinations in and of themselves are nothing new.) But I do find it important to emphasise that the choice to be vaccinated is not just an issue of personal preference, but a deeply ethical question of responsibility to others. In that sense, getting the jab is a form of loving your neighbour ;-)

Addendum 3: But natural immunity is so much more effective than vaccination! [5]

That's an interesting study! However, I don't quite understand why you see natural immunity as an alternative to vaccination? Basically, what the study says is: “If you were lucky enough to survive Covid once, you're almost certainly safe in the foreseeable future”. But of course the point of being vaccinated is that you don't have to play the Russian roulette that is a Covid illness. The study investigates ~100 000 Covid survivors - but doesn't mention that ~16 000 of them would have been quite seriously ill, or the ~2000 additional infected people who didn't make into the study cohort because they died of their illness (numbers based on hospitalisation and mortality data from the Israeli Ministry of Health [6]). Compared to the known short- and long-term risks of a Covid infection, a vaccine may not be 100% safe (few things in life are), but the overall risk is lower by several orders of magnitude. And, at the risk of repeating myself, that is before we consider the effect that my vaccination choice has on the safety of those around me.


  1. European Medicines Agency. “COVID-19 vaccines”.

  2. e.g. Dagan et al. (2021). “BNT162b2 mRNA Covid-19 Vaccine in a Nationwide Mass Vaccination Setting”. The New England Journal of Medicine.

  3. Mallapaty (2021). “Can COVID vaccines stop transmission? Scientists race to find answers”. Nature News.

  4. Robert Koch Institut, Epidemiologischer Steckbrief zu SARS-CoV-2 und COVID-19.

  5. Gazit et al. (preprint). “Comparing SARS-CoV-2 natural immunity to vaccine-induced immunity: reinfections versus breakthrough infections.” medRxiv.

  6. Israeli Ministry of Health, Covid-19 Frequently Asked Questions

Tagged as society, science, social media, covid-19

Unless otherwise credited all material Creative Commons License by Daniel Vedder.
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