Biology / Health / Medicine / Microbiology / Science

Herd immunity

What is herd immunity (& why does it help when healthy people get vaccinated)?

Image of a herd of sheep

This question came via Mordred on Habitica. The assumption for the rest of this post is going to be that you accept that vaccinations work, via allowing the body to make the right antibodies to kill the real pathogen through what is basically a practice run on a dead or weakened virus. That gives the answer right away to why it helps for healthy people to be vaccinated: being vaccinated once you’ve caught the pathogen for real isn’t going to train your system to tackle it. Vaccination can help once infection is already underway, but you’ll still be infectious and you’re already sick.

The reason herd immunity is useful and can even lead to eradication of a pathogen (smallpox, for instance) is that the pathogen needs to infect people to survive. Some pathogens can survive a little while outside the human body in certain conditions, but for the most part, they need a living host. If everybody who comes into contact with the pathogen is immune, it can’t infect them and spread to other people through them — and even if it did come across one unvaccinated person, it couldn’t spread to other people who have been vaccinated. That’s the power of the “herd”: if a lot of people are immune, the chances of the pathogen being able to find someone to infect are dramatically reduced.

Herd immunity is really important with some contagious illnesses because some people can’t be vaccinated. They might be allergic to some component of the vaccine, or they might have weakened immune systems which would mean that the vaccine would make them ill anyway. Herd immunity keeps them safe by making it really unlikely they’ll come into contact with someone harbouring the pathogen.

It can even be really useful to have herd immunity to a relatively non-harmless disease which causes severe problems for people with weak immune systems (e.g. people who are having chemotherapy, people with autoimmune disorders, the elderly). Most people would be able to shake off the disease because their immune systems would respond quickly and destroy the pathogen, even without the training run. But people with weakened immune systems can run into serious problems with such common illnesses as the common cold or flu. So getting your flu jab can also protect the people around you and potentially save lives — that’s why caregivers and doctors get the flu shot, for example, to prevent them catching something and passing it on to the vulnerable people they work with.

Due to the largely unfounded scares about the dangers of vaccination, people are increasingly opting out of vaccination programs. The effect is clear in the resurgence of the diseases we used to be protected against. People are dying or being permanently scarred or disabled by diseases which they never had to catch at all, and diseases we almost eradicated are returning at an alarming rate. At the same time, our antibiotics are becoming less effective as many pathogens gain immunity to them.

In conclusion: keep up with your vaccinations, not just for yourself, but for those around you as well. The personal risk is tiny. The benefit is bigger than you think.

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