Herd Immunity is pretty cool

Why you probably won’t get measles in Australia

The other day I shared a fantastic infographic, and I was asked by a few people to elaborate. You may have heard about herd immunity (when I was in high school we actually had a science project dedicated to it), but there’s a good chance that you have never really had it explained.

It really is a great infographic; there is also a cool simulation at The Guardian, who made the thing

Attack rates, reproduction numbers and scary maths

As well as being one of my favourite public health terms, attack rate is a key part of herd immunity. It is a measure of the proportion of people who are infected by a disease in the population. Say there is a measles outbreak in a school of 100 kids. If 10 of those kids get measles, the attack rate is 10/100=10%. It’s a pretty simple metric to see how many people are infected in a population by a specific disease.

Measles has a very high reproduction rate, at 12–18 people infected, mumps is much lower at 4–7. On average, each person who has measles will infect about 3x as many new people as each person with mumps

Herd Immunity

Most diseases are infectious for a week or so, although it varies greatly. Imagine you catch measles. You are infectious for a couple of days whilst only feeling minor symptoms (cough, runny nose). You then come down with a nasty fever, and after seeing the doctor stay at home for the rest of your infectious period.

Stock photos of sick people; when you need a picture of someone sick, but not THAT sick
God I love infographics

Why do people still get sick?

The first objection you’ll always hear when talking about herd immunity is “people aren’t cows”. This is stupid.

Why it matters

Herd immunity is important for one reason; not everyone can get vaccinated. For example, people who are undergoing chemotherapy for cancer have compromised immune systems and can get very sick if they receive certain vaccines.

Ironically sheep aren’t fond of vaccination at all, the silly buggers

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