Gary Silverman
Columnist
Puzzling to some when reading about the measles epidemic sweeping through the country may be the use of “herd immunity” as justification for requiring immunizations. This term that used to be found mainly in epidemiology textbooks now has managed to enter common usage. Perhaps it needs a little more explanation.
To provide context, let’s first consider how one role of government is to protect its people. Many readers will agree that people should be left alone to do whatever they want, unless it hurts other people. Yet, few would argue that government has a role in protecting people from the actions of others. Easy examples are on the highway. I may want to drive my new Corvette (certainly a hypothetical example) 100 mph on a country road, but the government does not allow that. It’s too dangerous to others. Same thing with drunk driving – it may be a major inconvenience to me to not be able to drive home after an evening at a local bar, but government doesn’t allow it – too dangerous to others. To pick another context – a restaurant may have limited refrigerator space and want to leave its food out on a counter overnight before reheating it the next day. Same idea – the government (in this example the local health department) doesn’t allow it as it could hurt others.
Now let’s turn to the schools – they have requirements that kids are vaccinated. Why not leave it up to their parents to decide? One reason is simple – it’s to protect others. Receiving the recommended two doses of the measles vaccine is about 97% effective in preventing measles. This means that in a school of 100 children, if all were vaccinated, about three of them would still be at risk if someone with measles enters the school. Let’s imagine that a plumber with measles (but with few symptoms yet) comes in the school to fix a water fountain – the risk of those three children passing by while the plumber is at work is relatively low. And if one of those three happens to pass by and contracts measles, the risk of that child passing by either of the other two kids is pretty small. Now let’s say that only about 75 of the kids in that school are vaccinated. That would leave 25 – 27 kids at risk of getting measles. The likelihood of one of those children passing by the infectious plumber would be relatively high. If one of those kids did get measles, then about every four classmates that each interacted with could get measles! The outbreak would most likely be widespread.
The “herd” here is the population of kids at potential risk. If any child is not vaccinated, that adds to the likelihood of getting measles and passing it on – even to the kids who were vaccinated but are part of the unlucky few who were vaccinated but are still unprotected. So – deciding not to get the vaccine puts the child at risk plus all other children in that “herd.” And, of course, not getting the vaccine greatly increases the risk of that child getting a serious illness (as physicians and disease scientists overwhelmingly agree) – denying it is sort of like saying that driving 100 mph is safer because it gets you off the road quicker. I guess there is a kernel of truth in that – but it’s simply not a reasonable argument. We know that the key to preventing measles outbreaks is to keep the unprotected part of the herd extremely small – as not being vaccinated not only puts the individual child at risk but classmates as well. Not very different from having speed limits.
