Vaccine skeptics fall prey to the law of large numbers

The Yukon’s chief medical officer recently warned local residents about two cases of pertussis – better known as whooping cough – here in the territory.

The Yukon’s chief medical officer recently warned local residents about two cases of pertussis – better known as whooping cough – here in the territory.

Whooping cough is a bacterial infection that – as far as vaccine-preventable illnesses go – is tough for public health officials to beat. The vaccine for it is not quite as effective as the vaccine for the measles and other

infectious diseases, and its effectiveness wanes over time, necessitating periodic booster shots.

We adults bear a good part of the blame for this disease’s continued recurrence in part of the world, because so many of us have neglected to get our recommended adult boosters for the disease. (If you want to know

if you have received your booster or would like to receive one you should contact Public Health.)

Whooping cough is usually not life threatening for adults and older children, but it is potentially deadly when we transmit it to infants and toddlers. Newborns, who are unable to receive the vaccine until they are two months old, are particularly vulnerable.

So the reappearance of the whooping cough in the territory is a worrying reminder for the parents of small children that it is still out there.

I’ve written previously on the subject of vaccines and some critics complained that my column didn’t actually engage with the various arguments of the anti-vaccine movement. So, in the interests of tackling this subject one argument at a time, my focus today will be what I see as a major driver of fear of vaccines – anecdotes.

Humans love anecdotes. Much of what we know about the world doesn’t come from studies or double-blind trials. It comes from things we hear from other people. Anecdotes have been an important mechanism for learning about the world for much of human history.

But anecdotes can be problematic when used inappropriately. A statistical rule called the “law of large numbers” offers one cautionary lesson for why we should be hesitant to draw broad conclusions from the anecdotes of someone, somewhere in our interconnected seven-billion-person globe.

The “law of large numbers” essentially says that given enough opportunities, even things that are rare will eventually occur. And depending on the number of opportunities and the relative rarity of the event they could occur over and over again.

The law of large numbers has important implications for misperceptions about vaccine safety.

The reality of life is that bad things happen. The random, sudden and often unexplained development of any number of illnesses (sometimes leading to death) is an unfortunate part of the human condition.

And when we hear that a perfectly healthy child developed some illness shortly after that child received a vaccine we want to know why and often erroneously conclude that it was a side-effect of a vaccine.

But we really shouldn’t. In our highly populated, interconnected world we need to seriously consider if it isn’t just coincidence whenever we hear the kind of anecdotes that the law of large numbers would predict.

Consider the role that anecdotes played in the development of the widely discredited notion that vaccines cause autism. There are numerous stories floating around the Internet of parents suddenly noticing symptoms of autism in their child shortly after that child received a vaccination.

The first time we hear one of these stories we say “that’s a weird coincidence.” But by the time we’ve heard the same story five or six times many of us are absolutely convinced that the two have to be related.

Anecdotes have a powerful effect that way.

But is such a small sampling really good evidence of anything?

Between birth and the age of two, children will receive multiple rounds of immunizations. These vaccines happen to be received around the same time parents tend to notice symptoms of autism. Today, one in 68

American children will be diagnosed with autism and in most cases symptoms will be detected before the age of two.

So while the odds of a particular person noticing symptoms of autism on the day a child got a vaccine just by chance are pretty slim, the odds that it will happen to some parent, somewhere are actually quite high.

After all, there are millions of children just here in North America receiving vaccines every year. If you crunch the numbers you can see that by statistical chance alone hundreds of parents would notice symptoms of autism on the day their child received a vaccine and thousands would notice within a week.

As these stories spread throughout the Internet and accumulate over the years they give the appearance that the two are connected. But ultimately these anecdotes are evidence of nothing. They are simply an example of the law of large numbers playing out in large scale across modern society.

This mistake is made over and over again when anecdotes are used to conclude cause and effect about vaccines. The United States government maintains an online database known as the Vaccine Adverse Event

Reporting System (VAERS) where anyone can record any “adverse event” – basically any negative health problem – that develops shortly after receiving a vaccine, whether it is related to the vaccine or not.

It was intended as a tool for researchers to carefully sift through using proper statistical techniques and detect the true rare side-effects we know vaccines have.

Unfortunately, when used improperly VAERS becomes a significant source of fear and misapprehension. It has been used by some in the anti-vaccine movement to suggest a connection between vaccines and just about every ailment under the sun because, as I said earlier, bad things happen and in our interconnected world you can find incidences of just about all of them happening shortly after receiving a vaccine.

So the next time you hear a story that you think is just too weird to be a coincidence, think again. Coincidences happen much more often than our anecdote loving brains care to admit.

And hopefully thinking twice about anecdotes will alleviate some of the fears we have about vaccines so that outbreaks of preventable diseases like whooping cough become less common. Next time it could be a small child who catches the disease here in the territory and the outcome could be much more serious.

Kyle Carruthers is a born-and-raised Yukoner who lives and practises law in Whitehorse.

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