Prevalence, Prevalence, Prevalence, Prevalence!

If you have an anti-vaccine agenda, and you want to scare people off vaccines by telling them that vaccines cause autism, and you want to scare them about autism, then all you have to do is get the definition of prevalence wrong. Then, take a national emergency like Hurricane Sandy and write some half-assed blog post about how autism is some sort of a national emergency that needs to be addressed immediately but is being hidden from the public by special interests.

How something that is emergent like that can be hidden remains a mystery to me, but — as always — facts don’t ever get in the way of a good anti-vaccine, anti-government, big conspiracy nut’s blog post. Like this one here. If you can stomach it, go read it, then come back for today’s breakdown of the [redacted] spewed there.

Let us begin with two quick definitions. “Incidence” is the number of new cases of a disease or condition divided by the number of people at risk. For example, the incidence of cervical cancer would be the number of new cases divided by the number of women with cervices. Note that we don’t include men in that rate/proportion because men don’t have uteri nor cervices.

“Prevalence” is the number of existing cases of a disease or condition divided by the total population. For example, the prevalence of diabetes is the number of total diabetes cases in a community divided by all of the people in that community. These two numbers, incidence and prevalence, tell you very different things epidemiologically. Only incidence can tell you if you have an outbreak, or national emergency, on your hands.

For a condition such as autism, where the person who has autism rarely, if ever, dies from it and can lead long, productive lives, the prevalence rate will continue to climb and climb as more people are diagnosed and more of them are living long. Even if the incidence (new cases) drops precipitously, the fact that there are new cases will mean that prevalence will continue to rise. I’ve explained this to you before, haven’t I?

I have.

I really wish the author of that post had an epidemiologist who she could ask about these things before looking foolish. All she has is an even more hardcore anti-vaxxer who is trying to become an epidemiologist. But that’s a whole other story.

Anyway, back to the post in question. In it, the author states the following:

“Starting in the 1980s the autism rate began an ever-ascending climb. 

1995 1:500
2001 1:250
2004 1:166
2007 1:150
2009 1:110
2012 1:88″

She quickly acknowledges having been told the reason for this climb in prevalence, but she immediately refutes it:

“For years the medical community has been credited with “better diagnosing” of a disability that’s always been around. In other words, we’ve always had people like this in society– we just didn’t call it autism… The trouble is, no one has ever had to prove the claim of “no real increasing—better diagnosing.””

Allow me to highlight the troubling part of her statement:

“…no one has ever had to prove the claim of “no real increasing —  better diagnosing”

No one? Really? What about this, this, this (.pdf), and this? Those don’t count because of [insert conspiracy theory here]? Oh, well, I tried.

And then she gets all conspiracist about it:

“That hasn’t stopped authorities from claiming that they’re out there somewhere, undiagnosed or misdiagnosed. It would be especially interesting to see the 40, 60, and 80 year olds with classic autism, whose symptoms are evident to all. It would be of real significance to find middle aged and elderly people whose health history also included normal development until about age two when they suddenly and inexplicably lost learned skills and regressed into autism.”

In other words, because the author doesn’t see them, they must not exist.

Tell me something. Do you “see” people with schizophrenia everywhere? Well, you should. You should see them because 1.1% of the world’s population suffers from it. As it turns out, 1.1% is 1 in 88.

Let that settle in for a little bit. Maybe get up and stretch and whatnot.

Based on prevalence, there are just as many people with schizophrenia as there are people with autism. In the cases of both conditions, the prevalence will continue to increase not because there is some “tidal wave”, “hurricane”, or “emergency” of number of incident cases. Nope. The prevalence will continue to increase because people with these conditions are being treated and accepted — diagnosed and intervened on — and allowed to be part of society. No longer are they being institutionalized in the same manner or proportion as they were in the past.

But we don’t “see” them everywhere because these kinds of conditions manifest themselves at A) a certain age, and B) as a spectrum. You don’t see kids with schizophrenia because it manifests in young adulthood. You don’t see a lot of schizophrenic adults because they are either being treated for their condition and lead “normal” lives or are institutionalized (e.g. sanatoria or even jail). Likewise, you don’t “see” autistic children everywhere because, well, seriously, how many of us wander around elementary schools? And the 1 in 88 adults? I’ll get to that in a second.

By the way, I have several friends with mental health issues, including schizophrenia, and central nervous systems that are not typical, and I love them to death. But I digress…

The author of the misinformed, misconstrued blog post then want to see the following:

“The problem is no one has ever been able to show us the one in 88 adults with autism.”

The author wants to believe — or make her readers believe —  that 1 in 88 adults has autism. I hope it’s an oversight on the author’s part because the prevalence rate on autism is for children. Here, I’ll show you:

“About 1 in 88 children has been identified with an autism spectrum disorder (ASD) according to estimates from CDC’s Autism and Developmental Disabilities Monitoring (ADDM) Network.”

It’s children. There are less children than adults in the United States. So you can’t extrapolate that number willy-nilly without use of some biostatistics. Again, if only she had a [expletive] epidemiologist to help her sort these things out and not read so idiotic.

Finally, if you can do me a favor and not even mention the author’s name in the comments. She’s been known to go all “decepticon” and have her bot fill comments sections with what can be best described as manure.

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To understand autism, you need to understand incidence and prevalence

In 2012, the CDC put out a prevalence study of autism in the United States. It changed the prevalence number from “1 in 110” to “1 in 88”. There were many who were alarmed at this statistic. They thought that the chances of a child being born with autism increased from 1 in 110 to 1 in 88. Well, they didn’t. This was only a rise in prevalence, not a rise in incidence. While the two are related, they are not necessarily tied to each other. That is, if one rises, the other one doesn’t have to. One can rise and the other can fall. Why?

It’s a little complicated, but I’ll try to explain it.

Let’s look at the definition of incidence. Incidence is the number of new cases in a population, per unit of time (usually a year), divided by the number of people at risk in that population. So, if your population is 100,000 and 100 people get the disease in one year, then your incidence is 0.001 or 0.1%. But what if we’re talking about cervical cancer? In a normal population of 100,000, only half of the people in that population — the women — would get cervical cancer. Men don’t have the right equipment for that. In that case, 100 cases in an at risk population of 50,000 is an incidence of 0.002 or 0.2%.

If you fully recover from the disease, then you move over to the “at risk” population again. If you don’t — because it stays with you forever or because it kills you — then you stay out of the at risk population. You could have 100 cases each year, no more and no less, and the incidence would continue to rise if no one recovers or your at risk population is not replenished by new births fast enough. In the example I just gave you, the population at risk for year two is 49,900. If you get another 100 cases, then your incidence is 0.00200401 or 0.2004%. It’s a small increase, but it’s an increase nonetheless.

So, remember this: If the disease is not curable (because it is chronic, pervasive, incurable, or deadly), then the population at risk dwindles if it is not replenished by births or immigration. Lower the denominator in incidence, and you will get a higher number. To decrease incidence, you either increase the number at risk or you decrease the number of new cases.

Now, let’s move on to prevalence. Prevalence is the number of existing cases in a population, per unit of time, divided by the total number of people in that population. That’s total population, regardless of whether or not they have the disease. So, if you have 100 cases of cervical cancer on year one, your prevalence will be 100 divided by 50,000, which is 0.002 or 0.2%. Year two, you get another 100 cases, and you will now have 200 existing cases divided by the same population of 50,000, which is 0.004 or 0.4%. Your prevalence doubled!

This assumes, of course, that no one died of the disease or that the total population stayed static through some means. In real life, population levels change.

In year three of the above scenario, you get another 100 cases, making it 300 existing cases in a population of 50,000, for an overall prevalence of 0.006 or 0.6%.

So, remember this: If a disease is not curable, then the prevalence will increase as long as there are new cases. Prevalence will decrease if the increase in population outpaces the new number of cases or the number of existing case decreases because of death or recovery.

Now, onto autism.

As far as science and medicine can tell us, autism is not curable. It is treatable. With the right interventions and depending on the level of severity of the autism signs and symptoms, autism is treatable. Plenty of people with autism go on to live happy and fulfilling lives. Again, it is not curable. Not at this time. So any new cases of autism will pile-on to existing cases and… Prevalence will increase.

Not only that, but the number of new cases per year can go down, but there will still be all those previously-diagnosed cases of autism which are still being added on to even if the incidence falls. Incidence would have to reach zero, the number of new births would have to continue (some countries have a negative birth rate), and people with autism would have to start passing away before the prevalence of the condition decreases.

Here are some theoretical numbers, as an example:

Note that there was a successful intervention in this example.

The column headings are self-explanatory, but let’s just go over them again for clarity.

  • New cases – Number of newly diagnosed cases that year.
  • Existing cases – The number of new cases for the year plus the number of existing cases the previous years. (Let’s pretend that there were no existing cases in 1999.)
  • Incidence – The number of new cases for the year, divided by the population at risk.
  • Prevalence – The number of existing cases (new cases plus existing cases) for the year, divided by the total population.
  • Population – The total population.
  • Population at risk – The total population minus the number of new and existing cases.

As you can see, we had a steady increase in the number of new cases from 2000 to 2009. From 2009 to 2015, the number of new cases declined. Appropriately, the number of existing cases continued to increase throughout because the condition is not deadly. (Again, this is theoretical. People with autism die from other causes, like the rest of us.) As you can see, incidence climbed along with the number of new cases until 2009/2010, then it began it’s decline. On the other hand, prevalence started its increase in 2000 and continued increasing to 2015. Also note that I increased the population every five years or so in our theoretical place (city, county, state) because that’s what populations in the United States have been doing. We don’t have a negative birth rate.

So, as cases dropped and population increased, incidence dropped. Because cases didn’t die, and the new number of cases outpaced the population increase, prevalence continued to increase.

On a side note, one of the criticism someone mentioned about HIV/AIDS treatment is that prevalence continues to increase. In their mind, HIV/AIDS treatment is not working if that particular measure of disease continued to increase. Can you see now why they were wrong?

Can you see why the prevalence of autism increasing from 1 in 110 (0.9%) to 1 in 88 (1.1%) is not a clear indicator that the number of new cases each year is rising? It’s only an indicator that people with autism are living and that the number of existing cases each year is outpacing population growth. For prevalence to decline, you would have to drop the new number of cases per year to zero and wait for the number of existing cases to drop on their own as people with autism get old and die.

That right there is what puzzles me when certain groups say they want the prevalence of autism to plummet. And their ranting and raving about an increase in autism signaling an “epidemic” of autism is also puzzling. Hopefully, it will be puzzling to you as well, now that you have seen how incidence and prevalence work.

Of course, this assumes that all things are equal when it comes to autism surveillance. But they are not. But that’s for another blog post at a later time.

By the way, here is the graph of the information in the table above, for those of you who are more visual:

Even with a theoretical, successful intervention in 2009, prevalence continued to increase. Why?