Peggy O'Mara misrepresents statistics

I have a sort of love/hate relationship with Mothering magazine. While they do have interesting articles, they have a tendency to twist scientific results to support their idealization of parenting. Take for instance Peggy O'Mara's recent editorial in the January/February 2009 issue titled 'The assault on freedom of conscience'.

Sometimes people will characterize the magazine as pro-homebirth or anti-vaccine because of our frequent coverage of these issues. In fact, we are pro-informed consent; we publish both sides of the story so that parents can be aware of all angles before they make a decision.

That's all well and good; but then Ms. O'Mara and shows her misinterpretation of scientific data regarding measles cases and the decline in vaccinations.

According to the Centers for Disease Control and Prevention (CDC), 63 of the 131 new cases of measles from January to July 2008 were among those unvaccinated. The majority of the cases (68), however, were among those vaccinated.

Face, meet palm. Okay people, a chi-square test really isn't that hard to understand. If you have two populations that are not evenly distributed, say vaccinated and unvaccinated, you would expect the outcome, say disease manifestation, to be similarly distributed unless there is something fundamentally different between the two populations. So, if you have about 95% of children vaccinated (which most states do based on the Healthy People 2010 guidelines), you would expect 95% of children with disease to have been vaccinated. But that is not what you see. Here, I will put it in table form (with hypothetical numbers).

Population size; Disease Prevalence ; Expected Prevalence
Vacci 95,000 (95%) ; 52 (52%) ; 95 (95%)
Unvac 5,000 (5%) ; 48 (48%) ; 5 (5%)
Total 100,000 (100%) ; 100 (100%); 100 (100%)

A majority of patients may have been vaccinated, but it is still significantly less than what it should be. Conclusion: unvaccinated children are more likely to come down with measles. Not complicated. The same sort of stuff they used to find out smoking causes cancer. Understand? Maybe not.

Interestingly, according to the CDC, 89 percent of the 131 new measles cases were "imported from or associated with importations from other countries, particularly countries in Europe, where several outbreaks are ongoing."

You mean countries like the UK, France, and Italy which have lower vaccination rates than those in North America?

Measles is depicted as a life-threatening disease instead of the mild illness that my friends and I all had as children. In the Private Practice episode, the child dies from measles, an occurrence that is so rare that, based on the current incidence levels (42 in 2007), a death from measles would happen once every 119 years. Even if the incidence of measles were to quadruple, we would not see a death for 30 years. The current death rate from measles is 1 in 5,000, yet it is portrayed in the show as though it happens frequently.

This of course ignores a very real problem. The occurrence of measles is now so rare that doctors may be inadequately trained to recognize it, meaning that the disease may reach a more severe stage before it is treated. And what can we thank for such a rare occurrence?

Vaccinations.

If you choose not to vaccinate, you can thank something else.

Herd immunity.

You're welcome.