Marco Cáceres di Iorio

Whooping Cough Outbreaks: Correlation is Not Causality

The outbreak of pertussis, or “whooping cough,” in California this year (about 8,000 cases so far) has again spawned countless articles in newspapers throughout the United States blaming the unvaccinated community for the outbreak. The headlines provide a sense of the obvious bias. Headlines such as “Did Poor Vaccine Response Contribute to California’s Whooping Cough Outbreak?” in WIRED, or “Anti-Vaccination Beliefs are Contagious Like a Disease” in The Washington Post.

The bias is troubling, because it reflects superficial thinking. That’s dangerous, particularly when it involves the health of one’s children. The articles imply or outright say that the reason there have been so many cases of whooping cough in California is that vaccination rates for children in the state are low relative to other states in the country, and that they’ve continually been dropping.

An article in Salon titled “California’s Whooping Cough Outbreak is Officially an Epidemic,” published on June 16, 2014, quoted the following from an NPR article, “Vaccine Refusals Fueled California’s Whooping Cough Epidemic“:

“They compared the location and number of whooping cough, or pertussis, cases in that outbreak with the personal belief exemptions filed by parents who chose not to vaccinate for reasons other than a child’s health. (Some children with compromised immune systems aren’t able to be vaccinated. … They found that people who lived in areas with high rates of personal belief exemptions were 2 1/2 times more likely to live in a place with lots of pertussis cases. “The exemptions clustered spatially and were associated with clusters of cases,” Jessica Atwell, a graduate student at Johns Hopkins Bloomberg School of Public Health and lead author on the study, told Shots. It was published online in the journal Pediatrics.”

The problem is that in all these articles there is a tendency to confuse correlation with causality. The two are not the same. In other words, the fact that there is a whooping cough outbreak in an area of low vaccination rates does not mean that the low vaccination rates were behind the outbreak. There may be a correlation between the two, but that does not prove that the former caused the latter. Not by a long shot.

You say it’s self-evident? Well, let’s see about that.

Let’s start with an interesting view from Dr. Anne Schuchat, who is the director of the Centers for Disease Control’s (CDC) National Center for Immunization and Respiratory Diseases. In an article titled “CDC: Whooping Cough Heading to a 50-Year High,” published by WebMD Health News on July 19, 2012, Dr. Schuchat is reported to have said that “better diagnosis and reporting of whooping cough may be contributing to the increased numbers, along with the fact that the disease tends to peak and wane in cycles. It does not appear that anti-vaccination sentiment among parents has contributed to either the national rise in cases or the Washington State epidemic.”

Hmm, wonder why.

Simple. It turns out that in many cases, people (both children and adults) who get whooping cough are up to date with their vaccinations. Note the following excerpt from “Immunized People Getting Whooping Cough” published by KPBS of San Diego State University on June 12, 2014: “Most of the people who got whooping cough in San Diego County so far this year were up to date with their immunizations, according to county data. Of the 621 people who contracted the illness, 85% had all their preventative shots — calling into question the efficacy of the vaccine.”

In a study reported by Reuters on April 2012 and published in the  Clinical Infectious Diseases journal, Dr. David Witt and other researchers looked at 132 patients at the Kaiser Permanente Medical Center in San Rafael, CA who tested positive for whooping cough during March-October 2010.

Get this… 81% of the patients were fully up to date on the pertussis vaccine, 11% had received at least one round of the vaccine, and only 8% had never been vaccinated.

More? In 2012, there was an outbreak of whooping cough in Vermont. About 600 cases were reported, and of those, 90% involved vaccinated children.

It only takes a quick scan of the literature online to notice that there are hundreds, perhaps thousands, of articles regarding outbreaks of whooping cough, measles, and other contagious diseases around the country in which the vast majority of the people infected had been fully vaccinated.

Uh, that’s astounding.

What’s the conclusion here? You figure it out. But remember, correlation is not the same as causality. Oh, and please look elsewhere to lay the blame, cause that dog just don’t hunt no more.


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