Refusing to Vaccinate Leads to Outbreaks

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August is National Immunization Awareness Month. I get it. The decision to vaccinate yourself or your child is a contentious subject. Stacy Morrison's post at the beginning of the month (or more specifically the comments it generated) proved that point. This conversation is important and the facts are important - even if it isn't (in some circles) popular. The comments section got very heated, and I think Stacy did a fantastic job of staying above the fray without backing down on her stance.


Image: Romana Klee via Flickr

A paper was published in February in PLOS ONE (not behind a pay wall, so I encourage you to read it if you're interested) titled "Positive Network Assortativity of Influenza Vaccination at a High School: Implications for Outbreak Risk and Herd Immunity." It's a bit of a mouthful, I'll admit. The authors also tend to speak about the outcomes in statistical terms, so it's not likely to show up on a NY Times bestseller list any time soon. For everyone's convenience I am going to summarize the main points that they made in plain English.

Work done by previous research teams has already shown something incredibly interesting (in my opinion). In the new study, the authors showed that when they evaluated the distribution of vaccinated and unvaccinated individuals in a high school, the groups tended to cluster. Now in this case, they were looking at the seasonal flu shot, and not the standard childhood vaccinations, but the principles can be extended (and have been in other instances) to all vaccinations.

What the authors found was that students tended to positively associate (as in, were more likely to) with students of a similar vaccination status -- more so than if they were to randomly associate. When the researchers ran outbreak simulations from the groups that they established from the surveys collected, they found that the clusters led to larger potential outbreaks than a random clustering would have -- 22% larger outbreaks in fact.

What this means is that clusters of unvaccinated individuals, who due to social stratification because of similar opinions, reduce the herd immunity of the community and lead to more frequent and/or larger outbreaks of said disease.

Now before you write this off as "oh it's just the seasonal flu," the exact same issue has cropped up time and again with more serious diseases. The Whooping Cough is a well known example, but it's pretty controversial so let's talk about something that I think everyone can agree on.


Measles is insanely contagious. It spreads through microdroplets in the air (airborne transmission), and a person is contagious up to five days prior to the onset of the rash symptoms. On average it takes about 2 weeks from the time a person is exposed until they begin to develop symptoms -- which means that this disease can spread extremely quickly and relatively silently at first -- if there are enough people around who are at risk for contracting it.

There have been outbreaks here in the US (despite being declared "eradicated" in 2000), as well as England, Switzerland, the Netherlands, and many other locations with high vaccination rates. It really puzzled the hell out of physicians and researchers, until they took a closer look at the distribution of vaccinations.

In the Netherlands, for example, it was noted that the outbreaks occurred mainly in populations that consisted of a religious group that did not vaccinate. In other countries it was shown that disease outbreaks also clustered with regions that were anti-vaccination for one reason or another, despite the overall high coverage nation-wide. Measles, by the way, requires a 90%+ vaccine coverage to be contained due to its highly infectious nature. Studies showed that the heterogeniety in vaccinations due to opinion makes a 90% coverage function more like a coverage of less than 70%.

Researchers have since realized (as I think all of us have at this point) that vaccination coverage in countries like the US which have excellent access to vaccines, is heterogeneous due to opinions on vaccination. And that this heterogeneity is linked to the outbreaks we have been seeing lately in these diseases.

In the end what this means is that people who are unvaccinated are putting themselves at more risk than they realize. So many people (myself included) have made the argument that it is socially irresponsible not to vaccinate due to loss of herd immunity, but it appears that even more than that, you're putting yourself at much greater risk by choosing not to vaccinate than you thought.


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