Measles and Immunity: It’s all In the Numbers
Apr 12, 2010

By Christine Vara
A recent NPR article I read chronicled a measles resurgence tied to people who intentionally choose to go unvaccinated.  In Vancouver, foreign travelers to the Olympics apparently brought with them more than luggage.  In fact, they were carrying two different strains of the measles from Asia.
It’s not surprising really.  The Olympics is the pinnacle of international sports and people swarmed to Vancouver to be a part of it.  However, with the global traveler comes the threat of contagious diseases and, so far, the disease has spread to 16 people in Vancouver, with half of them  residing in one large unvaccinated household.
Similarly in 2008, a seven year old boy, who was an intentionally unvaccinated US resident, returned from a visit to Switzerland and unknowingly exposed 839 people to the measles.  Of these 839, 73 were unvaccinated children (25 of these children from households that chose not to vaccinate) and 48 were children younger than 12 months old and yet to receive the measles vaccine. To limit the spread of the virus any further, parents of those 73 unvaccinated children were asked to isolate their kids by keeping them home for three weeks!  Trust me:  that’s no walk in the park for any parent.
Fortunately, in that case, the vigorous response from public health agencies helped to track and contain the outbreak. But what do we expect to find with this outbreak in Vancouver?  According to NPR, just a generation ago, up to 4 million U.S. children got measles every year, hundreds died, and thousands were left with permanent brain damage. It became a national health crisis and a vaccine was eventually developed that effectively protected people from contracting measles.  Nowadays, many people do not consider measles a threat.  After all, you hardly ever hear about it.  And many people have never seen it.  In fact, I wonder how effective today’s doctors would be in diagnosing it since it is not all that prevalent.  Perhaps that justifies the choice some people make to forego the available vaccine.
So, I had to ask myself the question: If the numbers of unvaccinated people influence the spread of diseases, but are not large, just how many people need to refuse vaccines in order for these diseases to resurrect themselves in greater numbers and with more severe outcomes?  In other words, what is the percentage of unvaccinated people that will tip the scales of social health away from overwhelming protection from a disease due to high rates of immunization—what health officials call “herd immunity”?
Shockingly, experts say measles is so contagious that more than 95 percent of a population needs to be immunized in order to prevent mass infection.  If as little as five percent of people go unvaccinated, the herd immunity becomes threatened, resulting in a higher number of vulnerable children.  It’s surprising that such a small percentage of unvaccinated population can contribute such great risk.  However, health officials say there are growing pockets of “vaccine refusers” across the country, giving viruses like the measles the opportunity to impact public health at much greater levels.
Now it’s important to note that in some communities in this country there are as many as 10 percent of the childhood population that goes unvaccinated so this is a definite concern in the U.S.  Consider another NPR article, dated April 5, regarding a recent measles epidemic in Zimbabwe where the official count is over 2,000 cases and about 200 deaths.  The Zimbabwean Health Minister highlights the importance of herd immunity saying, “estimated coverage for measles is well below 70 percent, so there’s no herd immunity.”  The article goes on to say that the epidemic began in two sects whose leaders oppose vaccination.  As more and more Americans refuse vaccinations, is this what we have to look forward to?
Currently, herd immunity, as unflattering as it may sound, might be one of the primary factors protecting the very people who choose not to immunize.  While I would not interfere with the fact that people should have a choice whether to vaccinate or not, I am concerned about the numbers.  Fortunately there are still enough people receiving vaccinations to protect the community at large.  Because others are vaccinated, it greatly reduces the risk of the unvaccinated.  However, as in the cases cited above, unvaccinated individuals are typically the first to fall victim to a case of preventable disease.
What are your thoughts on herd immunity and its effectiveness in protecting our population from major, preventable epidemics?

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