Measles Epidemics Could Return: Low Vaccination Rates to Blame

By Alison Silvis
Associate Editor

Measles, a highly contagious and deadly childhood illness, is on the rise. This year, 131 cases have been confirmed in the United States to date, the highest incidence reported since 2000. Almost 50 percent of those occurred in children who were unvaccinated due to their parents’ philosophical or religious beliefs, and another 15 percent had unknown vaccination status. Measles can have serious consequences including pneumonia, encephalitis and hepatitis. In developing countries, the fatality rate is 4-10 percent, making it one of the leading preventable causes of death worldwide.

The Centers for Disease Control (CDC) warns that the recent increase might portend a serious outbreak of measles. The last major outbreak was in 1989-1991, with 55,000 cases and 123 deaths. “Increases in the proportion of the population declining vaccination for themselves or their children might lead to large-scale outbreaks in the United States,” warned CDC in April.

Unfortunately, Americans who chose to place their children at risk of naturally acquiring measles also chose to increase the risk of other children becoming ill. Infectious diseases are held in check partly by relying on “herd immunity” – the threshold percentage of the population that must be immune to prevent an outbreak from burgeoning into a full-blown epidemic. If the percentage of vaccinated individuals falls below the herd immunity threshold, epidemics can strike. Analysis of the 1989-1991 outbreaks concluded that more than 80 percent vaccination coverage is required to prevent measles epidemics. (1)

So what is the herd immunity of American children? This fall, 2-3 percent of students in many states are returning to school without complete vaccinations, although there are communities in which as many as 30 percent of students are unvaccinated. More ominously, the CDC estimates that 25 percent of toddlers nationwide are unvaccinated. This means that in a few years, only 75 percent of students on average will be fully immunized. Pockets of very low immunization rates will undoubtedly exist, creating ideal conditions for epidemics.

Of the cases that have occurred this year, 25 percent occurred in children who were unvaccinated because they were too young or otherwise ineligible. These children would otherwise rely on herd immunity to be protected. Unfortunately, those who are too young to receive the MMR vaccine (less than 12-15 months old) are also at highest risk of severe measles or even death if infected.
Another factor explaining the rising incidence of measles is increased international travel. This year, 13 percent of cases were acquired overseas, in Western Europe, Israel, India, China, Russia, Pakistan and the Philippines. This trend is unlikely to change in the near future; our best defense against importing measles from abroad is to vaccinate at home.

What are the reasons that parents cite for not vaccinating their children against measles? Aside from religious beliefs, many parents fear that the MMR vaccine increases the risk of autism and/or inflammatory bowel disease. Although Jenny McCarthy will tell you otherwise, scientific evidence does not support these associations. Additionally, 23 peer-reviewed articles have been published refuting them, whereas three have been published in support. Of those three, one was subsequently retracted by its authors, and the other two were case-control studies that have not been reproduced.

Vaccination is unquestionably one of the most efficacious, low-risk, cost-effective medical interventions, period. But that rational argument isn’t enough to compete with Hollywood celebrities telling parents that immunization causes autism. Perhaps Americans need to witness an epidemic of measles to start vaccinating their children again. The old adage “out of sight, out of mind” may explain why parents who have never seen or known a child infected with measles are reluctant to vaccinate. I hope Americans have the foresight to realize that the benefits far outweigh the risks before an epidemic strikes. Otherwise, our children will pay the price for our poor judgment.

Alison Silvis is a second-year medical student.

1. Schlenker TL, Bain C, Baughman AL, Hadler SC. Measles herd immunity. The association of attack rates with immunization rates in preschool children. JAMA 1992;267(6):823-826.