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Herd immunity and Mickey Mouse

January 8, 2015

What is herd immunity? Some kind of Bovine Rock Band? No, not exactly. Herd immunity is a phenomenon that occurs when a critical number of people are vaccinated and the population gains protection from certain communicable diseases. Herd immunity protects people who can’t vaccinated: babies, pregnant women, and people with immune defects. The nice thing about herd immunity is that mass vaccination projects can lead to eradication of a virus, if there is no animal that carries it also. We did it for smallpox. We did it in the US for measles by the year 2000. However, since measles is still in other countries, cases are still reported in the US from visitors.
This week, the CDC reported an outbreak of Measles in California, traced directly back to the Disney Park in Anaheim. Reported vaccination rates for kindergartener (your typical Mickey Mouse fans) are about 92% for public schools and lower for private schools, with 13% (or about one in 9 kids) not being vaccinated with the MMR vaccine.

The “critical mass” needed for herd immunity varies depending on several factors, such as the virus, the infectious dose, the mechanism of spread and the density of people. Measles is an airborne virus; spread through the air at longer distances, and by sneezing and coughing. It lasts in the air for a very long, being a small unenveloped virus. So far, nine people have gotten sick from the Disney outbreak, not including the three visitors whom originally brought in the virus. Six of those were unvaccinated. 2 of those were babies, who were not eligible for vaccination. One adult was confirmed sick who had been vaccinated.

[Update 1/20/15- the outbreak is now confirmed in 51 people, with many of the ill being unvaccinated individuals]

Measles has several nasty complications: pneumonia, encephalopathy (brain swelling), sterility in men, and death. Babies and immunocompromised people are at a higher risk for complications from this virus, due to inadequate immune systems.
The Disney case is a prime example of why we need herd immunity. No brand-new parent wants a day at the park to become weeks in the hospital. There is also a risk that the measles virus may make its way into the school system in California. With approximately one in 10 kids unvaccinated and at risk, the disease could spread like wildfire, infecting even vaccinated children.

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References:
http://www.vaccines.gov/basics/protection/
http://www.usatoday.com/story/news/nation/2015/01/07/measles-outbreak-disneyland/21402755/
http://www.cdph.ca.gov/programs/immunize/pages/immunizationlevels.aspx

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