Today it’s popular to argue that measles would be totally defeated were it not for the Jenny McCarthys of the world. The only problem is that the MMR (measles-mumps-rubella) vaccine does not actually immunize — as most people understand the word — against measles. The most we can expect is temporary protection. That’s because vaccines are injected directly into the body [see update], bypassing the body’s natural immune response. Initially described as lifelong insurance, health officials realized in the 70s, when an uptick in measles diagnoses occurred among vaccinated high-school students, that the vaccine should probably be administered more regularly. The CDC now advises receiving the vaccine at 12-15 months, 4-6 years, and again as an adult. The U.S. is also using its third version of a measles vaccine, after the first two proved ineffective.

Which should probably make it no surprise that many of the people catching measles today were vaccinated. Today’s measles cases are occurring in heavily vaccinated populations. When a 2006 outbreak among college students in the Midwest struck, the fact that most of the affected were vaccinated seemingly made no difference. When an outbreak of the mumps hit the NHL this year, many reflexively blamed “anti-vaxxers.” Almost no one reported that every affected player appears to have received the MMR vaccine. The Penguins’ Sidney Crosby not only received the initial MMR, but also a booster just before the Sochi Olympics. The director of the Vaccine Education Center at the Children’s Hospital of Philadelphia, Paul Offit, would only say “we know that the short-term effectiveness of the mumps vaccine is excellent.”

Still, none of this would suggest there’s any reason to avoid regular vaccines — were it not for side effects. And here comes another wrinkle: The MMR vaccine can itself give you measles.

Update (Ed): The author of the article later corrected his piece to change “bloodstream” to “body,” and I have updated the excerpt by his request.