How Many People Must Be Unvaccinated Before You Have To Worry About Getting Measles?

By Francie Diep

A team of Boston researchers did the math, and confirmed that yes, low rates of vaccination are indeed to blame for the spread of measles across the United States from Disneyland. They published a short letter about their calculations today, in JAMA Pediatrics.

Those who fell ill during the recent outbreak, which affected 142 people from seven states, likely caught measles in places where anywhere from one in seven to one in two people were unvaccinated, the researchers found. People have caught measles that could be traced to Disneyland not only from the park itself, but also from returning family members, or health care settings, like emergency rooms, MyNewsLA reports.

Across the United States, more than nine out of 10 kids have had all their measles shots, which are given in combination with mumps and rubella vaccines, in the form of the so-called "MMR" shot. But certain pockets of lower vaccination rates can arise from a combination of factors, as Discover reported earlier this year. It's not just misguided parents who refuse to vaccinate their kids, although they certainly contribute, too. As social scientist Julie Leask, who was not involved in the recent JAMA Pediatrics study, wrote to Discover in an email:

There will be children whose parents refused vaccination; children whose parents were unwittingly not up to date for lack of access; affordability or awareness; adults and travellers who didn't get a needed booster; and babies who are too young to be vaccinated.

Because measles passes so easily between people, 96 percent to 99 percent vaccination rates are required for "herd immunity" to work, the Boston researchers wrote in their paper. That means no fewer than one in 25 people, including those who are too young or can't get the vaccine for other reasons such as allergies, can go unvaccinated before rendering their communities vulnerable to measles outbreaks.

Historically, before the invention of the measles vaccine, one person with measles usually made 11 to 18 other people sick. It's no surprise, then, that just a few not-fully vaccinated folks can lead to an outbreak.

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