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Yes, Low Immunization Rates Are Responsible For The Measles Outbreak, Study Suggests

Yes, Low Immunization Rates Are Responsible For The Measles Outbreak, Study Suggests

Yes, Low Immunization Rates Are Responsible For The Measles Outbreak, Study Suggests
March 16
15:41 2015

How can we be sure that the current measles outbreak is really due to too few people getting vaccinated? And what was the immunization coverage of visitors at Disneyland during the days the virus was circulating there?

A study in JAMA today addresses both these questions with some pretty clever calculations. The second question sounds like an impossible one to answer, but, in fact, we know enough about measles and its vaccine to come up with a pretty good estimate of coverage at Disneyland and in the various communities where cases are occurring.

“The authors have done an interesting and well thought out mathematical experiment to explore what coverage would have to be in order to produce the case counts we’ve seen, given what we know about the transmission dynamics of measles,” said Jessica Atwell, a Ph.D. candidate in epidemiology and global health at Johns Hopkins Bloomberg School of Public Health. And what they find, she said, is that the communities where these cases are occurring are well under the herd immunity threshold needed to prevent the disease from traveling, somewhere between 50 percent and 86 percent instead of the minimum 95 percent typically recommended. “It highlights the importance of making sure we can address the concerns of vaccine hesitant parents and continue our efforts to keep coverage as high as possible,” she said.

Atwell has similarly researched how much low vaccination rates contribute to outbreaks of a particular disease, but she has typically done what most researchers do: compare immunization coverage rates to clusters of disease cases to see if there’s an association between them. This study came at the issue from a different angle — practically in reverse, actually — and for good reason. We don’t really have the data we need on exactly how many people are immunized against measles in individual communities.
Most data on U.S. vaccine refusal rates are based on children’s non-medical exemptions to required school immunizations. But using these as a metric is often problematic: parents might seek an exemption if a child received no vaccines at all or even if the child received all but one vaccine, and information about which vaccines the child did and didn’t receive is not always available to researchers. It’s therefore impossible to know, at least across all communities, exactly what exemption rates mean in terms of how many vaccines those children did or didn’t receive and which ones we’re talking about. Further, even the data that are available usually aren’t reported at all geographical levels. Overall national and state data on exemptions and on kindergartners’ immunization rates (by vaccine) are available, and some states require counties or schools to publicly report these data. But in many states, data at the county level isn’t readily available, much less at the city, zip code or neighborhood levels. And knowing children’s immunization status in kindergarten doesn’t tell us their vaccination status when they’re older.

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