Because of lower vaccination rates, we’ve been seeing a lot more measles outbreaks in developed countries in recent years, including here in the United States. Measles is so contagious that if vaccination rates drop below 95%, the virus can find a foothold and begin circulating throughout the population.

The definition of a measles outbreak
A measles outbreak is defined as 3 or more linked cases of the disease breaking out in a particular area.

Recent measles outbreaks
Measles cases in the U.S. have been rising in recent years, and more and more outbreaks are occurring. In the first few months of 20 I I, the U.S. saw 118 new measles cases-nearly twice as many as the total for all of 20 1 O. Around 90% were unvaccinated, and 40% of patients required hospitalization for complications. Most cases arose from unvaccinated travelers to Europe, which was in the throes of a serious mt:as\t:s epiJernic. (Szabo, 5-25-2011) By 2019 lht:lt VVtlt 1l10rt tIlan 1,200 confirmed cascs of mcaslcs across 3 I U.S. states just from January through October, making it the worst measles outbreak in 25 years. (Abbott et aI., 2019)

Can vaccinated children get sick during a measles outbreak?
Yes. Although the odds of them getting sick are much, much lower, vaccinated children can sometimes get sick because no vaccine is 100% effective. It depends on your child’s immune response and how high of an immunity they built up through the vaccinations. On the bright side, the measles vaccine is around 97% effective when both doses are given, so the odds aren’t high. Vaccinated children who do get sick are also likely to have much milder cases and avoid the most dangerous complications.

What causes measles outbreaks?

For a disease that, thanks to vaccines, had once been declared eradicated in the U.S. in 2000, the cause of outbreaks is simple: Low vaccination rates in certain communities. A vaccination rate of roughly 95% is required to preserve the so-called “herd immunity” that keeps a virus from spreading.

Usually an outbreak starts when someone who had been traveling overseas returns home infected with the illness. Since measles has a reproduction number of between 12 and 18, that one person returning with the illness can infect as many as 18 other people. In a community with low vaccination rates, that’s sufficient to allow it to spread to other people who don’t have immunity from being vaccinated.

Jason L. Riley reports on just how little it takes for an outbreak to spread: “In 1994, a skier with measles at a resort in Colorado exposed five other people to the disease, including a teenage girl from Illinois, who proceeded to spread it to 51 people back home and 156 people at her boarding school in Missouri. As other vacationing skiers returned home, the outbreak spread to Texas, Maine, California, New York, Maryland, Michigan, Washington State and other parts of Colorado. Similarly, an outbreak in 2014 was traced to a single visitor to Disneyland in Southern California. It eventually reached more than 147 people in seven states.” (Riley, 5-15-2019)

In a Somali community in Minnesota where an outbreak occurred in 2017, vaccination rates had fallen from more than 90% in 2004  to a low of about 42% by 2014. This was largely driven by a fear that the MMR vaccine would give their kids autism–a notion that’s been repeatedly debunked. With such low vaccination rates, an infected person could give as many as 8 other people measles, more than enough for its spread.

Of the 55 measles cases reported as of April 11,2019, 329 of them occurred in New York City, primarily among Orthodox Jewish communities. (Evans & McKay, 2019) Many parents in these communities forego vaccinations for religious reasons. “New York City’s measles outbreak began when an unvaccinated child was infected on a trip to Israel,” write Betsy McKay and Melanie Grayce West. “It wasn’t huge at first, with fewer than 10 new cases every week. Early on, the city health department ordered more than 100 schools and day-care centers to exclude students who didn’t have the measles, mumps and rubella (MMR) vaccine.

“In February, an unvaccinated, infccted child at a Jewish school that didn’t enforce the health department’s exclusion order, infected other unvaccinated children, resulting in 28 new cases. That led to 17 secondary transmissions outside the school, fueling a surge of cases that continues.” (McKay & West, 2019)

A 2019 outbreak among an Amish community in Ohio was largely due to a lack of understanding about the risk measles presented. A 2019 outbreak among Eastern Europeans (Russia and its former states) seems to be driven by a distrust of government. “They think it’s a way for the government to make a profit,” says Dr. Tetyana Odarich, a family physician in Portland Oregon. “People from the former Soviet Union have a very difficult time understanding the difference between me, government and insurance.” (Reddy, 4-16-2019)

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