It's back. Three years after public health officials realized that they had been premature in declaring that measles was eliminated in the U.S., new outbreaks of the highly infectious disease are once again cropping up in cities across the country. And it would be a mistake, epidemiologists warn, not to take this extremely seriously.

As expected, the outbreaks have caused plenty of outrage directed against Jenny McCarthy and the crowd of parents who refuse to vaccinate their children. Writing in the Daily Beast, a pediatrician using the pseudonym Russell Saunders calls it "sheer lunacy": "Just over a dozen years ago this illness was considered eliminated in our country," he writes, "and this year people are being hospitalized for it. All due to the hysteria about a safe, effective vaccine. All based on nothing."

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But for the most part, the outbreaks are also being treated as a cautionary tale – an opportunity to chide anti-vaxxers while reminding parents to make sure that their children are up-to-date on their shots, as KJ Dell’Antonia does at the New York Times’ Motherlode blog:

This outbreak could serve as a reminder that we vaccinate our children, and ourselves, for our own protection and for the protection of our community. A spokesperson for New York-Presbyterian Hospital/Columbia University Medical Center said that the hospital is notifying 600 people who might have been exposed there (in what it calls “probably an excess of caution”). If the majority of those 600 people were not vaccinated, The Times could soon be running a very different article on its front page. Fortunately, statistics suggest that most of those 600 people will have been vaccinated against measles, and 95 percent of them will be immune as a result. That’s lucky for us all, including those who have made a different choice for their children.

It's true that we’ve yet to see measles outbreaks on a massive scale – what counts as a major outbreak, these days, is the 20 confirmed cases in New York City. The total number of confirmed U.S. cases for this year is in the 70s; in 2013, it reached 187. That’s nothing compared to the 500,000 cases per year that the U.S. saw before the vaccine was introduced in the 1960s. And it's nothing compared to other parts of the world where measles remains endemic.

But it is a lot compared to the United States in 2000, when we first managed to wipe out the disease. ”I think that these outbreaks are a really huge deal,” said Thomas Sandora, a specialist in infectious diseases at Boston Children’s Hospital. "Measles is essentially the most contagious disease on the planet” right now, he explained. It becomes contagious four days before the telltale rash appears, and can remain in the air for two hours after an infected individual has left the room. All it takes is for one case of measles to be introduced into a vulnerable group: About 90 percent of people who haven’t been immunized, if exposed, will become infected.

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"To me, the fact that we're seeing more cases is an indication of a problem that is increasing," agreed Stephen Morse, a professor of clinical epidemiology at Columbia University’s Mailman School of Public Health. Immunization rates in the U.S. remain high at about 90 percent, but they weren't high enough to prevent measles' resurgence. In Europe, all it took was for them to dip below 80 percent for the disease to once again become endemic -- as if it had never been eradicated in the first place. "When it reaches a tipping point ... when we have to start doing massive immunization campaigns again, the way we did in the 1960s, is very hard to say," Morse added.

Vaccines are the victims of their own success, the aphorism goes. Morse is old enough to remember what it was like in the pre-vaccination days: As a child, in 1957, he himself became infected. It was only in grad school, he said, that he learned of the serious complications he managed to avoid: they include pneumonia, loss of hearing, brain inflammation and blindness. But in the post-vaccine era, Morse said, our understanding of the balance between the benefits of vaccines and their potential to cause harm has shifted. While no vaccine is 100 percent safe, the side effects are extremely rare. When measles was eliminated, that made vaccination, for some, appear to be the greater risk.

The reminders, so far, haven't been strong enough: Even with the resurgence of the disease, anti-vaccination attitudes continue to hold strong. In New York, two of the children who contracted measles were refused vaccination by their parents. The same goes for nearly half of the 32 cases contracted in California. And that constitutes an emergency. If enough people continue to prioritize the risks of vaccines over the risks of an epidemic, "we will see measles come back" to an even greater degree, Morse said. "And I think people will feel very differently." The question now is how much damage they'll manage to do before that happens.