Why are babies dying of old-fashioned whooping cough?

In the past few years, diseases that vaccines are expected to prevent have flared across the country. Whooping cough killed 10 babies in California in 2010 and the next year measles sickened 21 people in an outbreak in Minnesota. Now this year, measles has struck 14 in Indiana, causing terror along the way with reports that one infected person had visited Super Bowl village. And whooping cough is on track to infect more people in the United States than it has in 50 years.

You can lay much of the blame for the measles outbreaks on the alarming number of parents who don’t vaccinate their kids. Both the Minnesota and Indiana measles episodes were traced to unimmunized people who had picked the disease up abroad and then spread it to others, many of whom were also unvaccinated (or unsure), according to the Centers for Disease Control and Prevention.

But the story of the whooping-cough outbreaks is more complex, with multiple — and unexpected — sources of risk. To be sure, the illness has struck unimmunized children. But the biggest problem is that the current vaccine wears off faster than researchers anticipated. So substantial numbers of vaccinated children are getting the disease. And since adults are supposed to get a booster, and many haven’t, they’re also vulnerable, even if they got all their shots as kids. The vaccine refusers aren’t helping, but the current epidemic is bigger than they are.

In the bad old days, whooping cough, like measles, infected nearly all children, often causing terrible sickness or even death (listen to the characteristic cough, especially harrowing in babies). It was with the advent of a vaccine for whooping cough, created in the 1940s, that the number of deaths plummeted. Known as DPT (it protects against diphtheria and tetanus as well as pertussis, the scientific name for whooping cough) the 1940s formulation also, however, caused serious side effects — perhaps more so than other childhood vaccines. Many kids developed fevers, some high and a small number had seizures.

The side effects gave rise to legitimate concern — and also to fear mongering. In 1982, a Washington, D.C., television station broadcast particularly irresponsible “claims of vaccine-induced brain damage, mental retardation and permanent neurological damage,” as Seth Mnookin relates, and debunks, in his superb book, “The Panic Virus.” The anti-vax movement didn’t need to hear more. The infamous vaccine skeptic Barbara Loe Fisher became active in the wake of the broadcast, convinced that DPT had caused her son’s developmental problems.

So under intense pressure, researchers set about making a vaccine with fewer side effects. In the late 1990s, the Food and Drug Administration approved a new formulation, called DTaP, for babies and children. Before this, the vaccine used dead whole cells of pertussis to stimulate kids’ immune systems. Now the newer version deployed only a few selected compounds, not cells. The good news is that it hasn’t caused as many side effects. Early clinical trials suggested that this newer, acellular vaccine was also highly effective.

But as Tom Clark, a pertussis expert at the CDC, told me, the studies fell short. They tended not to follow children for a long enough time. Or they defined cases in a way that missed milder infections. As a result, the studies missed a dire fact: The new vaccine doesn’t actually work for as long as the old one.

Now we’re feeling the painful effects. During the 2010 whooping-cough outbreak in California, the largest number of cases, age-wise, were infants under the age of 1. But a notable spike was also seen in kids aged 7 to 10, most of whom had received all of the recommended shots — at 2 months, 4 months, 6 months, 2 to 3 years, and 4 to 6 years of age. These kids were supposed to be safe. What’s more, their risk seemed to increase with age, with the 10-year-olds most likely to get sick. When the CDC picked up that pattern, it “leapt out at us,” Clark says. These were kids who hadn’t received any doses of the old, whole-cell vaccine, which had been phased out completely by 2000. So the uptick strongly suggested that the acellular vaccine’s effects were wearing off year-by-year as the kids got older — long before anyone had anticipated.

Data from this year’s epidemic tell a similar story. Kids receive a booster shot at age 11 to 12, and that helps; still, today’s 13- and 14-year-olds, who may have received only acellular vaccine, are also more likely to get the disease.

That’s the trouble with the current vaccine. Now here’s how parents who don’t give it to their kids, quite apart from those flaws, are making things worse for all of us. Unimmunized children are simply more likely to get the disease than their vaccinated peers, even with the limitations of the current formulation. And when they do, they are more apt to develop severe symptoms that last longer. This means they’re more likely to pass the disease on to others, including infants, who are at greater risk of dying. Nationally, the anti-vaxers may not be responsible for most of the cases in the spate of recent outbreaks. But that’s mainly because they make up a small fraction of the population.

In the long run, the most important step is a better vaccine. Researchers might add more of the components found in the old one, and try to create long-lasting effectiveness while skirting the old side effects. But that could take a while. The scientific challenges particular to a whooping-cough vaccine are daunting. Unlike measles (or rubella or varicella), pertussis mainly infects the respiratory tract rather than invading the bloodstream. So giving someone a shot intended to produce circulating antibodies, as other vaccines do, may not work as well, because that’s not where the pathogen is mainly found, as Sarah Long, chief of the section of infectious diseases at St. Christopher’s Hospital for Children in Philadelphia, explained to me.

For now, then, the best plan is to double down with the vaccine we have. CDC guidelines give physicians a window for administering whooping-cough shots to children: They can give the first one at six weeks instead of the standard 2 months. They can also offer the 11- to 12-year-old a booster at age 10. This might be a good idea in areas with high levels of disease, says Long.

Experts might also consider an updated schedule involving more booster shots, though Clark cautions that’s premature. The CDC advises pregnant women to get the vaccine, preferably in the third trimester, so that some antibodies will cross the placenta and continue to circulate in the newborn. Also, since babies are most likely to get pertussis at home, anyone in contact with a newborn, including grandparents and caregivers, should be immunized. (A smart New York law now requires hospitals to recommend the vaccine to new parents.) But with so much whooping cough in the air, no adult should go without the recommended booster (which for adolescents and adults is called Tdap). Only about 10 percent have done so currently, which is a far cry from herd immunity.

Like it or not, you are part of this herd. So if you haven’t gotten around to the shot, do it now. Seriously.

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Amanda Schaffer is a science and medical columnist for Slate.