Health and Science News for Parents

A weaker DTaP vaccine and other factors in the pertussis epidemics

written by Tara Haelle

You’ve probably seen the headlines this year of the pertussis epidemics in pockets of the U.S., such as the one in Washington state the CDC reported on in July. They’re looking at a 50-year high if the numbers continue. The numbers are also spiking over 3,000 in Minnesota.

Photo by Brian Hoskins

Last time we saw numbers like this for pertussis was just two years ago in California with the 2010 outbreak. So what’s going on? Why is whooping cough all over the place like it’s the 1930s again? A study published yesterday in the New England Journal of Medicine offers one good reason, explained here, but it’s helpful to look at the big picture.

First, we’re no where near the levels of the 30s, but some states are close to 1940s and 50s numbers, and there is cause for concern. Lots of blame and various excuses have been tossed around for these increasing epidemics — blame is tossed at people skipping vaccination and excuses include the cyclical trends of pertussis outbreaks. The newer consideration in just the past few years being tossed out there is the fading effectiveness of the pertussis vaccine, which the NEJM article focuses on.

As usual, the truth of the matter is a little of column A, a little of column B and a little of columns C, D and E, plus a bit of uncertainty that epidemiologists are still working to figure out. To make sense of all these factors, I put in a call to Paul Offit, Chief of the Division of Infectious Diseases and the Director of the Vaccine Education Center at Children’s Hospital of Philadelphia. Dr. Offit invented one of the rotavirus vaccines and is known as an outspoken vaccination advocate. Unfortunately, he won’t be available to discuss this in depth until Monday, so I’m publishing this post as part one and then I’ll post a follow-up after he and I talk next week.

Until then, let’s look at what we know, starting with the new study about the waning effectiveness of the DTaP shot. The DTaP stands for diphtheria – tetanus – acellular pertussis. It’s given to kids starting at 2 months with four more boosters up until a child is 4 to 6 years old.

It turns out, they just don’t make ’em like they used to. It helps to know a bit of vaccine history here. The previous pertussis vaccine came in the DTP (or DPT) shot, which is essentially the same as the DTaP except that the DTP used whole cells of pertussis in the formulation of the vaccine instead of just pieces of the virus. The problem was that DTP could cause high fevers — high enough to cause seizures. The seizures were not dangerous. They didn’t kill kids, they didn’t cause brain damage (despite what you may have heard) and they didn’t have any other long-term negative effects. But they sure looked scary as hell. And parents understandably raised some hell about it.

So out came the DTaP in the 1990s, which eventually replaced the DTP on the CDC’s schedule in 1998. The problem, as we’re finding out now, is that the new vaccine is less effective than the old one. It’s taken this long to figure that out because enough time needed to pass for the kids who only got the DTaP (and not DTP) to grow up. Now they have, and the NEJM study finds the vaccine’s effectiveness fades quite a bit in the five years following the fifth booster. In fact, it drops about 40 percent each year. That means each year after the fifth booster, your child is 40 percent more likely to get pertussis if they come into contact with it.

But that doesn’t mean the vaccine is worthless. First, having the vaccine can reduce the severity of a disease if the child does contract it. Second, the vaccine can still be pretty protective if the last round was good enough. The NEJM study authors calculate that a DTaP booster of 95 percent effectiveness ends up 71 percent effective five years later. If the last booster was only 90 percent effective, that becomes 42 percent effective five years later.

So, the decreased power of the new vaccine is playing a part in the current outbreaks. In California in 2010 — the period the NEJM study focuses on — many of the kids hit hardest with pertussis were 8- to 11-year-olds who had all their shots. Therefore, the weaker effects of the vaccine definitely played a part in keeping the epidemic going for longer.

However, there have also been decreasing rates of vaccination in general in some areas, seen in the increasing rates of vaccine exemptions granted to school children. But vax rates haven’t decreased dramatically and it’s unclear what part non-vaccinated children play in these epidemics.

It’s also true that pertussis tends to come and go in cycles of somewhere around five years. One of those expected five-year peaks did correspond to 2010. But it doesn’t correspond to 2012 two years later. And there are other reasons pertussis has been spiking, as outlined in another NEJM commentary two weeks ago.

The commentary authors point out that one reason for the higher prevalence of the disease is increased awareness. “What with the media attention on vaccine safety in the 1970s and 1980s, the studies of DTaP vaccine in the 1980s, and the efficacy trials of the 1990s comparing DTP vaccines with DTaP vaccines, literally hundreds of articles about pertussis were published,” they wrote. Pediatricians, public health officials and parents took notice.

Another reason is better testing. The NEJM authors write that it’s only been in the past ten years that polymerase-chain-reaction (PCR) assays have been used to test for the disease. Then, in addition to mentioning the weaker DTaP vaccine compared to the DTP, they suggest that genetic changes in the disease itself cannot be ruled out. Some changes have already been seen, though evidence so far does not support the idea that this has contributed to the outbreaks.

So, more awareness? Check. Better diagnostic techniques? Check. Weaker vaccine? Check. Decreasing vaccination rates? Somewhat check. Natural cycles? Somewhat check. Genetic changes? Maybe.

I see two overriding questions, one more academic and one more pragmatic.

The academic question is, to what extent do each of these factors play in the resurgence of pertussis over the past several years? More and more evidence is pointing to the weaker vaccine, but can the waning potency of the DTaP claim the lion’s share of blame? I don’t know.  I’m not sure epidemiologists know yet either, but this is one thing I’ll be discussing with Dr. Offit.

The more important question, which will be informed by the answer to the one above, is what do we do about it? (Another question for Dr. Offit.) There is a TDaP shot — the booster given to adolescents and adults. Should parents be giving this to their 8-year-olds? So far, we don’t see those recommendations coming out. Should we revert back to the DTP? After all, it caused scary seizures with no long-term effects while pertussis causes deaths. Still, that’s unlikely to be a palatable option to anyone.

The status quo isn’t acceptable. The deaths are mostly among infants, not the grade schoolers getting pertussis in record numbers, but that’s all the more reason for the rest of us to be well-protected: babies under 2 months can’t be vaccinated and rely on the immunity of the herd for protection. If the vaccine is wearing off for 8- to 11-year-olds, that puts babies in the community at risk as well. So should researchers come up with a new vaccine that’s both effective and absent the scary side effects? Well, ideally, yea, but it’s not clear if that’s in the works yet.

For now, all we can do is vaccinate our children, take them to the doctor if they start coughing like this, or even like this or this, and wait to see what develops. I’ll be back with an update after talking with Dr. Offit.

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