© 2016 Roy Benaroch, MD
A large, sobering study published in the March, 2016 edition of Pediatrics illustrates just how far we still need to go to effectively control pertussis.
Pertussis, also known as ‘whooping cough’, is a serious illness. Older children and adults get to enjoy a horrible cough for about three months—a cough that sometimes makes people vomit, break ribs, or pass out. Seriously. You haven’t seen a “bad cough” until you’ve seen the cough of pertussis. Worse: in little babies pertussis can cause breathing problems, seizures, and death. Though its caused by a bacteria, antibiotics (unless given very early) are ineffective at reducing the length or severity of pertussis. Prevention, in this case, is worth far more than a pound of cure.
Up until the mid-1990s, infants and children routinely received the whole-cell DTP vaccine (DTP = diphtheria, tetanus, pertussis.) It worked at preventing all three of these diseases, but had a relatively high rate of side effects, mostly fevers. Many of the suspected more-serious side effects (like encephalopathy and seizures) are now known to have been caused by genetic conditions, not the vaccine, but nonetheless parents and doctors alike welcomed a newer vaccine, the acellular DTaP. This newer vaccine, which replaced DTP in the United States by around 1998, caused fewer fevers, and was thought to cause fewer serious reactions, too.
The problem is that it just doesn’t work as well. And as the first generation of infants to get an all-DTaP series starts to go through adolescence, we’re starting to see the unintended consequence of that vaccine change.
In the current study, researchers used a huge database of information from the Kaiser Permanente system of Northern California. We’re talking solid, big-data research, here, the kind of study that requires consistent and reliable data across a huge set of patients. In this case, about 3.5 million patients across 55 medical clinics and 20 hospitals, using centralized labs and an integrated medical records system. If health things happen to this population, Kaiser knows it.
In 2010 and again in 2014, California experienced large epidemics of pertussis. A total of 1207 cases were among Kaiser teenagers, all with complete records of their pertussis vaccination status. And the results aren’t anything to be happy about. In the first year after an adolescent pertussis (Tdap) booster, the vaccine was about 70% effective in protecting against pertussis. Not great, but not terrible, either – until you look a few days down the road. The vaccine effectiveness drops off dramatically, year after year, down to only about 9% by four years after receipt of the vaccine.
Why does Tdap seem to provide such poor protection—much worse than was seen in the original licensing studies? It’s a generational change, and it goes back to the shift from DTP to DTaP in the mid-1990s. By now, these teens in California are old enough to have received DTaP, not DTP, as infants. The authors looked at the specific ages of pertussis cases during the 2010 and 2014 outbreaks, and the trends support the conclusion that teens who received DTP as infants get good, lasting protection from Tdap; teens who got DTaP do not.
Now what? Clearly, we need a more-effective vaccine, perhaps even resuming the use of whole-cell pertussis vaccine, at least for the earlier doses. But in the meantime, we have to do the best we can with what we have. Vaccinating pregnant women with Tdap does effectively prevent pertussis in their babies, especially when they’re the youngest and most-vulnerable. And adults (who got DTP as children) should get Tdap boosters too, to protect the children around them. Another idea (floated by the study authors) is to use Tdap in teens not as a routine booster, but as a strategy to control local outbreaks, taking advantage of the higher effectiveness seen for the first year after vaccination.
I don’t have the answers. I’m not happy to see studies like these, but examining and re-examining vaccine safety and effectiveness is something we need to continue doing, with an open mind, relying on solid evidence. Bottom line: with pertussis, we need to do better.