Disappointing Coverage of Pertussis Vaccine Findings

Pertussis has been in the news a fair bit recently. Actually, it's the vaccine against pertussis that has garnered a lot of attention after a recent presentation and study from the 51st Interscience Conference on Antimicrobial Agents and Chemotherapy. The presentation, Marked Acellular Pertussis Vaccine Failure in 8-14 Year-olds in a North American Outbreak, suggests that immunity from the pertussis vaccine can wane in as little as 3 years, covered a study by Kaiser Permanente Medical Center under the supervision of Dr. David Witt.

The story has been picked up by a lot of media outlets, such as the Associated Press and subsequently regurgitated in places like ABC News, The Boston Globe and The San Francisco Chronicle. While the story raises some valid questions, there are some problems with how it is being presented.

First and foremost, the spin on the study is that the waning immunity from the pertussis vaccine is somehow new and shocking, something that we did not know about before. We've know since at least 2005, if not earlier, that immunity from the vaccine ranges from about 4-12 years. Similarly, immunity following natural infection with Bordetella pertussis, the bacterium that causes whooping cough, wanes in about 4-20 years. This is not news. It's something we have known for a while, yet not one of the news stories covering this subject mentions the fact, nor does it seem that the Witt et al. acknowledge this fact. From the abstract, the authors state that "acellular boosters appear to be ineffective after 3 years." This seems to overstate things a bit. Are the boosters truly ineffective, or does the efficacy simply start to wane?

Speaking of efficacy, it is unclear from media reports and the presentation abstract whether the researchers controlled for the failure rate of the vaccine. According to the CDC Pink Book chapter on Pertussis, acellular pertussis vaccines have an efficacy of around 80%-85% (PDF). That means that for every 100 individuals who complete the vaccination series, there will be around 15-20 who will not develop immunity. The selection methods, picking anyone with a positive PCR for B. pertussis, may be sufficient to control for this, but it is something that is worth mentioning, to put things in context.

This brings us to a couple other issues: case definitions and population selection. Notably, the methods used by the authors to select cases for their study differ from the case definitions used by the CDC, which include the duration of cough and additional symptoms in addition to a positive PCR. It is possible for PCR to give a false positive, especially if samples are taken in a location which also administers or handles pertussis vaccines. The technique only requires a piece of genetic material, not the entire bacterium, to produce a positive result. It is therefore possible that PCR alone may indicate a patient is infected with pertussis when they actually have a different respiratory illness. It is not clear how Witt et al. controlled for false positives from the PCR.

What was also missing from both the study and the media reports was a sampling of the population who had received the vaccine in a similar timeframe to the 132 clinical cases identified, who had been exposed to the bacteria, yet who did not develop clinical illness. We know that in the 132 cases in the study, immunity waned in as little as 3 years. However, the level to which this represents the community at large is unknown. How many individuals were exposed to pertussis but remained immune thanks to the vaccine?

These are all very important questions which were not even glossed over in media accounts. They simply weren't considered at all. The down side of this is that the media reports play into anti-vaccine propaganda that the vaccines "don't work" or that because they may provide only short-term immunity, then "what's the point?" The acellular pertussis vaccines do have shortcomings; immunity can wane and efficacy is not as good as other vaccines. There is certainly room to improve it in terms of both efficacy and duration of immunity. However, the fact remains that it is pretty effective and immunity can last for much longer than 3 years. It has had a tremendous impact on overall pertussis rates, reducing incidence of disease by nearly 80% since it was first introduced (see the Pink Book link above). The triumph of this vaccine has meant that significantly fewer infants die from this disease and non-fatal complications are far less common in the population at large.

In the end, I have been extraordinarily disappointed in the media coverage of the new study. It has lacked basic investigation into what we already know of the science behind the vaccine and ignored the nuance of the subject. To all the science journalists out there, let's try to raise the bar a bit.