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Bordetella pertussis is the bacterium that causes whooping cough – the main clinical feature of which is a severe lingering cough that can last for weeks or even months. Right now we are in the midst of an epidemic of pertussis cropping up in pockets throughout the US, most notable California. According to the CDC:

During January 1– June 30, 2010, a total of 1,337 cases were reported, a 418% increase from the 258 cases reported during the same period in 2009. All cases either met the Council of State and Territorial Epidemiologists definitions for confirmed or probable pertussis or had an acute cough illness and Bordetella pertussis–specific nucleic acid detected by polymerase chain reaction from nasopharyngeal specimens.

In addition, if the trends continue through the end of this year, which they are likely to do, this will be the highest incidence of pertussis in almost 50 years. These numbers are not in question, but there is some discussion about what, exactly, is causing it.

The tempting conclusion is that pertussis is making its way back into the population due largely to vaccine refusal and anti-vaccine propaganda. However, there is yet no data to support that conclusion. It may or may not be the case – we will know once a more thorough analysis is done of the individual cases of pertussis. And in any case, there are many factors at work.

First, pertussis has a natural tendency to cycle every 5 years or so, and this year is the peak of the cycle. This is certainly a significant part of the increase this year, regardless of other contributors.

In addition, the lack of vaccine-induced immunity is also playing a role, but not necessarily from vaccine refusal. Pertussis is a very contagious illness, partly because people are often contagious with it for days or weeks prior to knowing they have it, or that their cough is not just a common cold. Prior to vaccination pertussis was a significant cause of childhood death, causing about 8,000 deaths a year in the US alone. After the wide availability of vaccination against pertussis there has been on average about 10 deaths per year.

The current vaccine is an acellular pertussis vaccine (part of the DTaP injection, which included diphtheria and tetanus). The aP vaccine is a toxoid vaccine – it contains inactivated toxin proteins which are themselves harmless. It is therefore a very safe vaccine with few side effects. Prior to 1996 the whole-cell pertussis vaccine was used – this was similar but still contained entire bacteria (although inactivated) and had a higher incidence of side effects. The DTaP vaccine is actually less effective than the older DTP vaccine, but a little bit of efficacy was traded for increased safety.

The childhood vaccine schedule requires 5 injections between 2 months and 6 years of age. Young infants are therefore most susceptible to pertussis because they have not yet had time to get vaccinated and develop immunity. Immunity does last for years, but wanes in teenagers and older adults. Therefore periodic boosters (with a vaccine called Tdap) are recommended to maintain lifelong immunity. Incidentally, immunity from the vaccine is not much different than immunity from the illness itself:

A review of the published data on duration of immunity reveals estimates that infection-acquired immunity against pertussis disease wanes after 4-20 years and protective immunity after vaccination wanes after 4-12 years.

The factors, therefore, that are contributing to the fact that the current epidemic is likely to be the biggest in 50 years are – the natural cycle of pertussis, a lower degree of immunity from the current DTaP vaccine vs the older DTP vaccine, and waning immunity in older children and adults with low rates of booster shots to maintain immunity.

Two other factors are currently under investigation. One is the rate of undocumented aliens in California that may not have been vaccinated. The CDC reports:

Incidence among Hispanic infants (49.8 cases per 100,000) was higher than among other racial/ethnic populations. Five deaths were reported, all in previously healthy Hispanic infants aged <2 months at disease onset; none had received any pertussis-containing vaccines.

So early indications are that the Hispanic population is disproportionately getting pertussis. But the burning question is – are there pockets of low vaccination rates among vaccine-refusers, lacking herd immunity, that are also contributing to the epidemic? A recent New York Times article by Tara Parker-Pope argues that vaccine refusal is likely not a contributor because there is no association between county-wide vaccine rates and pertussis incidence. However, this argument is not valid. Counties are a mostly arbitrary political boundary, not a meaningful population or social boundary. There are small pockets of low immunization rates in communities that have been centers of vaccine-preventable diseases in the past, and it is still possible (even probable) that pertussis is having an easier time spreading through these populations as well.

Further – we are on the cusp of a new school year. Once children go back to school, the pertussis epidemic may get into full swing. There are schools that, because of their culture and policies, have very low vaccination rates. We will have to see what happens with pertussis in these schools once the classroom doors open.

Conclusion

What we can say at this point for certain is that 2010 is an epidemic year for pertussis, and this cycle will be the worst in half a century. We know that vaccination with DTaP is safe and effective, but requires booster shots as adults, and that not enough people are getting this booster shot. This epidemic is still nothing compared to the pre-vaccine era of pertussis, but it highlights the ongoing need for vaccination and herd immunity against contagious and deadly diseases like pertussis.

Whether vaccine refusal is playing a significant role has neither been confirmed or rejected by current information, but eventually this data will be available. And unlike the anti-vaccine crowd, we will base our conclusions on the evidence, not rhetorical expediency.