At the time, certain sites suggested that the increase of cases of pertussis that we have been seeing over the last number of years were related to more and more parents refusing the vaccine. Or in simple terms, it was the "anti-vaccine" movement that was to blame.
Well, according to a new report from the CDC, we can put that myth to rest. According to this report, while it is still unclear exactly what is going on, there are several factors that are suggested -
1. Waning immunity
2. Suboptimal vaccine coverage
3. Improved surveillance and diagnosis
4. The switch from whole cell vaccine (WCV) to acellular vaccine (ACV)
5. Adaptation of circulating Bordetella pertussis strains.
The first reason is a no-brainer - the immunity granted from a vaccine does wear off over time (6-10 years), and no one really expects it to last for longer than that. Interestingly enough, the outbreaks that have been seen are in older children (10-19 year old), which fits well with the theory.
The second reason, well, I know the CDC publishes data on the uptake of the vaccine and I believe the coverage is very high so I don't know how much this contributes to the problem - especially since the majority of the increase is being seen in older children where the first point is more relevant.
The third reason I am not going to talk about other than to say that it seems that increases in everything are blamed on more awareness and better diagnosis. I am sure that it plays some role, but, at the same time, give it a rest already.
With the last two reasons, I think we come to the real problem. The vaccine was switched several years back from a whole cell vaccine to an acellular vaccine. What this means, according to an article in The New Scientist is that -
A key issue is that the whole-cell vaccine contained hundreds of antigens, which gave broad protection against many strains of pertussis. But the acellular vaccine contains only three to five antigens. Our findings suggest that the use of the acellular vaccine may be one factor contributing to these genetic changesThe new vaccine does not as create as broad of immunity as the old one. And, equally important, it looks like several strains of the pertussis virus have mutated so that the vaccine no longer protects against them.
So there you have it, the most likely reason that cases of pertussis are rising is that the vaccine that is supposed to prevent it is not as effective as it used to be.