Apr 04 2012
The Washington State Department of Health has released a statement stating that they are in the midst of a whooping cough epidemic, which will likely reach its highest levels in decades. So far this year there have been 640 cases, compared to 94 cases over the same time period last year. This is a dramatic increase. Whooping cough is a vaccine preventable disease, and so the resurgence of this infection raises questions about the efficacy of the vaccine program – specifically, to what extent is this increase due to vaccine refusal vs waning efficacy of the vaccine itself?
Whooping cough is caused by the Bordetella pertussis bacterium (a Gram-negative, aerobic coccobacillus, for those who are interested), which produce a toxin that paralyzes respiratory cells and causes inflammation. The result begins like an ordinary upper respiratory infection (a common cold) but then develops into a severe cough which can last for weeks. The name of the disease, whooping cough, comes from the sound made by the sudden inhalation after a sustained cough. The disease can be severe at any age, but is especially pernicious in infants, in whom it can cause apnea, or brief pauses in breathing. In infants less than 1 year of age half will need to be hospitalized and 1 in 100 will die.
The pertussis bacterium was first isolated in 1906 by Belgian scientists Jules Bordet and Octave Gengou. In 1939 researchers at the Michigan Department of Public Health demonstrated the efficacy of a vaccine against Bodetella pertussis. The vaccine reduced the incidence of whooping cough from 15.1 to 2.3% and reduced the severity of the illness in those who contracted it. In 1948 the whole cell pertussis vaccine was combined with vaccines for diptheria and tetanus to make the DTP vaccine.
In the 1990s the DTP vaccine was replaced with the DTaP vaccine, which is still used. The whole cell pertussis component was replaced with a acellular vaccine. In the whole cell version the entire Bodetella pertussis bacterium is included in the vaccine in an inactivated state. The acellular vaccine contains only components of the bacterium. The change was made because the acellular vaccine has fewer side effects. However there was also unsubstantiated concerns about the risks of the whole cell pertussis vaccine that may have contributed to the decision to change. Both versions of the vaccine are effective, but some studies suggest that the acellular version provides less immunity than the prior whole cell vaccine. Today there is also the Tdap vaccine, which is intended as a booster vaccine for adolescents and adults.
Historically the introduction of the pertussis vaccine resulted in a 92% decrease in morbidity and 93% decrease in mortality from whooping cough.
The Return of Pertussis
Up until a few years ago pertussis was under good control in developed countries with a vaccine program. Pertussis has not known host outside of humans so it is even possible to achieve eradication. But in the last decade pertussis has been making a comeback. There is likely no single cause for this, but several can be identified.
There is evidence that the bacterium is evolving new strains that are less well covered by the DTaP vaccine. A recent study, for example, found that a new strain is emerging in Australia that is not well covered by the vaccine and is therefore spreading, because the existing strains are being selected against by the vaccine. The study also found that the older whole cell version of the vaccine produced wider coverage (better coverage for different strains) than the newer acellular vaccine. So it is possible that the return of pertussis is partly due to the narrowing of coverage that is allowing for the spread of newer strains.
To the extent that this is true it implies that we need to update the vaccine to cover the newer strains. This, however, may be only a temporary fix and still more strains may develop. Therefore we either need to chase new strains as they emerge, or we need to develop a pertussis vaccine that has broader antigen coverage, more similar to the older whole cell vaccine, but without the increased side effects.
Another contributing factor is the waning immunity provided by either infection or vaccination. Antibodies against pertussis do not last a life time, so adults who were either infected or vaccinated as children may have lost their immunity. They then provide a vector for the infection of young infants who are not yet old enough to be vaccinated. For this reason the CDC is recommending booster vaccines for teens and adults, especially health care workers and those exposed to young children.
Finally there are serious concerns about vaccine refusal as a contributor to the resurgence of whooping cough. Thanks to the anti-vaccine movement there is unsubstantiated fear about the safety of vaccines. In particular there are pockets of vaccine refusal resulting in a loss of herd immunity. Herd immunity results when enough of the population is immune so that an infectious disease cannot spread, so cases become isolated and do not cause an epidemic. Without herd immunity pertussis infections can spread through a population.
The evidence indicates that unvaccinated children are at higher risk of developing whooping cough than vaccinated children. Existing herd immunity is not sufficient to protect the unvaccinated, even in areas of high vaccine compliance. Further there is early evidence that whooping cough is spreading the most in counties that have a high vaccine refusal rate. The correlation between whooping cough and vaccine refusal needs to be studied in more detail, but certainly vaccine refusal is not helping and is contrary to the goal of increasing coverage to achieve herd immunity.
Historically controlled infections, like whooping cough, have the potential to make a comeback. There is a certain amount of complacence today toward diseases that are thought to be largely a thing of the past. However we are now experiencing a resurgence of some of these diseases, like whooping cough, and while the causes are complex and need to be teased apart, they are worsened by vaccine refusal which in turn is spawned by an anti-vaccine movement that is spreading misinformation and unwarranted fears.
With regard to the current whooping cough epidemic, the CDC has many specific recommendations for the public and health care workers. In short – vaccinate your children and get a booster.
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