Nov 02 2010

The Chicken Pox Story

Prior to 1995 about 95% of the US population would get chicken pox by age 18. It was essentially a rite of passage, and almost everyone had their pox scar to show for it. In 1995 a chicken pox vaccine was introduced, and about 60% of children receive the vaccine. This has dramatically decreased the rate of chicken pox, with some interesting results.

Some Background on Chicken Pox

Chicken pox is caused by the Varicella zoster virus (VZV), a member of the herpes family of viruses. It causes a primary infection which results in a day or two of fever and malaise followed by a week of a macular papular rash – pustules filled with fluid. In children the disease is commonly mild, but not without complications. According to a review by Dr. Lichenstein:

Varicella also accounts for significant morbidity (4000 hospitalizations per year) and mortality (50-100 deaths per year) in otherwise healthy children; moreover, the annual cost of chickenpox has been estimated at $400 million in medical costs and lost wages in the past.

In immunocompromised children, such as those with leukemia, mortality rates from varicella have ranged from 7-28%. The case-fatality rate in the general population is 6.7 case per 100,000.

In addition, once the primary infection is over the virus is not eradicated. It moves into the dorsal root ganglia (sensory nerve cells just outside the spinal cord) and becomes dormant. But it can be reactivated later in life, usually (but not necessarily) in times of physiological stress, and can cause shingles, or herpes zoster.

Shingles is a rash in the distribution of 1 to a few spinal nerves. It can be painful, and when the rash is oozing it is infectious and can cause chicken pox in those who are not immune. Also it can be complicated by a condition known as post-herpetic neuralgia – a burning type of nerve pain in the distribution of the rash. PHN can be very painful and very difficult to treat.

The burden of disease like chicken pox is also disproportionately born by those of low income. Imagine a household with 2-3 children, each suffering from chicken pox lasting about a week but with an incubation period of 2-3 weeks. The household can be sick for a month, with the children having to stay home from school. This can be a significant financial burden on working families.

Imagine also being a child with a chronic medical condition leading to immunocompromise. Every time there is a reported case of chicken pox in your school, school becomes a potentially fatal place.

In short, while chicken pox is a mild disease for most children, it comes with a significant burden of morbidity, mortality, pain and suffering, financial costs and lost productivity.

The Chicken Pox Vaccine

The current chicken pox vaccine is a live attenuated virus – essentially it works by giving a mild infection, usually subclinical (which means it causes few or no symptoms), but sufficient to provoke an immune response. A single dose vaccine is about 75% effective in preventing chicken pox. In the 25% that do get a breakthrough infection, the infection is mild with less morbidity and mortality. However such infections are still contagious, and so the single-dose vaccine does not prevent outbreaks. Further, the risk of breakthrough cases appears to increase over time. Effective immunity significantly decreases after about 10 years following a single dose.

For this reason the current recommendation is for a second booster injection with the vaccine, which increases antibody titers and reduces breakthrough infection by 2/3 (in one study from 7% to 2.2%).

Because Varicella zoster infection is a lifelong disease once contracted, concerns arose about unintended consequences from a chicken pox vaccine program: shifting of the average age of contraction to older individuals, and increases in shingles.

Chicken pox is a more serious illness in older children and adults. Therefore protecting children from chicken pox may result in acquiring the disease as an adult, with a resultant increase is morbidity. Further, the fact that chicken pox is so common means that older children and adults (prior to the vaccine) were routinely exposed to the virus, resulting in a periodic natural booster to immunity. By preventing outbreaks, these natural booster shots are reduced, resulting in waning antibody titers (even in those who had the full infection and not the vaccine).

This question has been studied. Since there are so many variables there is considerable uncertainty as to what the net effect of a vaccination program would be. However, epidemiological modeling has shown that the overall morbidity would likely decrease with a vaccine program. Reduction in total chicken pox cases would be partially offset by a shifting upward of the age of infection, but there would still be a net benefit.

Further, the reduction in outbreaks of chicken pox leading to reduced exposure and immunity in the older population would likely lead to an increase in shingles cases. However, this effect would be temporary (essentially a generation) and eventually shingles cases would be reduced as we get an older population that never had chicken pox (remember, shingles only occurs in those who previously had the chicken pox infection, not the vaccine).

Because of the nature of Varicella zoster creating an optimal vaccine program has some challenges, but these challenges are being studied carefully. At present the recommendations are for a two-dose vaccination schedule to produce the best results and longest immunity. There may be the need for a booster in older children or young adults to maintain immunity. Further there is now a vaccine for those over 50 to reduce the risk and severity of shingles.

Essentially, in order for the vaccine program to be most effective we may need to replace a lifetime of multiple exposures to the wild version of the virus with multiple vaccinations over one’s lifetime to maintain immunity.

Let the Crank Fest Begin

Because of the complexity of the chicken pox vaccine there is a lot to cherry pick from if one is interested in spreading fear and misinformation in order to scare the public off the vaccine. Don’t worry – Joseph Mercola is on the job.

He begins, as many anti-vaccinationists do, by downplaying the severity of the disease the vaccine is meant to prevent:

It is estimated there were about 3.7 million cases of chickenpox annually in the U.S. before 1995, resulting in an average of 100 deaths (50 children and 50 adults, most of whom were immunocompromised). This hardly represents a dire, life-threatening epidemic that requires mass vaccination of all children!

Of course, if you are one of the 100 people who die, that is quite a dire and life-threatening epidemic. But Mercola also omits all the other aspects of the total chicken pox burden that I outlined above.

As with all such questions, the bottom line is – what is the risk vs benefit of getting the vaccine. The evidence strongly suggests that the total benefit outweighs the total risk – but some of the risk/benefit is subjective, like the risk of pain, the loss of work and school time, or financial loss. People are free to decide for themselves if the risk/benefit is worth it, but that requires full and fair information, not Mercola’s dismissiveness.

Incidentally, Mercola in this article recommends honey for treating Herpes zoster, claiming it works better than “the drugs.” He cites a prior article he wrote which in turn cites two sources, both anecdotal and dealing with oral and genital herpes outbreaks, not zoster. So the evidence is very low grade and not even relevant. There is no data with honey and herpes zoster or, more importantly, post herpetic neuralgia.

Mercola goes on to discuss the same issues I discuss above, but with an anti-vaccine spin rather than a sober analysis. He cherry picks the data, and ignores those analyses which show a net benefit for the vaccine, even with these issues, and the offered solutions.

Conclusion

Chicken pox is far from the worst infectious disease that plagues humanity, but it does come with a significant burden of morbidity and mortality. We have the ability to reduce this burden with a rational vaccine program. Such a program has some interesting challenges, such as the consequences on long term immunity and the incidence of herpes zoster, but these are manageable.

The relevant issues require further monitoring and study, and optimal recommendations will likely be tweaked going forward.

Unfortunately, amidst this science-based conversation, there are anti-vaccine cranks who seem to be interested only in poisoning the well and spreading confusion.

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