Last month, a study in the New England Journal of Medicine declared victory for the chickenpox vaccine. Immunization against chickenpox—"varicella" to the cognoscenti—was introduced in the United States in 1995, about 10 years after a similar vaccine was licensed for use in Japan. Since then, according to the February study, chickenpox-related deaths in the United States among people aged 50 and under have dropped from an average of 145 a year to an average of 66. Four million kids and adults a year used to get sick with the chickenpox, and about 12,000 went to the hospital with complications. Now the incidence of the illness has declined by as much as 90 percent.
So, what's not to like? A separate study, also in NEJM, reported that the effectiveness of the chickenpox vaccine wanes over time, from 97 percent in the first year to about 84 percent in the eighth year. Other studies have reported about a 30 percent rate of "breakthrough" cases of chickenpox six or seven years after vaccination. Fortunately, the vaccine tends to fail partially and gracefully—immunized children will likely have a shorter and milder illness. But what if giving children the vaccine might increase the risk that non-immunized adults could develop a different disease that is very unpleasant? And if older children end up needing a second dose of the vaccine to maintain their immunity, is it possible that, at least in economic terms, the chickenpox vaccine may not be worth the cost of administering it?
Vaccinating against the chickenpox wasn't an obvious call: Perceived as a universal and mild childhood malady, the disease didn't naturally lend itself to the trouble or expense of immunization. So, as usually happens when a potentially iffy product is introduced, a case for it was made. First, the disease was shown to be not as universally mild as people thought, as evidenced by the 100-plus annual deaths attributed to it. In addition, Japanese studies suggested that the new vaccine might help prevent the painful adult disease called shingles. (The chickenpox virus causes shingles by insinuating its genes into the genetic code of the cells that it infects, remaining there in a latent state, and then sometimes being reactivated.)
As a product, chickenpox vaccine has proved to be safe and reasonably effective, and now it is pretty much universally recommended by pediatricians and public health authorities. Along with social benefits, public health economists generally justify the introduction of a new vaccine by comparing the aggregate price of the product with the money it will save in reduced medical and other costs. The chickenpox vaccine is expensive—the pharmaceutical industry charges my pediatric practice about $46 per dose to recoup its research and development costs and to turn a substantial profit. Because the illness is mostly mild, it's hard to make a good economic case for it based on medical savings. But the ledger balances in favor of vaccination when you take into account another potential savings: parental work time. Itchy, infectious kids often have to stay home from school for five or six days, and someone has to stay home with them. Six days multiplied by almost-every-child comes to a whole lot of lost time at the office—a significant drain on the economy.
But now the questionable durability of the immunity produced by the vaccine may alter the cost-benefit calculus. Older studies have shown that immunity to chickenpox (which historically has been virtually perfect after an attack of the virus) seems to depend on re-exposure. Those findings have been borne out in Japan, where some kids are immunized against chickenpox and others are not. It turns out that the vaccinated kids keep up high levels of protection because they are exposed over and over again to unprotected kids who catch the disease and pass it on. Each time such an exposure occurs, the immunized kids get a little "boost," which stimulates their immunity. Doctors think that the same thing happens to older patients who are at risk of shingles because they once had chickenpox—every time they're exposed to poxy youngsters, their immunity gets a kick, which helps to suppress the reactivation of the latent virus as shingles.
But now that practically every child in the United States has been given a dose of chickenpox vaccine, neither kids nor adults will have the opportunity for re-exposure. The initial hope that the vaccine would help prevent shingles (because immunized people are less likely to develop shingles than people who have had the natural infection) may unravel, both for the large number of people who had chickenpox before the vaccine was introduced and for the children who are now being immunized. It is too soon to tell, but we may well need to routinely re-immunize children and perhaps even adults to prevent recurrent chickenpox and shingles—a far more costly proposition than we originally bargained for.
Perhaps we should have seen this coming. Though a few vaccines (measles and yellow fever are good examples) seem to give lifelong protection after a single dose, most need to be repeated at regular intervals to maintain their effectiveness. Tetanus boosters are given every 10 years (when did you last have one?). That was also the recommended interval for the smallpox vaccine, back in the days when we used to give it. I keep wondering if our early-on enthusiasm for the new chickenpox vaccine might have led us to ignore our past experience.
Now what? To ensure an uninterrupted supply of vaccines, the government has set up an extraordinary mechanism to protect manufacturers from the threat of lawsuits. Every dose of each vaccine is taxed and the proceeds are set aside to compensate any vaccine recipient who is harmed by immunization. Manufacturers cannot be sued—an unusual benefit. Maybe the manufacturers should have to offer something in return for this special gift in the event that a new vaccine turns out to cost much more than anticipated: lower profits. There is a limited amount of money available in the United States for children's health care in general and for vaccines in particular. If society is expected to simply swallow the extra cost of multiple doses of the chickenpox vaccine, there is a serious risk that in the future we won't be able to afford more immunizations like it.