WASHINGTON – As the number of children vaccinated against varicella has risen, the number of chicken pox cases has proportionally declined.
No news there.
But, as the number of kids getting chickenpox gets smaller, the number of adults getting shingles is growing – and no one really knows why, Dr. Vikash Oza said at the annual meeting of the American Academy of Dermatology.
“Varicella vaccination has had a dramatic impact on the incidence of chickenpox,” said Dr. Oza, a pediatric dermatologist at New York University Langone Medical Center. “In fact, as a resident I saw not one case of chickenpox. That’s pretty astounding considering how common it used to be.”
Introduced in 1995, the vaccine now covers about 90% of children in this country – an achievement almost exactly mirrored by the decrease in disease incidence. In fact, according to a 2013 study, the bang for the vaccine buck may be even bigger, achieving 98% decreases in two regions examined (Pediatrics. 2013 Nov;132:e1134-40).
That study also found that vaccinated children who did get chickenpox had much milder infections that resolved more quickly.
Again, that’s good news, but not particularly surprising, said Dr. Oza. The more intriguing change is the steady increase in herpes zoster among older adults occurring in tandem with the decreased incidence of chickenpox. Cases were up a total of 39% from 1992 to 2010 among adults 65 and older, according to the Centers for Disease Control and Prevention (Ann Intern Med. 2013;159:739-45).
“This thing that’s happening in adults is quite interesting,” Dr. Oza said. “Although zoster in children is uncommon, we do know that children are much less likely to develop it if they’ve been vaccinated against varicella. What we don’t really know is what this means for adults. One of the effects of the vaccine, of course, is less circulating varicella zoster virus in our communities. It’s possible that a benefit of having some circulating VZV gives adults an immune boost that keeps zoster in check, and without that, developing shingles is more likely.”
Several recent studies have documented the association. A 2005 study found that during 1998-2003, the incidence of chickenpox in Massachusetts went down by 79%, while the incidence of shingles increased by 90% (BMC Public Health 2005;5:68. doi: 10.1186/1471-2458-5-68). But a shifting age-related immune response is only one possible explanation, the authors wrote. “There are several possible explanations, which may be operating in combination. Other possible explanations include increases in the proportion of people with immunosuppressive conditions and therapies, in the duration of those conditions and treatments, and/or in the prevalence of other triggering factors.”
The shifting incidences of chickenpox and shingles, however, were predicted years before they occurred. A mathematical model from 1992 predicted a long-term elevation of up to 20% in the shingles rate, relative to pre-vaccination incidence (Epidemiol Infect. 1992 Jun;108:513–28). A 2001 paper predicted that shingles would increase for 30-50 years after mass varicella vaccination, rising to a maximum of about 50% above pre-vaccination rates, before falling below baseline levels (Epidemiol Infect. 2001 Oct;127:305–14).
The CDC refutes any causative link between varicella vaccinations and shingles. “This proposed explanation seems unlikely based on two CDC studies which found that shingles rates started increasing before chickenpox vaccine was introduced in the United States, and after the routine chickenpox vaccination program started,” the agency’s surveillance pagenotes. “Other countries without routine chickenpox vaccination programs, have observed similar increases in shingles rates.”
Regardless of the “why” behind the association, Dr. Oza expects it to level out as vaccinated children come of shingles age. “As the Millenials take over, with their heightened immunity to varicella virus, we will eventually get to the point where zoster has less of an impact on clinical practice and the healthcare industry.”
He had no financial disclosures.
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