News of the Day ... In Perspective3/17/2007
Chickenpox vaccine protection waning
While chickenpox vaccine has substantially reduced the incidence of varicella, protection wanes with time, and older children and adults are at greater risk of complications from the disease.
Chickenpox is 36 times more common 9 years after vaccination than in the first year after vaccination, according to a study of active surveillance data from 350,000 persons. The annual breakthrough rate increased from 1.6 cases per 1,000 person-years within 1 year after vaccination to 9.0 per 1,000 person-years at 5 years and 58.2 per 1,000 person-years at 9 years (Chaves SS et al. N EnglJ Med 2007;356:1121-1129).
Unvaccinated persons who get the disease later in life have a 20 times higher risk of death compared with a child, and a 10 to 15 times higher risk of needing hospitalization, according to Dr. Jane Seward of the Centers for Disease Control and Prevention (CDC). With fewer natural cases of the disease, unvaccinated children or those whose vaccination fails to work are getting chickenpox later in life (NY Times 3/15/07).
It has been hypothesized that exogenous reexposure to the virus may be needed to boost humoral and cellular immunity to varicella-zoster virus, write Chaves et al.
The United States has been vaccinating against chickenpox since 1995. Before 1995, 73 percent of cases occurred in children under the age of 6 years. In 2004, only 30 percent of cases occurred in children this young. Peak incidence in vaccinated children is between 6 and 9 years, and in unvaccinated children, the peak is between 9 and 12 years.
One dose has an efficacy of 80 to 85 percent. A second dose is now recommended for children between 4 and 6 years, with catch-up doses to children, adolescents, and adults who only received one dose.