Presented by: Anne Gershon. Columbia University, New York, NY, USA.

There is no question that varicella may be a serious illness in some individuals, and its identification prior to illness may be difficult. In part for this reason, universal immunization against varicella was recommended by the Centers for Disease Control and Prevention (CDC) in 1995, in the USA. Since that time, there has been a dramatic decrease in the incidence of varicella and its complications. Originally, one dose of vaccine for 12–15-month-old infants was administered, usually at the same time as measles–mumps–rubella (MMR) vaccine. Since about 2000, however, the incidence of varicella has not continued to fall, and many outbreaks of chickenpox in vaccinated schoolchildren have been observed. In some studies, the effectiveness of vaccine has been as low as 44%, although in most studies, reported effectiveness is about 85%.

Although antibodies to varicella zoster virus (VZV) have been reported to persist for many years, there are two possible confounders. One is that the wildtype virus continues to circulate and provide a boost in immunity to vaccinees. Another is that the techniques used to measure VZV antibodies may have overestimated the number of children who responded initially to the vaccine. Recent studies of responses of healthy children to one dose of Varicella vaccine suggest that primary immune failure may occur in 10–20%, using the sensitive and specific FAMA test. When primary immune failure occurs, secondary immune failure (waning immunity) may be difficult to document, but a number of studies suggest that children vaccinated recently may have better protection after an exposure to VZV than those who were vaccinated 3–5 years before an exposure took place. When vaccinees develop breakthrough varicella, wild-type VZV may spread to susceptibles, and on occasion, severe disease may result. Due to the increasing numbers of outbreaks of breakthrough varicella in day care and schools, continued transmission of VZV, failure to continue to decrease the incidence of varicella since 2000, and the potential accumulation of susceptibles (vaccinees who had primary vaccine failure) into adulthood, the CDC instituted a two-dose schedule for varicella vaccine in June 2006. Because it is known that second doses of the vaccine provide a significant boost in humoral and cellular immunity, it is hoped that this approach will decrease both primary and secondary immune failure that may occur. Due to the development of a combined MMR–varicella (MMRV) vaccine in 2005, most children will be given two doses of this vaccine, beginning at 12–15 months of age.

It is known that zoster caused by the Oka (vaccine) strain is unusual or rare. The question has also been raised, however, as to whether decreased circulation of VZV due to vaccine use will increase the incidence of zoster in middle aged adults who were not vaccinated as children. Currently, the incidence of zoster in vaccinated and unvaccinated populations is under study. There is no firm evidence of a cause-and-effect relationship between an increase in zoster and vaccine use at this time, but there is ongoing research in this area.