When Should We Order Varicella Serology Tests?
ABSTRACT & COMMENTARY
Synopsis: Testing for varicella antibodies is recommended before immunization with the varicella vaccine in 9-12 year-olds who have an uncertain history of chicken pox, since 48% of these children are seropositive.
Source: Lieu TA, et al. Ped Inf Dis J 1998;17:120-125.
Lieu and associates from the north california kaiser-Permanente Medical Group studied varicella seroprevalence among 7- to 12-year-old children with a negative or uncertain history of chicken pox to determine whether it was more cost effective to immunize all such children or only those who were found to be seronegative on testing. Nine hundred thirty-seven children aged 7-12 years were seen in regular, non-emergent medical visits between 1995 and 1996. In this practice, varicella serology was routinely obtained on all children 7-12 years of age who had either a negative or uncertain history of chicken pox, as well as some children with a definite negative history. Varicella seroprevalence in children whose parents believed that they definitely did not have varicella averaged 25% (9% in 7-year-olds to 33% in 12-year-olds). Varicella sero-prevalence in children whose parents were uncertain about a history of chickenpox averaged 48% (20% in 7-year-olds and 52% in 12-year-olds). Even among children with a definite history of not having chicken pox, seropositivity averaged 25%.
Lieu et al addressed the clinician's dilemma of whether to recommend testing of varicella serology or to vaccinate preemptively without testing children 7-12 years of age whose parents believe there is no history of chicken pox or who are uncertain. Cost is an important consideration in large medical groups like Kaiser-Permanente. They estimate that it would be most cost effective to recommend serologic testing before immunization in children 7-12 years of age with uncertain histories of chicken pox. Seven to 12-year-old children with negative histories should receive the vaccine.
COMMENT BY ROBERT BALTIMORE, MD, FAAP
The Committee on Immunizations of the AAP recommends universal varicella immunization for children 12-18 months of age as well as all children 18 months to 13 years who have not been immunized previously and who do not have a reliable history of chicken pox.1 However, what should be done if the parents of a child 7-12 years of age believe he or she either has not had or is unsure whether they have had chicken pox? The alternative is to immunize all such children or to test for varicella seropositivity and then only immunize those children who are seronegative.
Lieu et al, working in a large California HMO, surveyed parents of children in whom a varicella serology had been ordered during a one-year period and compared the seropositivity with a definite or uncertain history of clinical chicken pox. Interestingly, nearly 25% of children whose parents were certain that their child had not had chicken pox were seropositive. This study extends earlier ones that demonstrated that young adults with a negative or uncertain history of chicken pox are usually seropositive.2 Chicken pox can be a mild disease with few noticeable pox lesions, which probably accounts for these findings.
However, one cannot automatically assume that an adult is immune in the absence of a positive history of chicken pox. We have seen too many medical students and student nurses who have contracted chicken pox while working in pediatrics. Serotesting followed by immunization of those who are susceptible is mandatory for individuals with negative or uncertain history who are at high risk of exposure due to occupation. Of course, the increasing rate of varicella immunization may change these recommendations in the future. (Dr Baltimore is Professor of Epidemiology and Public Health, Yale University School of Medicine.)