Practical questions and answers about childhood immunization

This new feature will appear regularly in Paediatrics & Child Health and is meant to address practical questions without ready answers in standard references such as the Red Book or the Canadian Immunization Guide. Readers are invited to submit questions to the journal office. A timely response will be provided, whenever possible, from one member of a panel of experts. The most interesting exchanges will be selected for publication. Submitters should identify themselves, but will be given the option of anonymity in the published version. Submitted questions may be edited for clarity and brevity. David Scheifele MD Associate Editor, Paediatrics & Child Health QUESTION: A 17-year-old male who was susceptible to varicella was given the first dose of the live attenuated varicella vaccine. One week later, he developed a generalized varicella-like rash, with approximately 50 vesicular lesions. He had no known chickenpox exposure in recent weeks. Is a second dose of vaccine necessary? Should he be tested for seroconversion? Kevin Cho MD family physician Toronto, Ontario ANSWER: The two-dose schedule of varicella vaccine for adolescents and adults is based on an overall seroconversion rate of 75% to 95% after the first dose and 99% after the second dose (1). Whether a vaccine induced varicella-like rash implies a ‘take’ and obviates the need for a second dose is not known for certain. A sensitive, reliable antibody test for immunity to the virus would provide an answer, but commercially available assays are not sensitive enough to detect vaccine-induced antibody levels, which are lower than after natural infection. Therefore, the decision to provide a second dose needs to be made on other considerations such as safety, cost and likelihood of protection. A generalized varicella-like rash is a recognized side effect of the live attenuated varicella vaccine. It is known to occur in 5% of adults and adolescents after the first dose and in 1% after the second dose of Varivax III (Merck Frosst, Canada) (1). Because wild-type varicella is circulating in our communities, it is possible that any single case of postvaccination varicella-like rash is due to exposure, recognized or otherwise, to wild-type varicella, rather than vaccination. How often this occurs was addressed in a postmarketing survey of varicella vaccine safety (2). Among 62 cases of generalized post-vaccine rash in which the strain present in vesicles was characterized, 38 wild-type and 24 vaccine strains were identified. Wild viruses were more likely to be identified in rashes occurring in the first two weeks after immunization (31 of 33 samples), and caused a median of 100 lesions (range 10 to 1000). Vaccine strains were more often identified in rashes occurring three to six weeks after vaccination (22 of 29 samples), and caused a median of 51 lesions (range 1 to 500) (2). Thus, the vaccine virus was more frequently associated with rashes occurring later and with fewer lesions. This makes intuitive sense, since one would expect rashes caused by the vaccine virus to appear after the incubation period for varicella zoster virus, which is usually 14 to 16 days. However, the clinical features of the rashes overlap substantially, so it is not possible to distinguish between vaccine and wild type associated rashes with confidence. In this case, the rash, which occurred one week after vaccination, was more likely to be caused by an unrecognized exposure to circulating wild-type varicella zoster virus before immunization. If so, the patient is probably immune. Even if his rash was caused by the vaccine, the number of lesions does suggest a ‘take’, and again, he is likely to be protected. However, there are no available data on the reliability of vaccination-related rash as a predictor of immunity. If cost is an important consideration in this case, a second dose can be forgone. If cost is not a significant barrier, giving a second dose is the better option because completing the two dose series would virtually guarantee protection. A second dose is safe and recurrence of the rash is highly unlikely. Finally, it is important that this case be reported to the local public health unit as a vaccine-associated adverse event. Our understanding of what constitutes an expected side effect relies on consistent reporting from vaccine providers. We thank Dr Anne Gershon of Columbia University Medical School, New York, for expert advice.

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