Don't Confuse Varicella Virus Vaccine with Varicella-Zoster Immune Globulin

Author: Kate Kelly, PharmD, Editor, ISMP Medication Safety Alert! Community/Ambulatory Care Edition, and Allen J. Vaida, PharmD, FASHP, Executive Director, Institute for Safe Medication Practices

Problem
To prevent or attenuate illness from chickenpox, varicella-zoster immune globulin (VZIG) should be given intramuscularly within 4 days of significant exposure to susceptible pregnant women or others at elevated risk for severe disease or complications. Some pregnant women, however, have accidentally received varicella virus vaccine (Varivax), which is specifically contraindicated in pregnancy. These cases continue to happen despite a 1996 Centers for Disease Control and Prevention (CDC) publication (Unintentional administration of varicella virus vaccine?US 1996. MMWR. 1996;45:1017-1018) and an ISMP alert (December 4, 1996) that warned health professionals about 7 prior cases.

Infection with wild varicella-zoster virus during the first half of pregnancy may result in congenital varicella syndrome, characterized by limb atrophy, extremity skin scarring, ophthalmologic abnormalities, and brain damage. According to the CDC, the risk of congenital abnormality is about 2% for infections occurring during weeks 13 to 20 of pregnancy. Because varicella virus vaccine is a live, attenuated product, labeling specifically notes that its use is contraindicated in pregnancy and that pregnancy should be avoided for 3 months after vaccination.

CDC and Merck, the vaccine?s manufacturer, established the Varivax Pregnancy Registry to monitor cases where immunization occurred just before or during pregnancy. Among 600 reports in the registry, 22 involved medication errors in which the vaccine was given to women who should have received VZIG. In 17 of these cases where follow-up was obtained, 2 pregnancies ended in a first-trimester spontaneous abortion, and 1 involved a male infant with unilateral renal dysplasia. In at least 9 cases, VZIG was properly prescribed, but Varivax was given 4 times in an emergency department, twice in a pediatrician?s office, and twice in an obstetrician?s office. Also, the FDA is aware of a case where a patient erroneously received Varivax when varicella antibody testing was ordered.

Safe Practice Recommendations
Mix-ups with Varivax and VZIG most often occur due to confusion between their similar names or lack of understanding of the appropriate indications for use. Because mix-ups persist despite prior publicity, regulators, manufacturers, and others concerned with medication safety need to continuously bring this problem to the attention of clinicians through ongoing education. Also, clinicians should have a clear understanding of the indications for using VZIG. Administration of VZIG to susceptible, pregnant women has not been found to prevent viremia, fetal infection, congenital varicella syndrome, or neonatal varicella. The primary indication for pregnant women is to prevent complications of varicella in the mother, rather than to protect the infant.

Clinicians should read package inserts if they are uncertain of appropriate indications for use, but stronger warnings are needed in the product labeling to draw attention to the problem. ISMP is contacting the FDA to suggest an alert on the package label itself, with reference to the package insert for details. For now, pharmacists should affix their own warning labels. Computer alerts also should warn practitioners about error potential. Obtaining a detailed pregnancy history on all women of childbearing age when these products are ordered can avert problems. For more information on the Varivax Pregnancy Registry, visit www.cdc.gov/epo/mmwr/preview/mmwrhtml/00040693.htm.



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