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The effectiveness of vaccinating children and adults following exposure to varicella was elucidated at a homeless shelter for women and children in Philadelphia in 1998. Barbara Watson and colleagues describe the scenario in their paper in the January 2000 issue of Pediatrics.1
Following the eruption of two cases of vari -cella in a 27-year-old mother (case A) and her 11-month-old son (case B), the shelter director notified the Phila del phia Department of Public Health's Division of Disease Control of the cases. Both patients had been prescribed antiviral medication when they were seen in the emergency department, but they failed to have the prescriptions filled due to cost of the drug. Instead, they returned to the shelter and the restrooms and common area for classes, recreation, and eating they shared with the other residents.
Because all residents use the common room and had been in that room with the identified mother and child for the two-day period before diagnosis of the cases, it was assumed that all residents had been exposed to the virus. Upon notification, staff at the Division of Disease Control offered varicella vaccine to all susceptible residents of the shelter. At that time, the shelter was at its full capacity of 154 residents and no new (i.e., unexposed) residents were allowed into the shelter.
Within a 36-hour period, all susceptible residents were offered the varicella vaccine free of charge. First, Watson and colleagues interviewed all the residents to determine susceptibility by history of chickenpox and of varicella vaccination. "All unvaccinated persons with negative or unknown varicella history status were considered susceptible," the authors wrote. "Although a specific postexposure recommendation was not in force at the time, this practice was, nonetheless, consistent with varicella vaccination recommen dations of both ACIP [the Advisory Committee on Immun ization Practices] and the American Academy of Pediatrics."1
Within 36 hours, 67 individuals (25 women, 42 children <13 years old) were vaccinated. Ten children determined to be susceptible were not vaccinated; nine were under the age of 12 months, and one remained unvaccinated due to an inaccurate reporting of history followed by development of chickenpox before the child could be vaccinated.
Two children developed a vesicular rash 12 days after administration of vaccines; both children were sons of case A and were considered to have had longer initial exposure to the virus. Both of the cases in children who received the vaccine were mild in intensity (<50 skin lesions). No other cases of chickenpox developed from the initial exposure.
"Using an attack rate of 100% in the 1 unvaccinated child," the authors wrote, "the vaccine was 95.2% (95% CI, 81.6%-98.8%) effective in preventing all disease and 100% effective in preventing moderate and severe disease."1
Residents were monitored for up to 42 days after the last diagnosed case of chickenpox. Though still monitored, the shelter was reopened just six weeks after diagnosis of the initial case of chickenpox with no susceptible individuals in residence. On the other hand, a varicella outbreak in another shelter in Philadelphia caused the second shelter to close its doors for six months. Therefore, the varicella vaccine was effective not only in protecting individuals from infection, but in opening the shelter to newcomers earlier than previously reported.
The authors conclude that use of the varicella vaccine postexposure "and for outbreak control should limit the spread of varicella, prevent its associated complications and dramatically shorten outbreaks."1
1. Watson B, Seward J, Yang A, et al. Postexposure effectiveness of varicella vaccine. Pediatrics 2000; 105:84-88.