Varicella re-infection may be more common than previously thought in the United States.
An active surveillance project was undertaken in Antelope Valley, California, between January 1995 and December 1999.
The health-services district is in eastern Los Angeles County, with a population of approximately 300,000 people.
The investigators say only 4.5 percent of varicella cases registered in 1995 had recorded previous varicella. In 1999, 13.3 percent did. “Our study provides the first population-based data on the frequency of clinical varicella re-infections and suggests that re-infections may occur more commonly than previously recognised,” say researchers from the University of North Carolina at Chapel Hill, the US Centres for Disease Control and Prevention in Atlanta, Georgia and the Los Angeles County Department of Health Services.
They point out an assumption that natural varicella confers lifelong immunity is basic to current vaccination policy in the US: “A history of varicella, a rash illness easily recognised by the lay public, is considered a reliable indicator of immunity.”
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