Study finds lower-than-expected conversion rates

Universal vaccination reaps savings of $53,000

In a study conducted at Memorial Sloan-Kettering Cancer Center in New York City, researchers found a considerably lower rate of varicella vaccine-induced seroconversion compared with most other reports in published literature. Despite these findings, universal varicella vaccination is a cost-effective alternative to furloughing exposed health care workers.1

David M. Weinstock, MD, lead author of the study, reported that seropositivity rates among test subjects were only 81.6%, compared to rates of 94% to 99% commonly reported in the literature.

Of 263 seronegative health care workers at Sloan-Kettering, 96 began the vaccine program. Of the 57 who ultimately received two doses, 38 returned for follow-up serology. Thirty-one (81.6%) of those subjects were seropositive for varicella-zoster virus antibodies, and seven remained seronegative. The total cost of vaccinating all 263 seronegative health care workers was estimated and compared with the cost of varicella-related furloughs at the facility.

Weinstock and his colleagues estimated that savings at Sloan-Kettering from a program of universal varicella vaccinations exceeded $53,000 in the first year alone, with expenses totaling $47,514 for universal vaccination and projected expenses of $101,000 in furloughs without vaccination. He writes that "universal vaccination remains a cost-effective proposition as long as the average salary of furloughed health care workers is above $21,000."

Weinstock proposes that the relatively low seroconversion rates were due to the fact that his research team used a latex agglutination (LA) assay to measure serum antibodies. The test is not as sensitive as the gpELISA assay, which was used during FDA clinical trials of the vaccine (Varivax).

According to Weinstock, the vaccine manufacturer, Merck & Company of Whitehouse Station, NJ, and independent authors recommend the LA assay as a "prompt, sensitive, specific and cost-effective alternative to the gpELISA." The gpELISA test is neither "commercially available nor optimal for single-sample testing or rapid turnaround time, as is often necessary after potential exposure," writes Weinstock.

He adds that even though most subjects who receive the vaccine may have a positive gpELISA, if commercially available assays cannot verify the presence of a predetermined level of immunity immediately following an exposure, the potential savings from a vaccination program are reduced. Weinstock also cautions that the study sample was small, that women outnumbered men 1.7 to 1, and that the relatively young age of the subjects (mean of 33 years) may have skewed the seroconversion rate higher than might have been expected with a sample more representative of the general population.

Reference

1. Weinstock DM, Rogers M, Lim S, et al. Seroconversion rates in healthcare workers using a latex agglutination assay after varicella virus vaccination. Infect Control Hosp Epidemiol 1999; 20:504-507.