Background and epidemiology: Be-

25 (28%) of 88 children at a daycare centre in New Hampshire came down with chickenpox.1 Perhaps not a sur-prising finding in the pre-vaccine era, but in this outbreak 73 % of the chil-dren old enough to receive the vaccine had been vaccinated. The index case was a healthy 4-year-old boy who had been vaccinated 3 years previously. This outbreak raises new concerns over the effectiveness of the varicella vaccine and whether a booster dose is needed.2 In the United States the varicella vaccine was approved in 1995 by the US Food and Drug Administration. By 2000 more than 75 % of young Ameri-can children had been vaccinated and the incidence of varicella and varicella-related hospital admissions had de-clined by 80%.3 The vaccine was licensed in Canada in 19984 and its use recommended by the National Advisory Committee o