The incidence of varicella: correction of a common error.
BACKGROUND
Interest in the incidence of varicella (chickenpox) has increased since the discovery of an effective vaccine, but calculations to date have incorrectedly ignored the question of susceptibility.


METHODS
We studied the occurrence of varicella in Italy on the basis of 33,343 children (age 0-14 years) cared for by 35 pediatricians between 1 October 1997 and 30 September 1998. The life-table technique was used to calculate the number of susceptible children. On this basis, we estimated the corrected age-specific and cumulative incidence.


RESULTS
We identified 1749 cases among the estimated 21,783 susceptible children, for a crude incidence of 8.0% (95% confidence interval [CI] = 7.7-8.4). The rate age-standardized to the Italian population 0-14 years old was 6.8 (CI = 6.5-7.2). The incidence was more than 16% among children age 3-4 years and more than 4% for those age 1-10 years. Comparison of the usual method and our corrected method showed that the uncorrected method underestimates the crude annual incidence (5.2% 8.0%), shifts the peak incidence to earlier ages, and underestimates cumulative incidence (at age 14, 49% 67%).


CONCLUSIONS
The use of our corrected method provides more valid estimates of the incidence of varicella than the ones that are currently available. Corrected estimates should be preferred to uncorrected ones in models to study the cost-effectiveness of universal vaccination against varicella.
Nicolosi
Alfredo
A
Department of Epidemiology and Medical Informatics, Institute of Biomedical Technologies, National Research Council, Milan, Italy. nicolasi@itba.mi.cnr.it
Sturkenboom
Miriam
M
Mannino
Salvatore
S
Arpinelli
Fabio
F
Cantarutti
Luigi
L
Giaquinto
Carlo
C
D000293:Adolescent; D017677:Age Distribution; D002644:Chickenpox / Q000453:epidemiology*; D002648:Child; D002675:Child, Preschool; D015340:Epidemiologic Research Design; D006801:Humans; D015994:Incidence; D007223:Infant; D007231:Infant, Newborn; D007558:Italy / Q000453:epidemiology