Streptococcal Infections and Varicella

Le résumé fourni par la source

To the Editor—Frère et al [1] recently reported the effects of a universal varicella immunization program on admissions to a pediatric hospital for invasive group A streptococcal infections (IGASI). We wish to draw readers’ attention to 2 points concerning this study, hoping to help interpret its results. The first point is that the quantities the authors call “rates of IGASI per 1000 hospital admissions” are in fact proportions of admissions due to IGASI. As such, they are as dependent on changes in numbers of hospitalizations for any other cause as they are on hospitalizations for IGASI. Only true population-based hospitalization rates, with person-time denominators, would generally be independent of each other. The reservation expressed in the last sentence of the article is therefore not supported by the results reported: the varicella immunization program could have decreased the hospitalization rate for IGASI; we simply do not know, based on these proportions, if it did. The second, related point is that it is not meaningful that the difference between the proportions of admissions due to IGASI before and after the implementation of the varicella immunization program was not statistically significant, because the study had a very low power for detecting as statistically significant a difference in proportions of the size observed. After the denominators had been retrieved by a rule of three, one calculator [2] estimated that the study had a power of 6%, another [3] a power of 7%. A statistical power of about 80%, with a significance threshold of 0.05, is the usual target, giving a 20% risk of failing to detect as statistically significant a true difference in effect of the size judged important (type II error). Here the risk of a type II error is about 93%. These reservations bear only on the epidemiologic findings of the study and not on the clinical finding that the number of pediatric patients admitted each year to the study hospital for varicella-associated IGASI decreased from a yearly mean of 3.4 (24 in 7 years) before the implementation of the varicella immunization program to 0.9 (7 in 8 years) afterward. Testing the significance of the apparently large difference between these 2 means is a very straightforward matter, given the corresponding sample sizes and variances. The authors’ data on varicella-related IGASI suggest that the decrease is significant. Confirming this with absolute numbers instead of proportions would support the hypothesis of a population effect of the program in the absence of an alternate explanation, such as a decrease in the pediatric population served or a change in consultation patterns. Potential conflict of interest. Both authors: No reported conflicts. Both authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.