CORRESPONDENCE Re: Cancer Incidence in Denmark Following Exposure

We read with interest the recent ar-ticle by Engels et al. (1) in which the authors performed a retrospective birth-cohort analysis in Denmark following exposure to poliovirus vaccine contami-nated with simian virus 40 (SV40) to clarify whether SV40 infection in-creases risk of mesothelioma, choroid plexus tumors, and non-Hodgkin’s lym-phoma, or of cancers arising in children. The authors concluded that “exposure to SV40-contaminated poliovirus vaccine in Denmark was not associated with in-creased cancer incidence.” It is important to point out that sev-eral limitations have been recognized for this and similar epidemiologic stud-ies addressing exposure to SV40-contaminated poliovirus vaccines and the incidence of human cancers (Table 1) (2–4). Indeed, an evaluation by the Institute of Medicine Immunization and Safety Review Committee found that the epidemiologic data used in birth-cohort studies to examine cancer rates in individuals potentially exposed to SV40-contaminated vaccines are inad-equate to evaluate a causal relationship (2). The validity of observational stud-ies, such as the retrospective analyses by Engels et al. (1), depends on the accu-racy of the existing knowledge of the biologic properties of SV40 and the identification of the human population infected with the virus (2,4,5). There-fore, supportive evidence from experi-mental studies is required to draw causal inferences in human disease (4,6). In-deed, a recent case–control study (5) of 1793 cancer patients indicated that there is a statistically significant excess risk of SV40 associated with primary brain cancers (odds ratio [OR] 3.8, 95% confidence interval [CI] 2.6 to 5.7), primary bone cancers (OR 24.5, 95% CI 6.8 to 87.9), malignant mesothe-lioma (OR 15.1, 95 % CI 9.2 to 25.0), and non-Hodgkin’s lymphom