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Interrupted time series (ITS) analysis is being increasingly used in epidemiology. Despite its growing popularity, there is a scarcity of guidance on power and sample size considerations within the ITS framework. Our aim of this study was to assess the statistical power to detect an intervention effect under various real-life ITS scenarios. ITS datasets were created using Monte Carlo simulations to generate cumulative incidence (outcome) values over time. We generated 1,000 datasets per scenario, varying the number of time points, average sample size per time point, average relative reduction post intervention, location of intervention in the time series, and reduction mediated via a 1) slope change and 2) step change. Performance measures included power and percentage bias. We found that sample size per time point had a large impact on power. Even in scenarios with 12 pre-intervention and 12 post-intervention time points with moderate intervention effect sizes, most analyses were underpowered if the sample size per time point was low. We conclude that various factors need to be collectively considered to ensure adequate power for an ITS study. We demonstrate a means of providing insight into underlying sample size requirements in ordinary least squares (OLS) ITS analysis of cumulative incidence measures, based on prespecified parameters and have developed Stata code to estimate this.

We show here that BALB/c mice inoculated with murine cytomegalovirus (MCMV) express viral antigens in the endothelial and smooth muscle cells of the aortic wall, and that accumulation of inflammatory cells in the aortic lumen, similar to that seen in early atherosclerotic lesions in humans, colocalizes with the site of virus antigen expression. Immunosuppression of the mice at the time of virus infection increased the expression of viral antigens and the size of early atherosclerotic lesions in the intima. The percentage of the low-density lipoprotein cholesterol (LDL-C), the major lipid contributor to atherosclerotic plaques, was significantly increased in the serum of MCMV-infected mice, whether or not the mice were fed a high cholesterol diet. Human cytomegalovirus (HCMV) significantly increased the esterified cholesterol component of the total cholesterol in a human arterial smooth muscle cell line infected in vitro with HCMV. These results suggest that CMV infection is involved in two of the major mechanisms that lead to development of atherosclerosis, i.e., immune injury and high LDL-C.