Does spatial proximity drive norovirus transmission during outbreaks in hospitals?

Harris, John P; Lopman, Ben A; Cooper, Ben S; O'Brien, Sarah J

We also performed sensitivity analyses using different assumptions. First, we used an estimate from a study of a community outbreak of norovirus at a scouting jamboree, giving a mean serial interval of 3.6 days, with an assumed gamma distribution [13] . We then considered gamma distributions with the same variance (4.1) but with the mean serial intervals varying between 0.5 and 5 days in half day increments. Data were analysed using R [16] . Figure 2 shows the observed proximity metric and the distribution of proximity metrics obtained under the assumption that proximity was not associated with transmission (from the simulated permutations). This shows how the proximity metrics observed relate to the distribution of proximity metrics if distance were not important. The dashed line indicates the observed proximity metric (P) and the bars indicate the distribution of proximity metrics from the simulated permutations. For the model using the serial interval taken from the observed onset dates, the observed metric is outside of the range of the simulated distributions and is highly statistically significant (P = 153.34, p <0.001). With serial intervals of less than 2 days proximity is either outside or at the extreme right of the simulated proximity metrics and the p values ranged from < 0.001 for serial intervals of 0.5 days to 0.01 at a serial interval of 2 days. If we increase the assumed serial interval, the proximity metric moves to within the range expected from the simulated values, and at 3 days the p value was 0.2 (figure 2). Using the gamma probability distribution derived from a community outbreak by Heinje et al [13] (mean serial interval 3.06 days), the proportion of observed proximity values fell within the range that would be expected if proximity were not important.