Quantifying the roles of vomiting, diarrhea, and residents vs. staff in norovirus transmission in U.S. nursing home outbreaks

Carly Adams; David Young; Paul A Gastañaduy; Prabasaj Paul; Zach Marsh; Aron J Hall; Benjamin A Lopman

The copyright holder for this preprint (which was not peer-reviewed) is the author/funder. . https://doi.org/10.1101/707356 doi: bioRxiv preprint 326 possible explanations for this greater infectiousness of index cases. First, as an outbreak 327 progresses and more individuals become ill and later immune, there is a natural decrease in the 328 proportion susceptible. However, we found that index cases generally had substantially greater 329 R Ei 's compared to cases with onset dates only a few days after outbreak initiation, before a 330 sufficient number of susceptibles could accumulate to explain this pattern. We also found that 331 R 0,1 (the basic reproduction number for index cases) tended to be substantially larger than R 0,2-4 332 (the basic reproduction numbers for cases on days 2-4), even under the extreme assumptions that 333 all individuals were initially susceptible and that the total population consisted only of reported 334 cases in the outbreak. If the observed declines in R Ei had been due to a natural decrease in 335 susceptibles alone, we would expect the calculated R 0i values to remain relatively constant over 336 time. Therefore, these results suggest that index cases are more infectious than subsequent cases 337 for reasons other than the natural decreases in susceptibles alone. Second, index cases may have 338 been more infectious than non-index cases due to intrinsic case characteristics (e.g., vomiting).