Calculating the probability that a previously susceptible individual is infected as a function of time following exposure to SARS-CoV-2

Reliable assessment of disease state probabilities for an individual following a specific exposure event, such as occupational exposure, is critical for managing isolation and quarantine to reduce exposure of further susceptible individuals to infection. We provide a method, programs, and web site for calculating the disease state probability of an individual immediately following an exposure to SARS-CoV-2 that may or may not have resulted in a transmission. We illustrate the utility of these computations by calculating the time at which an exposed individuals risk of being infectious reaches an acceptable level, evaluating the benefit of a second test for a symptom free individual who has an initial negative test, assessing the value of PCR and antigen testing for case counting, and calculating the time at which the risk of an infected individual being still infectious reaches a level comparable with general background risk. Our results make it clear that evaluation of test results should not be done naively: a test may be negative because a transmission has not occurred, because the test result is false negative, because the individual has passed quickly through the course of the infection, or because the individual is experiencing an unusually long latent phase. Clearly, each of these situations has different implications and the value of our software is in differentiating, accurately assessing and clearly displaying the probabilities for all these disease states.