How to evaluate a schedule for SARS-CoV-2 swab test timing: a mixed model approach to study how probability of SARS-CoV-2 positivity changes over time.

Background During the COVID-19 pandemic, swab tests proved to be effective in containing the infection, as a means for early diagnosis and contact tracing strategy. However, little evidence exists regarding the correct timing for the execution of the swab test, especially in asymptomatic individuals and healthcare workers. Objective The objective of this study was to analyze changes over time of individual SARS-CoV-2 swab test positivity during a health surveillance program. Methods The study was conducted on 2,071 healthcare workers at the University Hospital of Verona, with a known date of close contact with a SARS-CoV-2 infected case, between February 29, and April 17, 2020. The healthcare workers underwent a health surveillance program with repeated swab tests to track their virological status. A generalized additive mixed model was used to investigate how the probability of a positive test result changes over time, since the last known close contact date, in the overall sample of individuals who tested positive for SARS-CoV-2 and in the subset of individuals with a first negative swab result before proving positive in order to test different surveillance time intervals. Results Among the 2,071 healthcare workers under study, 191 (9.2%) tested positive for SARS-CoV-2; 103 (54%) were asymptomatic with no sex or age differences. In 49 (25.7%) cases the first swab after the close contact was negative. Sex, age, symptoms, and time of sampling were not different between individuals with a first negative swab and those who initially tested positive after the close contact. In the overall sample, the estimated probability to test positive was 0.74 on day 1 after the close contact, increasing to 0.77, between days 5 and 8. In the three different scenarios to scheduled repeated testing intervals (3, 5, and 7 days) in the subgroup of individuals with an initially negative swab, the probability peak was on the sixth, ninth and tenth, and 13th and 14th days, respectively. Conclusions Swab tests can result in initially negative outcomes. The probability of testing positive increased from day one, reaching a peak on fifth to the eighth day after close contact. Early testing, especially in this last time window, is recommended together with a health surveillance program scheduled in close intervals.