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 change over time. (Preprint)

BACKGROUND During COVID-19 pandemic, swab tests proved to be effective in containing the infection, as mean 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 subjects and in healthcare workers. OBJECTIVE The objective of this study is to analyse changes over time of individual SARS-CoV-2 swab test positivity during a health surveillance program. METHODS The study was conducted on 2071 healthcare workers of the University Hospital of Verona having a known date of close-contact with a COVID-19 case. A generalized additive mixed model was used to investigate how the probability of a positive test result changes over time in the sample of subjects who tested positive to SARS-CoV-2 and in the subset of subjects having a first negative swab result before proving positive in order to test different surveillance interval times scenario. RESULTS Among the 2071 healthcare workers under study, 191 (9.2%) tested positive to SARS-CoV-2: 103 (54%) of them were asymptomatic and 49 (25.7%) had a first negative swab. The analysis on the association between days since close-contact and probability to test positive showed the highest probability (77%) between the 5th and 8th day. In the three testing intervals (3, 5 and 7 days) the probability peak was on the 6th day, between the 9th and 10th and between the 13th and 14th day respectively. CONCLUSIONS Swab tests can result in false negative outcomes. Probability to test positive resulted higher in the first 10 days after close-contact. Early testing and a health surveillance program with close intervals in this temporal window would be recommendable.