Time-varying Reproductive Rates for SARS-CoV-2 and its Implications as a Means of Disease Surveillance on Lockdown Restrictions

To quantify the time-varying reproductive rates for SARS-CoV-2 and its implication in Louisiana. SUMMARY OF BACKGROUND DATA: Basic reproductive number (R(0)) and effective reproductive number (R(e) or R(t)) are 2 measures of the ability of an infectious agent to spread in the environment. They differ in that R(0) assumes zero immunity in the population, while R(e) or R(t) accounts for change over time. Reproductive number modeling is influenced by several factors, including serial interval, the time between the onset of symptoms in an infector, and a secondary case. Quantification of the ability of a pathogen to spread is essential in guiding policy. METHODS: Here, we construct epidemic curves and calculate daily R(t) values for the state of Louisiana and each of its 9 regions. RESULTS: Our results demonstrated variation over both time and geography in calculated R(0) and R(t) values. Generally, as time has progressed, predicted R(0) and R(t) values have decreased. In Louisiana, mean R(t) was calculated at 3.07 in March and 0.82 by May. A reproductive number less than one is important as it indicates infectious spread will decline with time. The most recent finding of mean R(t) = 0.82 is important. It stands in stark contrast to the situation in April when New Orleans, Louisiana, had the highest per capita coronavirus mortality rate in the United States – twice that of New York City and 4 times the rate in Seattle. CONCLUSION: As locations around the world begin to lift restrictions, monitoring of infectious spread will be essential.