biostudies-literatureUnknown18(2)Baunez CWe provide a novel way to correct the effective reproduction number for the time-varying amount of tests, using the acceleration index (Baunez et al., 2021) as a simple measure of viral spread dynamics. Not correcting results in the reproduction number being a biased estimate of viral acceleration and we provide a formal decomposition of the resulting bias, involving the useful notions of test and infectivity intensities. When applied to French data for the COVID-19 pandemic (May 13, 2020-October 26, 2022), our decomposition shows that the reproduction number, when considered alone, characteristically underestimates the resurgence of the pandemic, compared to the acceleration index which accounts for the time-varying volume of tests. Because the acceleration index aggregates all relevant iPloS onee0281943https://www.ebi.ac.uk/biostudies/studies/S-EPMC9970098biostudies-literatureCorrecting the reproduction number for time-varying tests: A proposal and an application to COVID-19 in France.PMC9970098Pintus MLPintus PADegoulet MTeschl MBaunez CLuchini SfalseCorrecting the reproduction number for time-varying tests: A proposal and an application to COVID-19 in France.We provide a novel way to correct the effective reproduction number for the time-varying amount of tests, using the acceleration index (Baunez et al., 2021) as a simple measure of viral spread dynamics. Not correcting results in the reproduction number being a biased estimate of viral acceleration and we provide a formal decomposition of the resulting bias, involving the useful notions of test and infectivity intensities. When applied to French data for the COVID-19 pandemic (May 13, 2020-October 26, 2022), our decomposition shows that the reproduction number, when considered alone, characteristically underestimates the resurgence of the pandemic, compared to the acceleration index which accounts for the time-varying volume of tests. Because the acceleration index aggregates all relevant i2023-01-01T00:00:00Z20232025-04-18T14:21:20.93Z2025-04-07T00:29:32.324ZS-EPMC99700983684835510.1371/journal.pone.0281943