Estimating the reproductive number R0 of SARS-CoV-2 in the United States and eight European countries and implications for vaccination

ABSTRACT

SARS-CoV-2 rapidly spread from a regional outbreak to a global pandemic in just a few months. Global research efforts have focused on developing effective vaccines against SARS-CoV-2 and the disease it causes, COVID-19. However, some of the basic epidemiological parameters, such as the exponential epidemic growth rate and the basic reproductive number, R0, across geographic areas are still not well quantified. Here, we developed and fit a mathematical model to case and death count data collected from the United States and eight European countries during the early epidemic period before broad control measures were implemented. Results show that the early epidemic grew exponentially at rates between 0.19-0.29/day (epidemic doubling times between 2.4-3.6 days). We discuss the current estimates of the mean serial interval, and argue that existing evidence suggests that the interval is between 6-8 days in the absence of active isolation efforts. Using parameters consistent with this range, we estimated the median R0 value to be 5.8 (confidence interval 4.7-7.3) in the United States and between 3.6 and 6.1 in the eight European countries. This translates to herd immunity thresholds needed to stop transmission to be between 73% and 84%. We further analyze how vaccination schedules depends on R0, the duration of vaccine-induced immunity to SARS-CoV-2, and show that individual-level heterogeneity in vaccine induced immunity can significantly affect vaccination schedules. SignificanceWith the global efforts to develop vaccines for COVID-19, it is important to understand the contagiousness of the virus to design regional vaccination policy. To that end, we fit a mathematical model to data collected from the early epidemic period in the United States and eight European countries, estimating that the early epidemic doubles between 2.4-3.6 days. This suggests that SARS-CoV-2 is highly transmissible in the absence of strong control measures irrespective of heterogeneity in geographic and social settings. We estimated the median basic reproduction number, R0 to be 5.8 (confidence interval 4.7-7.3) in the United States and between 3.6 and 6.1 in the eight European countries. The herd immunity needed to stop transmission is high (between 73% and 84%).