This study calculates the current reproduction number for COVID-19 for each state in the United States.
The basic reproduction number is the average number of people to whom an infected person transmits the infection when virtually all individuals in a population are susceptible. A reproduction number greater than 1.0 means an epidemic is growing while a number less than 1.0 means an epidemic is slowing down and may eventually end. The reproduction number for the novel coronavirus responsible for COVID-19 was estimated to be 2.2 in Hubei Province, China, based on cases in December 2019 and January 2020.
Using data on the number of daily cases, the authors of this study found that the reproduction number for COVID-19 in the US declined from 4.02 to 1.51 between March 17 and April 1, 2020. They also found that the reproduction number has declined in most states over the past two weeks, which suggests that social isolation measures may be having a beneficial effect. However, their analysis also found that most states still have reproduction numbers substantially higher than 1.0. This means it would be premature to relax restrictions and resume normal social and economic activities.