  We analyze changes in the reproduction number, R, of COVID-19 in response to
public health interventions. Our results indicate that public health measures
undertaken in China reduced R from 1.5 in January to 0.4 in mid-March 2020.
They also suggest, however, the limitations of isolation, quarantine, and
large-scale attempts to limit travel. While the world-wide reproduction number
briefly dropped below 1 as China implemented extensive public health measures,
the introduction of the virus to other nations swiftly led to an increasing
world-wide average value of R. In Italy, the nation hardest-hit following
China, social distancing measures brought the local value of R down from 3.71
to 2.51. Nonetheless, the value of R in Italy persisted at levels well above 1,
allowing for ongoing transmission. By mid-March 2020, as COVID-19 spread in
areas without extensive public health interventions in place, the world-wide
value of R increased to a level similar to that of late January.
