  In March of 2020, many U.S. state governments encouraged or mandated
restrictions on social interactions to slow the spread of COVID-19, the disease
caused by the novel coronavirus SARS-CoV-2 that has spread to nearly 180
countries. Estimating the effectiveness of these social-distancing strategies
is challenging because surveillance of COVID-19 has been limited, with tests
generally being prioritized for high-risk or hospitalized cases according to
temporally and regionally varying criteria. Here we show that reductions in
mobility across U.S. counties with at least 100 confirmed cases of COVID-19 led
to reductions in fever incidences, as captured by smart thermometers, after a
mean lag of 6.5 days ($90\%$ within 3--10 days) that is consistent with the
incubation period of COVID-19. Furthermore, counties with larger decreases in
mobility subsequently achieved greater reductions in fevers ($p<0.01$), with
the notable exception of New York City and its immediate vicinity. These
results indicate that social distancing has reduced the transmission of
influenza like illnesses, including COVID 19, and support social distancing as
an effective strategy for slowing the spread of COVID-19.
