Funding for this research was provided by:
U.S. Department of Health & Human Services | Centers for Disease Control and Prevention (5U01EH001301)
Received: 12 February 2018
Revised: 20 July 2018
Accepted: 26 July 2018
First Online: 6 November 2018
: A.S., P.R., and E.G. are directly employed by Google. The remaining authors declare no competing interests.