Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
Simulated cases represent mean incidence across 500 simulation iterations. Zero cases were observed during 2012, 2013, 2016, and 2017; zero cases not plotted due to use of log scale.
