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.
The data points for the 2-week interval fell between those of the 1 and 4 weeks; those of the 8-week interval fell between those of 4 and 12 weeks. Error bars represent the standard deviation of the mean.
