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.
Figure 2 in the PDF does not precisely match the image of Figure 2 in the online version of the article. The online version is correct.
Competing Interests: No competing interests declared.
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