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.
There were three typographical errors in the second and third paragraphs of the Introduction. The values "7.7%", "55%", and "60%" are incorrect. The correct values are "7.7‰", "55‰", and "60‰".
Competing Interests: No competing interests declared.
