Correction to: Nature Medicine https://doi.org/10.1038/s41591-018-0002-1, published online 16 April 2018.

In the version of this article originally published, a URL provided in the Methods section was incorrect. The URL had a solidus at the end but should have appeared as http://www.nature.com/authors/policies/image.html. The error has been corrected in the PDF and HTML versions of this article.