https://www.nature.com/articles/s41562-019-0632-4 Hmm this isn't free either. This is a summary article, which is: https://www.nature.com/articles/s41562-019-0746-8.pdf Here are authors' slides: https://www.who.int/docs/default-source/epi-win/presentations-of-all-speeches/webinar-14-ps-ue-8-april-2020.pdf?sfvrsn=7725b501_2