Published on in Vol 2, No 2 (2021): Apr-Jun
                                    Preprints (earlier versions) of this paper are
                                    available at
                                    https://www.medrxiv.org/content/10.1101/2021.01.13.21249749v1, first published
                                    .
                                

1 London School of Hygiene & Tropical Medicine, London, United Kingdom
