diff --git a/README.Rmd b/README.Rmd
index 39520f9..468b92c 100644
--- a/README.Rmd
+++ b/README.Rmd
@@ -39,14 +39,14 @@ A. Lieber, K. Schulman, Y. Shao, V. Qarkaxhija, A. K. Nandi, S. H. Keddie, S.
 Rumisha, P. Amratia, R. Arambepola, E. G. Chestnutt, J. J. Millar, T. L. Symons,
 E. Cameron, K. E. Battle, S. Bhatt, and P. W. Gething.* **Global maps of travel
 time to healthcare facilities.** (2020) Nature Medicine.
-https://doi.org/10.1038/s41591-020-1059-1
+<https://doi.org/10.1038/s41591-020-1059-1>
 
 *D. J. Weiss, A. Nelson, H.S. Gibson, W. Temperley, S. Peedell, A. Lieber, M.
 Hancher, E. Poyart, S. Belchior, N. Fullman, B. Mappin, U. Dalrymple, J. Rozier,
 T.C.D. Lucas, R.E. Howes, L.S. Tusting, S.Y. Kang, E. Cameron, D. Bisanzio, K.E.
 Battle, S. Bhatt, and P.W. Gething.* **A global map of travel time to cities to
 assess inequalities in accessibility in 2015.** (2018). Nature.
-doi:10.1038/nature25181.
+<https://doi.org/10.1038/nature25181>
 
 ## Installation
 
diff --git a/README.md b/README.md
index d384ede..7c8df8e 100644
--- a/README.md
+++ b/README.md
@@ -41,7 +41,8 @@ Lieber, M. Hancher, E. Poyart, S. Belchior, N. Fullman, B. Mappin, U.
 Dalrymple, J. Rozier, T.C.D. Lucas, R.E. Howes, L.S. Tusting, S.Y. Kang,
 E. Cameron, D. Bisanzio, K.E. Battle, S. Bhatt, and P.W. Gething.* **A
 global map of travel time to cities to assess inequalities in
-accessibility in 2015.** (2018). Nature. <doi:10.1038/nature25181>.
+accessibility in 2015.** (2018). Nature.
+<https://doi.org/10.1038/nature25181>
 
 ## Installation