I didn't have a particular paper in mind that I remembered.
Just remember reading several articles in the past month about new strains. Some examples are easy to find:
https://www.abc.net.au/news/2020-05-06/new-coronavirus-covid...
https://www.cnbc.com/2020/05/05/the-coronavirus-mutated-and-...
https://www.usnews.com/news/health-news/articles/2020-05-06/...