Based on the difference in virulence of the virus in Western Europe and US versus Asia, it is hard to believe there is only one strain of SARS-CoV-2.<p>My hypothesis is that while Asia was originally infected by Wuhan originated strain, the Western Europe and US may have been infected by a mutated strain. The differences in impact in different countries is very difficult to explain without the hypothesis of multiple strains.<p>Edit: There was a decent article recently (I am unable to find now, most probably from NYT, WSJ) that addressed several factors mentioned in replies and showed none of those could explain the discrepancy in cases across countries. For example, high temperature and warm weather don’t explain differences in cases between Singapore, Thailand, Indonesia, Malaysia, and Amazon region of Brazil.<p>The closest article, I could find using google search is from Brookings.<p><a href="https://www.brookings.edu/blog/future-development/2020/05/05/the-unreal-dichotomy-in-covid-19-mortality-between-high-income-and-developing-countries/" rel="nofollow">https://www.brookings.edu/blog/future-development/2020/05/05...</a>