In December, British researchers discovered that a new variant was sweeping through their country. When it arrived in other countries, the variant, now known as Alpha, tended to become more common in its new homes as well. By April, it had become the dominant variant in the United States, and it has remained so ever since.

A sudden, sharp rise in coronavirus cases in many parts of Africa could amount to a continental third wave, the World Health Organization warned on Thursday, a portent of deeper trouble for a continent whose immunization drives have been crippled by shortfalls in funding and vaccine doses.