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Yet another coronavirus variant has public health officials around the globe scrambling to control its spread.

The delta variant, which first emerged in India, has now spread to more than 80 countries and is quickly becoming the dominant version of the virus (SN: 5/9/21). In places like the United Kingdom, delta has dethroned the highly transmissible alpha variant, which was first identified in that country, as the most common form of the virus.

That rapid spread of the delta variant has forced health officials to react. U.K. officials, for instance, delayed plans to reopen the country, pushing the date back to mid-July. And health officials in Israel, a nation where nearly 60 percent of the population is fully vaccinated, reinstated its requirement that residents wear masks indoors — a public health measure that had been lifted 10 days before. In the United States, places like Los Angeles County recommend that even vaccinated people still wear masks indoors. The World Health Organization also urges everyone to continue wearing masks, though the U.S. Centers for Disease Control and Prevention’s guidelines that vaccinated people can go without masks in most situations remain in place.

Delta poses the biggest threat to unvaccinated people, the latest studies suggest. In the United States, delta is responsible for an estimated 26.1 percent of cases across the country. Its prevalence is doubling every two weeks. Narrowing in on regions that include states with low vaccination rates like Missouri and Wyoming reveals that delta is already causing the majority of infections in some places. On July 1, the Biden administration announced that teams of experts equipped with testing supplies and therapeutics would be sent to U.S. hot spots to control outbreaks of delta.