The COVID-19 pandemic is increasingly becoming an arms race among the emerging variants of the virus, and at the moment, there’s no question which one is winning: the Delta variant—formally known as B.1.617.2—one of four strains to have emerged originally in India. It was just last month that the World Health Organization labeled Delta a “variant of concern”—joining with the Alpha strain, which emerged in the U.K.; the Beta strain, from South Africa; and the Gamma strain, first seen in Brazil. But Delta is fast becoming the most worrisome of the bunch.

Health officials are sounding the alarm that Delta threatens to reverse the progress made in countries, like the U.S. and U.K., that have lately been beating the pandemic into retreat and worsen conditions in countries, like India, that are still deep in crisis. Researchers have found that Delta is at least 60% more transmissible within households than the Alpha strain, the dominant variant in the U.S., according to the Public Health of England.

According to accounts from doctors on state-run television in China—which were first reported in English media by the New York Times—Delta-variant patients there have seen symptoms develop more quickly and grow more severe than those in people infected with other variants. Viral loads also climb faster and decline more slowly. Still, epidemiologists say it may be too soon to know for certain if Delta causes more severe illness, and it’s important to recognize that other factors, like lockdown restrictions and vaccination rates, may be affecting disease spread as well. “I’m pretty wary of putting too many eggs in the basket of ‘the variants are making things worse’” says Dr. Gigi Gronvall, senior scholar at the Johns Hopkins Center Health Security at the Bloomberg School of Public Health. “It’s very convenient for some political leaders to blame variants like an act of God for policy decisions that have led to the situation that we find ourselves in.”