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.”

In the U.S., the Delta variant now represents roughly 6% of all cases, according to Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Disease. Those numbers are likely to climb. “I think that with the data we have, there’s a good chance that it could take over the 117 [Alpha strain] as the primary variant just because it’s more infectious,” says Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. “It’s going to create a real additional challenge.”

At a White House briefing last week, Fauci noted that the Pfizer-BioNTech vaccine is 88% effective in preventing disease caused by the Delta variant. “The good news is that the Delta variants, just like the other variants, do not appear to escape the protection afforded by the vaccines available in the U.S.,” says Dr. Gronvall. In addition, the AstraZeneca vaccine, which is authorized for use in the E.U. but not in the U.S., has been shown to be 60% effective against the Delta variant, according to a study published in Nature, though scientists expect even greater protection against hospitalization and death caused by the strain.

But in the interim, as populations remain significantly unvaccinated, the Delta variant’s infectiousness is cause for concern. The U.K. government had set June 21 as the target date for reopening the country, but earlier today, Prime Minister Boris Johnson announced it will be postponed at least a month. The extra time will allow at least 10 million more doses of vaccine to be administered, according to U.K. Health Minister Edward Argar. So far, 62% of the U.K. population has received at least one vaccine dose and 44% are fully vaccinated.

But even if 10 million doses are indeed successfully administered over the next month, there’s no guarantee breakthrough infections won’t occur.

In response to the spread of the Delta variant in the U.K., other European nations have imposed additional travel restrictions on visitors from the country.

In China, the Delta crisis is centered around the city of Guangzhou in the southeastern part of the country, where the government has imposed restrictions not seen since the earliest days of the pandemic. The government has administered an estimated 32 million COVID-19 tests in the southeast, including to the entire 18.7-million-person population of Guangzhou (the region’s largest city) in an effort to get the outbreak under control. The tests in Guangzhou were conducted over just three-days from June 5 to 7.

So far, 800 million doses of vaccine have been administered across all of China, according to the Chinese news outlet XINHUANET. But that is total doses—not necessarily including second doses, which the widely used Sinovac vaccine requires—and in any event represents just 57% of the population.

And vaccination numbers are much lower in much of the rest of the world, exposing millions to highly transmissible and potentially more dangerous variants, and creating fresh opportunities for the SARS-CoV-2 to shape-shift yet again. “Every time you’re giving the virus a chance to replicate, you’re giving a chance for another variant to take hold, which may have different properties that are not to our advantage,” says Dr. Gronvall. “That is why it is in our self interest to help vaccinate the world.”

In the U.S., cases of the Delta variant are doubling every two weeks, according to Dr. Scott Gottlieb, the former director of the Food and Drug Administration, who spoke to CBS News’ “Face the Nation” on Sunday. “It’s essentially taking over,” warned Fauci flatly at a June 8 White House briefing. The answer, he reiterated, is to get people vaccinated—at least up to the 70% level needed to achieve herd immunity.

Osterholm is not optimistic that can happen, given the low rate of vaccination to date, especially in southern and Appalachian states like Georgia, West Virginia, Mississippi, and Alabama. Some of those states—Alabama, Arkansas and Missouri, for instance—have seen cases rise in recent weeks, and Osterholm says the situation is particularly worrying in so-called “vaccine deserts” within those states where vaccination rates are even lower. There, it’s possible that Delta transmission could fuel local COVID-19 case spikes that overwhelm the regional medical system.

“We have to be careful not to automatically just assume what’s happened in England will happen here,” he says. Nonetheless, he adds, “The risk is surely greater when you have more unvaccinated people together. I think that as we open up everything we’re kind of back in many ways to where we were back in pre-March 2020.”