Vaccines are effective against the variant, but experts worry about states where fewer people are inoculated.

By many metrics, the US is doing relatively well against the pandemic coronavirus. National tallies of cases, hospitalizations, and deaths are all at lows not seen since early last year. Several states have hit the goal of having 70 percent of adults vaccinated with at least one dose, and many areas are easing or lifting health restrictions in response to suppressed transmission levels.

B ut those hard-fought gains were accompanied by a grim milestone Tuesday: The national death toll reached 600,000. That’s roughly the population of Milwaukee or Baltimore. And experts are voicing concerns that the state of the pandemic could once again take a turn for the worse in the US.

The menacing coronavirus variant B 1.617.2, first seen in India—now dubbed Delta by the World Health Organization—is spreading rapidly around the globe, including in the US. It is estimated to be even more contagious than the worrisome B.1.1.7 variant first seen in the UK. That variant, now dubbed Alpha, is estimated to be about 50 percent more transmissible than the original coronavirus that mushroomed out of Wuhan, China, at the start of 2020. Delta is estimated to be 50 to 60 percent more contagious than Alpha.

In the UK, infections with the Delta variant began popping up at very low levels in early April and then rose swiftly. By early June, the Delta variant was accounting for more than 60 percent of cases.

Earlier this year, Alpha out-spread other versions of the virus to become the predominant strain in the US. Now, experts expect Delta will take its place, just as it did in the UK. The US Centers for Disease Control and Prevention on Monday officially dubbed Delta a “variant of concern.”

“Right now, in the United States, [Delta accounts for] about 10 percent of infections. It’s doubling every two weeks,” said Scott Gottlieb, former commissioner of the Food and Drug Administration, Sunday on Face the Nation. “So it’s probably going to become the dominant strain here in the United States. That doesn’t mean that we’re going to see a sharp uptick in infections, but it does mean that this is going to take over. And I think the risk is really to the fall—that this could spike a new epidemic heading into the fall.”

Adding to the worry is new data that suggests Delta may also cause more severe disease—in addition to spreading to more people. Early findings out of Scotland suggest infections with the Delta variant were associated with nearly double the risk of infected persons ending up hospitalized compared to infections with the Alpha variant. The data was published Monday as correspondence in the Lancet. Experts say they’ll need more data to confirm that risk.

T he good news in all of this is that being fully vaccinated appears to protect against Delta. At the end of May, researchers at Public Health England posted data (which had not been peer-reviewed) indicating that two doses of the Pfizer-BioNTech vaccine were 88 percent effective at preventing a symptomatic infection with the Delta variant. Meanwhile, the data said, two doses of the Oxford-AstraZeneca vaccine were 60 percent effective. (Notably, just one shot of either vaccine was not protective, offering only 33 percent efficacy against symptomatic Delta infections. Experts emphasized the importance of not skipping the second dose.)

Data out of Scotland on Monday likewise suggested that two doses of the Pfizer-BioNTech vaccine were 79 percent effective against the Delta variant, while two doses of Oxford-AstraZeneca vaccine were again 60 percent effective.

Also on Monday, PHE released another analysis (also not peer-reviewed) that finds that two doses of the Pfizer-BioNTech vaccine were 96 percent effective against hospitalization, and two doses of the Oxford-AstraZeneca vaccine were 92 percent effective against hospitalization.

“So we have the tools to control this and defeat it,” Gottlieb noted.

But experts are still concerned. The pace of vaccination has slowed significantly in the US, and many states—particularly in the South—are far behind the goal of getting 70 percent of adults at least one vaccine. Pockets of low vaccination are fueling fears among experts, including Gottlieb, that cases could once again spike as Delta continues its spread.

Peter Hotez, director of the Texas Children’s Hospital Center for Vaccine Development, echoed that concern Tuesday. He told CNN that he is “extremely worried” about the Delta variant. He emphasized that right now is “crunch time” to get fully vaccinated—which takes five to six weeks—before Delta spreads further.

In a press briefing last week, top infectious disease expert Anthony Fauci made a similar plea, pointing to the rapid spread of the Delta variant in the UK. “We cannot let that happen in the United States,” he said. This “is such a powerful argument … to get vaccinated.”