The UK's battle with the Delta variant may serve as a crystal ball for the US: Cases and hospitalizations could soar again

• The UK's daily coronavirus case total has risen again due in large part to the Delta variant.
• In a matter of weeks, Delta will likely become the US's dominant strain, too.
• Experts worry the US could soon see a similar uptick in cases and hospitalizations.

Summer was supposed to bring a return to normal in the UK - until coronavirus cases started trending in the wrong direction.

The UK's daily coronavirus cases have increased fivefold in the last month, from around 1,500 to 9,200 per day. In that time, daily hospitalizations have doubled from around 100 to 200 per day. That's due in large part to the spread of the Delta variant, the most transmissible to date.

The UK detected its first Delta cases in February. The variant is now thought to make up 99% of new coronavirus cases there, according to Public Health England. The spread of Delta and the resulting increases in coronavirus cases prompted a four-week delay in the UK's reopening plans: Public venues like restaurants, clubs, and festivals will need to maintain social distancing and capacity restrictions until at least July 19.

The US, meanwhile, is expected to see almost all restrictions lifted by the 4th of July. But experts worry that the nation could be on the verge of an uptick in cases and hospitalizations similar to the UK's. The US's Delta cases appear to have tripled in just 11 days, from just 10% of all coronavirus cases sequenced in early June to 31% of all cases last week, according to a recent estimate by the Financial Times. At that rate, experts predict that Delta will become the nation's dominant strain in a matter of weeks.

At the same time, the vaccination rate continues to slow and Americans are steadily ditching their masks and resuming social gatherings.

"It's a set of circumstances that sets us up for more serious surges," Bob Wachter, the chair of the Department of Medicine at the University of California, San Francisco, told Insider.

"Particularly in low-vaccine states, it sets up unvaccinated people to be at significantly higher risk," he added. "I was worried about them before. I'm much more worried for them for the fall and the winter."

The UK and US could be on parallel tracks

It took just a few months for Alpha, the coronavirus variant first spotted in the UK, to become the dominant strain there. The US similarly identified its first Alpha case in December, and the strain had become dominant by April. Now experts think the two countries could be on parallel tracks yet again.

"We saw [Alpha] quickly become the dominant strain in a period of one or two months, and I anticipate that is going to be what happens with the Delta strain here," Rochelle Walensky, director of the Centers for Disease Control and Prevention, told "Good Morning America" on Friday.

Research from Public Health England suggests that Delta is associated with a 60% increased risk of household coronavirus transmission compared to Alpha - which is already around 50% more transmissible than the original coronavirus strain, according to the CDC.

Researchers in Scotland also found that getting infected with Delta doubles the risk of hospital admission relative to Alpha. (Previous studies have suggested that Alpha may be 30% to 70% deadlier than the original strain.)

What's more, emerging research indicates that a single vaccine dose doesn't hold up as well against Delta as it does against other coronavirus strains. Recent Public Health England analyses found that two doses of Pfizer's vaccine were 88% effective at preventing symptomatic COVID-19 from Delta, while a single shot was just 33% effective. That's compared to 95% efficacy against the original strain, with 52% after one shot.

Nearly 64% of UK residents and 53% of US residents have received at least one vaccine dose, but both countries' vaccine rollouts have slowed recently. The UK's weekly vaccination rate dropped 22% in the last month, while the US's weekly rate dropped 67% over the same period.

On Friday, President Joe Biden said the US likely wouldn't return to lockdowns because so many people have been vaccinated already. But states with particularly low vaccination rates, like Alabama or Mississippi, might benefit from the UK's more cautious approach to reopening.

"It's going to really come down to a state-by-state level," Vivek Cherian, an internal-medicine physician in Baltimore, told Insider. "If in an individual state, the vaccination rates are dropping and then we're finding that this Delta variant is starting to go back up, then that individual state or a county may go back into lockdown."