A 22% surge in US coronavirus deaths is hitting unvaccinated people hardest. Experts worry about the long-term effects for vaccinated people, too.

• The US's daily coronavirus deaths surged 22% in the past week - mostly among unvaccinated Americans.
• Disease experts worry about breakthrough cases in older people or those who are immunocompromised.
• Increased transmission could also allow the virus to mutate into a more dangerous strain.

The US is far removed from the deadliest point in its coronavirus outbreak: The country reported more than 3,000 daily coronavirus deaths in January, compared with less than 275 daily deaths, on average, in the past week.

But average daily deaths surged 22% in the past seven days, following a record low of 30 deaths on July 11. In the past two weeks, average daily deaths rose 33%.

The vast majority these deaths are among unvaccinated Americans: Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, told NBC earlier this month that unvaccinated people represented more than 99% of recent coronavirus deaths. The Centers for Disease Control and Prevention also reported Friday that more than 97% of people entering hospitals with symptomatic COVID-19 hadn't received shots.

The US is now dealing with a "pandemic of the unvaccinated," CDC Director Rochelle Walensky said at a press briefing.

"We are seeing outbreaks of cases in parts of the country that have low vaccination coverage because unvaccinated people are at risk, and communities that are fully vaccinated are generally faring well," Walensky said.

But disease experts worry that allowing the virus to spread among unvaccinated people could give it more opportunities to mutate. That could pose a long-term risk for vaccinated people, too. Already, the Delta variant - now the dominant strain in the US - appears to be more transmissible than any other version of the virus detected so far.

"The worst-case scenario is if Delta mutates into something completely different, a completely different animal, and then our current vaccines are even less effective or ineffective," Vivek Cherian, an internal-medicine physician in Baltimore, told Insider last month.

Experts also worry that increased transmission could result in more severe breakthrough infections - cases of COVID-19 diagnosed at least two weeks after someone is fully vaccinated - among older people or those who are immunocompromised, since vaccines may already be less effective among these groups.

People over 65 represent about 75% of breakthrough cases that result in hospitalization or death, according to the CDC.

The UK offers insight into what to expect in the US

Disease experts worry that the US could soon follow in the footsteps of the UK, where average deaths have more than doubled in the past two weeks, from 17 to 40 a day. The UK's average hospitalizations have also increased about 60% during that time, from about 380 to 615 a day.

That's despite the fact that nearly 70% of UK residents have received at least one vaccine dose.

In the US, about 44% of the population remains unvaccinated. (That includes about 48 million children under age 12, for whom vaccines haven't been authorized yet.)

The country is now administering as many daily vaccine doses as it was in late December, when vaccines were available only to healthcare workers and residents of long-term-care facilities. Just 384,000 daily doses were given out on average over the past week.

Some Americans, particularly in rural counties, may still struggle to access shots, while others can't afford to take time off work to get vaccinated. But, for the most part, widespread vaccine hesitancy has slowed down vaccination rates.

About 18% of adults surveyed in a recent YouGov poll said they didn't plan to get vaccinated, while 11% said they were unsure. These rates were significantly higher among Republicans and people in the Midwest and South.

Most vaccine-hesitant people in the survey said they were worried about side effects from coronavirus shots - though studies have shown that vaccine side effects are generally mild and fleeting. The vast majority of them also said they believed that the threat of the virus was exaggerated for political reasons.

Lifting mask and social-distancing mandates could delay herd immunity

Despite lagging vaccination rates, most US states have lifted mask and social-distancing mandates. In states such as Delaware, Florida, Missouri, and South Carolina, masks are recommended but not required for unvaccinated people.

Some disease experts said removing these restrictions too soon could send the wrong message about the state of the pandemic.

"The concern is if you're on the fence, and then you go outside and you see, 'Hey, things are back to normal,' that may decrease the chance of you wanting to even get vaccinated," Cherian said.

For now, experts are hopeful that the US can still vaccinate at least 70 to 85% of its population - a threshold that may allow the country to reach herd immunity. But a new variant that evades protection from vaccines or prior infection could push that goal even further from view, so public-health officials remain determined to vaccinate more Americans as quickly as possible.

"If you get to that situation, then you essentially get us back to a level" that we were in before March 2020, Cherian said, adding: "That's just not a place that you want to be."