The coronavirus might be receding in much of the United States, but it continues to spread in communities with low Covid-19 vaccination rates, where highly contagious virus variants pose a threat to those who have not had shots.

In Smith County, Tenn., where only 20 percent of people are fully vaccinated, there has been an almost 700 percent increase in hospitalizations for Covid-19 over the past two weeks, according to a New York Times database. In Trousdale, Tenn., where only 23 percent of people have had two vaccine doses, hospitalizations have also surged by 700 percent in the same period.

The increase is not a coincidence, said Dr. Ted Delbridge, executive director of the Maryland Institute for Emergency Medical Services Systems. People who become ill with Covid-19 now are, “in most age groups, twice as likely to end up hospitalized as people who got the virus earlier in the course of the pandemic,” Dr. Delbridge said.

In Maryland, of those between the ages of 50 and 59 who contracted Covid-19 over the winter, about 8 percent were hospitalized, he said. From the end of April through the beginning of June, the hospitalization rate in that group was 19 percent.

Dangerous virus variants are likely to be to blame, Dr. Delbridge said. The variant first found in Britain, now known as Alpha, is deadlier and more contagious than most others and is now dominant in the United States. Last month, Dr. Rochelle P. Walensky, the director of the Centers for Disease Control and Prevention, said the variant, also known as B.1.1.7, made up 72 percent of U.S. cases at the time.

But vaccines have proven to be effective against the Alpha variant. A spring surge that scientists had warned of largely failed to materialize in the United States.

“I think we got lucky, to be honest,” Nathan Grubaugh, an epidemiologist at Yale University, told The New York Times last month. “We’re being rescued by the vaccine.”

Through Tuesday, about 172 million Americans had received at least one dose of a Covid-19 vaccine, according to a Times database. But vaccine distribution across the country has slowed in recent weeks. About 1 million shots are being administered nationwide each day, down from an April peak of 3 million.

In Michigan, one of the few states that saw a surge in cases this spring, Alpha struck younger people who were returning to schools and playing contact sports.

“Because it’s more transmissible, the virus finds cracks in behavior that normally wouldn’t have been as much of a problem,” said Emily Martin, an epidemiologist at the University of Michigan.

At a White House press briefing on Tuesday, Dr. Anthony Fauci, President Biden’s chief Covid adviser, said the Delta variant, which was originally identified in India, was emerging as the dominant variant in Britain.

“We cannot let that happen in the United States,” Dr. Fauci said, adding that the Delta variant now accounted for 6 percent of sequenced cases in the United States.

Dr. Fauci urged young people to get vaccinated, citing a study that found that two doses of the Pfizer-BioNTech vaccine or the AstraZeneca vaccine appeared to be effective against the Delta variant.

One way of limiting the spread is for those who are vaccinated to wear masks around those who are not, doctors say. At least one state is making that a rule in some places: When California reopens next week, fully vaccinated colleagues working in a room together will be allowed to work maskless. But if one person is unvaccinated, everyone in the room will need to wear a mask.

“If I’m in close proximity to other people, and I don’t know their vaccination status, I put a mask on,” Dr. Delbridge said. “It’s just too easy.”