The three sizable urban centers in the United States where the coronavirus is spreading fastest right now have something in common: They are major warm-weather tourist destinations.

Miami-Dade County, Fla., Honolulu County, Hawaii, and San Juan, P.R., are all averaging at least 85 new cases a day per 100,000 residents, with test positivity rates above 20 percent, according to a New York Times database. By contrast, the nation as a whole is averaging 34 newly reported cases a day per 100,000 residents, with a positivity rate of 13 percent.

As of Wednesday, new confirmed cases in the United States have been roughly flat at around 110,000 a day on average over the past two weeks, according to a New York Times database, after rising from lower than 30,000 a few months ago. And those are just the reported cases; widespread use of at-home testing means that many positive test results never make it into official tallies, experts say, and many people with mild or no symptoms may never be tested at all.

“Much of the U.S. is experiencing summer weather, yet Covid-19 cases are surging,” said Dr. Sandra Albrecht, an assistant professor of epidemiology at Columbia University. “So I wouldn’t expect to see this pattern look any different for warm weather destinations.”

The only places in the country with higher recent figures than those three urban centers are smaller communities in Puerto Rico or Hawaii and a few isolated rural counties elsewhere.

Some U.S. regions that were hit early by the latest surge, like the Northeast, have been showing signs of improvement lately. But Miami-Dade has gotten steadily worse since early April, with its daily new-case average rising more than tenfold, hospitalizations more than tripling and deaths ticking upward.

The C.D.C. now considers it, along with much of Florida, to be a high-virus-level area where extra precautions are recommended, including wearing masks on public transportation and in indoor public spaces.

Dr. Mary Jo Trepka, who heads the epidemiology department at Florida International University, pointed to several factors that could be driving the surge, including flocks of spring-break tourists, recent big events like the Miami Grand Prix race, and widening public apathy about the pandemic.

“I think people are no longer taking precautions as they did before,” Dr. Trepka said. “People were masking more here in the county, and we are seeing less of that. People are being less careful, because they are tired.”

Mayor Daniella Levine Cava dropped all of Miami-Dade’s pandemic safety mandates in February, around the time the C.D.C. shifted its strategy on measures such as masks and social distancing. At the time, the federal agency said, many such restrictions were no longer needed in most of the country, although counties should calculate their own risk as conditions change.

When the latest surge took hold in April, Ms. Levine Cava urged residents to get vaccinated, wear masks, disinfect surfaces and maintain social distance, but the county has not made any of those measures mandatory.

In a statement on Wednesday, Ms. Levine Cava once again called on residents to take precautions on their own: get vaccinated and boosted, get tested if they show symptoms, and stay home if they feel sick. “The best tools to fight the virus are the same ones we know and continue to use,” she wrote.

Ms. Levine Cava noted that Miami-Dade was “the most vaccinated county in Florida: and those efforts have paid off with fewer hospitalizations than in past surges. But caution was still warranted.

“We have not beaten this virus, but we know how to control it,” she wrote.

Mayor Rick Blangiardi of Honolulu County has taken a similar approach.

“Coronavirus is not going away,” he wrote in a statement on Wednesday. “I encourage everyone to continue to demonstrate personal responsibility and wear your masks when around others, get tested if you are not feeling well and please make sure to get your booster.”

Mr. Blangiardi said his administration was not considering reinstating mask mandates or other restrictions but that it would “consider all possible solutions to any situation that warrants a response.”

The county, which includes the islands of Oahu, Molokai and Lanai, has experienced a significant surge akin to Miami-Dade’s since early April, but in Honolulu’s case there are signs that it may have peaked. New virus cases have declined slightly over the past two weeks to 85 cases a day per 100,000 residents, and the positivity rate stopped climbing in mid-May.

Hawaii had some of the strictest travel restrictions in the country, requiring everyone arriving to the state to complete a 14-day quarantine. In March, it lifted its travel restrictions, allowing travelers from the continental United States to enter without testing and became the last state in the nation to remove its indoor masking requirement.

A month later, the state’s tourism industry recorded its highest traffic figures since the beginning of the pandemic, with more than 800,000 visitors arriving in the Hawaiian Islands, according to the state’s Department of Business, Economic Development and Tourism.

Mike McCarthy, the department’s director, said in a statement that the tourism sector was “showing strong recovery from the Covid-19 pandemic.” He said he expected a gradual resumption of international travel by Japanese citizens — typically a major share of visitors to Hawaii — to strengthen the rebound.

In Puerto Rico, Gov. Pedro Pierluisi lifted nearly all pandemic restrictions in March, and new confirmed cases soon started rising. But tourism to the island has bounced back: Though arrivals from cruise ships had not yet reached prepandemic levels, business travel for meetings and conventions was improving, Discover Puerto Rico, the island’s official tourism website, said in late April.

Kenira Thompson, president of the coalition of scientists and vice president for research at Ponce Health Sciences University in Puerto Rico, said that older and immunocompromised people there should consider continuing to wear masks in crowded places, and that those who are eligible for booster shots should seek them out.

Dr. Alain Labrique, the director of the Johns Hopkins University Global Health Initiative, said the summer tourism season meant large gatherings and increased contacts between people, a recipe for the easy spread of infection, even if fewer people are experiencing serious illness.

“Covid-19 hasn’t disappeared as much as our patience for precautions has,” he said.