Public health officials were already struggling with how to persuade coronavirus vaccination holdouts to get the shot. But declining case rates and a highly contagious variant have made their work at once more difficult — and more urgent.

In month 16 of the pandemic, local governments have closed most large-scale clinics and are homing in on the hardest-to-reach individuals, with modest goals of vaccinating a handful of people at a time.

The increasing prevalence of the delta variant, first discovered in India, underscores the importance of getting vaccinated, experts say, because the highly contagious strain could trigger outbreaks in still-vulnerable communities where vaccination rates are low.

Virginia, where as few as 30 percent of residents in some southwest counties are vaccinated, had reported at least 48 cases of the variant through Friday. Maryland had seen 48 cases of the same variant as of June 22, officials said.

Officials are turning to behavioral health scientists to figure out the best way to persuade the hesitant, using cellphone data to identify places to send mobile vaccination units and relying on the tried and true: one-on-one conversations with trusted community leaders.

“We have done such a good job of getting to the masses. We should just accept that the next few months of work will be slower and harder,” said Danny Avula, Virginia’s vaccine coordinator. “That’s how we chip away at the remaining portion of our population who are unvaccinated.”

The low infection rate — cases and deaths have dropped precipitously in Virginia, Maryland and the District as statewide vaccination rates approached 70 percent of all adults — could undermine the effort to vaccinate others.

“There’s no doubt that it is extremely difficult to motivate the remaining part of the population that has not been vaccinated to get vaccinated when infection rates are low,” Avula said. “We don’t want fear to be the motivation for health behaviors but want people to know we are not out of the woods.”

D.C. has not reported a new death from the virus in more than a week and has had three two-day stretches with no new cases this month. Maryland reported two consecutive days last week without a death.

Across D.C., Maryland and Virginia, the seven-day average of new cases has not cracked 300 in more than two weeks and is the lowest since the beginning of the pandemic. The average rate of new cases per 100,000 residents is 1.42 in the District, 0.93 in Maryland and 1.93 in Virginia.

In less-vaccinated areas, however, the per capita average of new daily cases is higher.

Greensville County, on the border with North Carolina, is reporting about two new cases a day, or 17.2 cases per 100,000 people. Less than 33 percent of the population is vaccinated, according to Virginia Health Department data. In much more populous Fairfax County, by contrast, there is an average of nine new cases a day — less than one case per 100,000 people. About 54 percent of the population is vaccinated.

But even in most of the less-vaccinated parts of the region, case numbers are much lower than at the height of the pandemic. And with mask-wearing and other restrictions easing, health providers say, the public’s sense of urgency about getting vaccinated has dissipated.

“I rarely see masks anymore on the Eastern Shore,” said Dimitri Cavathas, chief executive of the Lower Shore Clinic, a community health provider in Maryland that has not had a covid-19 case among its 2,000 patients since May 27. “It’s definitely different from before.”

He guessed that about one-third of residents on the Eastern Shore will never get the shot.

In addition to southwest Virginia, there are also pockets of vaccine resistance in the most rural sections of Page and Warren counties in the Shenandoah Valley, leaving residents vulnerable to an uptick in cases in the fall and winter.

“If that does happen, we expect it’ll be in the unvaccinated community,” said Colin Greene, the health district’s director.

The top reason for hesitancy is concern over side effects, according to analysis of Facebook survey data performed weekly by a team of researchers at the University of Virginia Biocomplexity Institute.

But respondents are increasingly citing distrust of the government or vaccine science, and the feeling that the vaccine is unnecessary, as reasons to take a pass for now, said Bryan Lewis, a computational epidemiologist at the institute.

“The threat seems diminished when you don’t know anyone getting actively sick,” said Lewis, whose team has a state contract to use cellphone data to identify places where mobile clinics might find people who are willing to get the shot if it doesn’t disrupt their day.

Jeff Feit, the community and population health manager at Valley Health, a health-care system based in Winchester, said conservative rural residents who decided to get vaccinated tend to be the best messengers for their community.

“I understand people are nervous about something that is new and there are lots of trust issues in the world right now,” he said, “but the truth is if you get vaccinated, you’re less likely to die of covid.”

Maryland Health Secretary Dennis R. Schrader told a legislative panel this week that 97 percent of all new covid cases in the state between May 10 and June 8, and 89 percent of hospitalizations and deaths, involved unvaccinated residents. “It’s very sobering,” he said.

Victoria Romero, chief behavioral scientist at the defense contractor CACI, advises Montgomery and Anne Arundel counties in Maryland on the best messages to persuade people to be vaccinated.

“The longer we go, the harder they are” to persuade, she said of vaccine holdouts. “The people who are left are left for a reason.”

Romero encourages doctors to avoid debunking unfounded theories or parsing political motivations as a way of persuading reluctant patients. Instead, she said, they should empathize with a patient’s fear and explain why the medical community has confidence in the vaccines.

If a person’s religious community is hesitant, Romero said, another tactic is to emphasize the people in their lives who do trust the vaccine, such as their work or school colleagues.

“To the extent that it’s true, find a way you can show that person’s group is all vaccinated,” she said.

Allen Jessee, pastor of Highlands Fellowship Church, a large congregation in southwest Virginia and upper east Tennessee, hosted a vaccination clinic at the church’s Bristol location and recently shot a TV commercial for the local health-care system.

He said he assumes that worshipers at his church have been vaccinated, but he makes a point of bringing it up if he’s not sure.

“It’s not like they are trying to make a political statement,” he said of the hesitant. “They are good people. They are just apathetic about the vaccine.”

His church followed guidance from Virginia Gov. Ralph Northam (D) on shutting down in-person services and later requiring masks and social distancing, even when it triggered anonymous messages from people who questioned his belief that God would keep them safe.

“’ ‘Where’s your faith?’ ” he said they would ask. “We just chose to sort of stay above that,” he continued. “We’re not going to get into that. That’s not who we are.”

Jessee said he conducted about a dozen funerals in two months last winter and wrote 30 letters to people who lost loved ones, such as the late state senator A. Benton Chafin Jr. As a result of those experiences, Jessee said, he couldn’t ignore what the virus had done to the community where he has lived all his life.

“I’m so pro-vaccine because I have walked a different path than some people,” he said. “I have seen the grief.”

Rebecca Tan and Ovetta Wiggins contributed to this report.