Families got the first worrisome email on July 16. A fully vaccinated resident of the Good Samaritan Society nursing home in rural Clear Lake, S.D., had tested positive for the novel coronavirus, a rare breakthrough infection. In subsequent days, notifications from the nursing home kept coming, each progressively more alarming.

On July 18, with six cases confirmed, a relative responded to the nursing home’s email with an emoji of Edvard Munch’s painting “The Scream.” Then came the first death, on July 20. By Tuesday this week, the facility said it had 14 covid-19 cases and two deaths — all among residents who were fully vaccinated in January.

The outbreak — the largest the national chain has seen among vaccinated residents — helped spur Good Samaritan to impose a vaccine mandate on its 16,000 staffers working in 22 states to try to avoid a repeat of the devastating covid-19 surges in spring 2020 and last winter, which contributed to a total of 185,000 pandemic deaths among people in facilities providing long-term care.

As the threat of the delta variant grows, other large nursing home chains also are beginning to adopt vaccine mandates, including the largest for-profit chain, Genesis HealthCare, which is requiring employees to get their first vaccine shot by Aug. 23. Massachusetts said Wednesday that it is imposing a state vaccine mandate on nursing home staffers. These moves are part of a building wave of requirements in health-care settings nationally.

Good Samaritan’s vaccine mandate, announced last month, goes into effect Nov. 1. Good Samaritan is the largest nonprofit long-term-care chain in the nation, with 200 nursing home, assisted-living, and other long-term-care services. It is affiliated with the large Midwestern provider chain Sanford Health, which also adopted the vaccine mandate. Counting Sanford’s 46 hospitals and medical network, the system has nearly 50,000 employees who are required to get vaccinated.

Coronavirus infections in nursing homes nationally remain extremely low after horrific waves of infections and deaths during earlier phases of the pandemic. High rates of vaccination among residents of long-term-care facilities quelled that category of outbreaks. Still, nursing home operators said sealing off the chance that unvaccinated workers will introduce new infections is crucial to preventing another surge.

“Clear Lake is the biggest, most active outbreak we have. You’ve got to plan like this is going to happen, as if it’s going to be just like we saw with the original outbreak,” Randy Bury, the chief executive of Good Samaritan Society, said in an interview. “Most experts will agree we’ve been wrong as much as we’ve been right in trying to predict this virus.”

Good Samaritan’s facility in Clear Lake has about 31 residents, and all have been vaccinated. But fewer than two thirds of the 61 staff members have been vaccinated, which is in line with the national vaccination rate for workers in nursing homes. Six employees of the facility have tested positive during the outbreak, none of them vaccinated, Good Samaritan confirmed.

The crisis shocked family members who had expected that their loved ones would be protected by vaccinations.

“We’ve been scared for all these weeks, basically just praying that our mom doesn’t get covid,” the daughter of an uninfected resident, who spoke on the condition of anonymity to protect the resident’s health-care privacy, said in an interview.

“Even though they were supposed to be masking, I saw people coming in with masks below their noses all the time. I didn’t say anything because my mom was vaccinated, I was vaccinated,” the daughter said. “I’m blaming the people who did not get vaccinated, and I feel they wantonly put these at-risk old people in danger.”

Bury said the episode at Clear Lake was frustrating. Vaccinations have been proved to dramatically stifle covid outbreaks, he said, but not enough staffers have been vaccinated.

“Certainly, Clear Lake was one of the catalysts for us,” Bury said. “We’ve been trying to keep it out, but now we’ve got this more-contagious variant that can break through some of those defenses.”

The rapid spread at Clear Lake suggests that the outbreak was triggered by an unvaccinated staff member coming into contact with multiple elderly residents, Bury said. He said the state of South Dakota, which says on its website that it performs “sentinel” genomic testing to track coronavirus variants, has not confirmed what strain of virus infected the Clear Lake nursing home residents.

Bury added that it is a “logical conclusion” that the delta variant is responsible for the facility’s outbreak. Faced with so many unknowns about the variant’s strength, he said, health systems need to plan for the worst to prevent a repeat of last winter.

Good Samaritan’s Clear Lake facility had avoided outbreaks previously in the pandemic with stringent isolation measures that prevented visits by residents’ families. The introduction of vaccines allowed those restrictions to be eased in March.

Now, a ban on family visits is back in force at the nursing home. Infected patients are quarantined in a “red zone” within the building. Uninfected residents have been ordered to remain isolated in their rooms; bingo is being played over the intercom system, according to the daughter of the woman who has not been infected.

The South Dakota Department of Health is investigating the outbreak and working with the home’s staff to assess testing and protective equipment needs, provide advice on how to halt the spread and review procedures, said agency spokesman Daniel Bucheli.

But South Dakota Gov. Kristi L. Noem (R) is a staunch opponent of vaccine and mask mandates. She indicated her continued opposition to mask requirements in tweets on July 28, saying school administrators should take into account the impacts of masks on learning.

Asked in an email whether the state supports vaccine mandates for staffers in nursing homes, Bucheli wrote back, “Gov. Noem has been very clear about vaccine and mask mandates — there will be none in South Dakota.”

Bucheli did not answer emailed questions about whether the state has tested for the presence of delta variant in Clear Lake, citing the privacy interests of the facility’s residents and the staff.

About 47 percent of the state’s total population is fully vaccinated, which ranks it in the middle of the pack among states.

The Centers for Disease Control and Prevention did not respond to requests for comment and perspective on the South Dakota nursing home outbreak.

CDC data shows that after vaccines crushed the number of viral infections in nursing homes this year, there has been only a slight increase in infections from the very low numbers.

Nursing home infections hit a low of 326 in the week ending June 27, according to CDC data, and climbed back to 1,312 in the week ending July 25. There were 208 deaths in the week ending July 25, up from 121 in the week ending June 27.

At the peak of the winter surge in December 2020, infections in nursing homes reached a peak of 35,000 in a single week, with 6,358 deaths.

A growing number of nursing home and assisted-living operators say now is the time to prevent a more significant surge in the facilities as the delta variant sweeps through unvaccinated populations in outside communities. The industry’s trade groups are divided by business model. The group representing nonprofit nursing homes, LeadingAge, has endorsed mandates, while the representative of for-profit and nonprofit operators, the American Health Care Association, said it supports strong vaccination education efforts aimed at nursing home staffers.

One of the factors that made the industry reluctant to impose vaccine mandates on staffers previously was the fear of causing people to leave their jobs. Bury said leaders of Sanford and Good Samaritan Society expect that they will lose some staffers, but he added that acceptance of vaccines among nursing home workers has grown with time.

In New Jersey, CareOne, a for-profit long-term-care chain, is imposing a mandate with a deadline of Sept. 30.

“We hope that people get vaccinated, not terminated, because we’re not looking to lose people to the mandate,” said Toya Casper, CareOne’s chief clinical officer.

But other operators say a mandate could make a shortage of workers throughout the industry even worse. Single-facility and family-owned operators could be especially disadvantaged because they lack the scale of their larger competitors, industry advocates said.

“The challenge for many long-term-care providers is that we are at Defcon 1 when it comes to our staffing crisis,” said Brendan Williams, president and chief executive of the New Hampshire Health Care Association. New Hampshire’s nursing homes have a 76 percent staff vaccination rate.

“To lose even one vaccine-hesitant worker, let alone several, could be debilitating to a facility’s operation,” Williams said.