After declining for months, largely because of the federal effort to vaccinate residents, the number of deaths rose sharply from July to August. Nursing homes reported nearly 1,800 deaths among their residents and staff in August, which represented the highest monthly toll since February.

The findings underscore the ongoing vulnerability of nursing home residents, who are old and in poor health, and highlights the importance of getting booster shots to people in this population. The rising number of infections could fuel more calls to vaccinate nursing home workers ahead of the federal requirement announced in August by President Biden.

“While the vast majority of Covid-19 deaths happened outside of nursing homes in July and August, the high rate of increase within nursing homes indicates that residents and staff in these settings are at risk of death during the Delta surge,” the researchers said. The study, published Friday, did not break out how many of the dead were unvaccinated.