Case rates in central and western Maine continue to climb to levels last seen during the latter end of the spring surge, just one month after trends bottomed out at prewinter levels.

As of Saturday, Androscoggin, Franklin and Oxford counties had a moderate level of community transmission, but Androscoggin and Oxford could spill over into substantial transmission in the coming days if the number of new daily cases continues to increase. A substantial or high transmission level triggers the state’s recommendation that all people, regardless of vaccination status, wear a mask indoors.

On Saturday, state health officials reported 164 new cases across the state and one additional death, a woman 80 years or older from Cumberland County. The state no longer updates case numbers on Sunday and Monday.

Of those cases, 10 were in Androscoggin County, one in Franklin County and six in Oxford County.

The seven-day rolling average of new daily cases per capita was 0.66 per 10,000 residents in Androscoggin County, 0.53 cases in Franklin County and 0.67 cases in Oxford County.

Statewide, there were 0.9 cases new cases per 10,000 individuals over the past seven days as of Saturday.

The last time case averages stood at their current levels in the tri-county area was in early June, at the tail end of the spring surge that peaked in mid-April. During the first weeks of July, case rates stood at levels not seen since late October.

In Franklin County, the seven-day average per capita was almost a 1500% increase from one month earlier on July 10, when the case rate was nearly zero.

Still, trends are far below where they stood at the worst of the spring surge. By mid-April, the seven-day rolling average in Androscoggin, Franklin and Oxford counties was 8.4, 5.0 and 5.73 cases per 10,000 residents, respectively.

What has changed in the past month is the delta variant: According to a report on genome testing from the Maine Center for Disease Control and Prevention published Monday, the delta variant accounted for 86% of all samples collected in July. In June, the delta variant was identified in just 3.3% of cases sequenced for variants.

Three cases were identified in Franklin County in July. Another case was identified in July in Oxford County, bringing the total there to three. No cases from Androscoggin County were sequenced in July and no cases of the delta variant have been identified there thus far.

Cumulatively, 160 cases of the delta variant have been identified in Maine since April, when the state began sequencing samples for variants. Eighty-five of those cases were identified from July 30 to Aug. 6, according to the report.

The variant — nearly twice as contagious as other strains of the virus — prompted the U.S. CDC to recommend late last month that even fully vaccinated people should wear masks indoors in areas with higher case rates. Maine followed suit the following day.

And although new research suggests that fully vaccinated people may spread the delta variant as easily as unvaccinated people, the risk of contracting COVID-19 and developing symptoms decreased 18-fold in vaccinated people and the risk of hospitalization or death due to COVID-19 decreased 25-fold.

While 69% of all Mainers eligible to receive the vaccine are fully vaccinated, the vaccination rate in the tri-county area ranged from 58% to 62%.