Updated at 6:20 p.m.

Coronavirus infection rates continue to increase across Massachusetts, with the number of cities and towns in the highest risk "red" category nearly doubling in the last week.

According to the latest weekly report from the Department of Public Health, which considered cases recorded from Sept. 20 to Oct. 3 and was released Wednesday, 40 communities across the state are in the red category — meaning their infection rates are above eight cases per 100,000 people. The week before, only 23 municipalities were at that level.

The report released Wednesday shows the state's highest infection rate was in Middleton, at 58.1 infections per 100,000 population. That is significantly higher than Middleton's COVID rate the week prior, which was 12.5 infections per 100,000 people — though that still placed the town in the red zone. That follows an outbreak at the Middleton Jail and House of Correction, where 139 prisoners have tested positive for the virus in the last several days.

Middleton Town Administrator Andy Sheehan addressed the outbreak in a recorded message posted to YouTube on Tuesday, and emphasized that the spike in cases was confined to the jail.

"It's certainly good news that the recent spike is not the result of community spread in the at-large population of Middleton residents," Sheehan said. "The bad news is that there's an increase in cases within Middleton's borders. And due to the way that the state reports its COVID data, it is difficult to separate those isolated cases that we see — in our case, at the jail — from the at-large population data."

In order to proceed with the reopening process, Sheehan pointed out, communities must be out of the red "higher risk" category for three consecutive weeks. So, the outbreak in the jail could stall the town's progress. Sheehan said town officials are reaching out to the state in an effort to keep the town from being in that category.

In Boston, a positive test rate of 4.1 percent led Mayor Marty Walsh to postpone the scheduled reopening of in-classroom learningby a week, to Oct. 22.

The state Department of Public Health declined to make anyone available for an interview on the updated case numbers, instead releasing a statement saying they’re closely monitoring data to analyze the impact of the virus every day.

“The COVID Enforcement and Intervention Team will continue to work closely with high risk communities, collaborating with local officials to understand the impact that unique factors like higher education or long term care facilities may have on their case numbers,” the statement says. “The Administration recommends cities and towns review several weeks of data before making any updates to local policies, including schools.”

"Schools are reopening, workplaces are reopening, the economy is reopening. And those things are going to lead to more interactions between people," said infectious disease doctor and researcher Mark J. Siedner of Massachusetts General Hospital and Harvard Medical School. "And when we have more interactions, there's going to be a higher risk for cases."

Siedner recently authored apaper in the journal Clinical Infectious Diseases that looked at infection rates in all 50 states between March and mid-July, and tracked the impact as social distancing measures were relaxed. He said the researchers hoped to see that people continued to take necessary precautions, even as mandated social distancing rules were relaxed.

"But we didn't find that," he said. "Unfortunately, pretty much across the country, what we found is that when these statewide policies were lifted, epidemic growth recurred."

Although the paper tracks infection rates from several months ago, Siedner suggested the current increase in Massachusetts cases results from the same relaxation in vigilence.

"I think what we're seeing today in Massachusetts is likely a combination of large returns to school, some relaxation in measures — whether they were things like restaurants or bars ... and people feeling a little bit more comfortable," he said. "As our epidemic improved with test positivity rates falling to less than 1%, it's possible that behaviors also loosened. All of those things are going to contribute to an increase in cases."

Despite that, Siedner said the answer to reducing cases again isn't just shutting everything down again. Tools like testing and contact tracing should be used, he said, to identify outbreaks in places like schools, prisons or manufacturing plants.

"Test, isolate, quarantine and control small epidemics," Siedner sad. "Failing that, if we get to a situation, which we may get to, where that is not enough and we can no longer contain it with testing and tracing, that's when these more broad policies need to come into place to get us back down to the safe level."

Andrew Lover, an assistant professor of epidemiology at UMass Amherst, cautioned that the COVID-19 infection rates could continue to increase as the winter weather sets in.

“Other countries with cold weather are starting to see some pretty rapid expansion,” Lover said. “Switzerland, especially in the last few weeks, has started to have really cold weather. So people are spending more and more time indoors and in closer proximity. And we kind of have to assume that that will start contributing to some growth here in the Northeast.”