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New COVID cases in New Hampshire continue to climb.

COVID-19 continues to spread rapidly in New Hampshire, with the delta variant driving infection rates to levels not seen since April.

The N.H. Department of Health and Human Services announced 337 new cases Friday, the highest one-day total since April 22. The state averaged 252 new cases per day over the past week, again comparable to the rate in late April.

Other metrics are also headed in a troubling direction. New Hampshire’s hospitals held more than 80 confirmed COVID-19 patients on Friday, up from 63 a week earlier and 28 on Aug. 1.

And a higher portion of tests are coming back positive. The seven-day average positivity rate was 6 percent as of Friday, compared to 3.6 percent at the start of the month and below 1 percent in late June.

As of Friday, Cheshire, Sullivan and Hillsborough counties were all experiencing high transmission, according to the Centers for Disease Control and Prevention.

In June, following the winter’s devastating surge, the rate of new cases had fallen to its lowest level in 10 months, raising hopes for a semi-normal summer. The week ending June 25 averaged just 16 new cases per day.

But the rapid spread of the more contagious delta variant, combined with plateauing vaccination rates, has led to a sharp spike over the past several weeks.

“In June 2021, I was thinking that we were seeing a lull in cases similar to the summer of 2020; however, I was worried that things would take off again in the fall, if vaccination rates did not increase,” Dr. Michael Calderwood, an infectious-disease doctor and chief quality officer at Dartmouth-Hitchcock Medical Center in Lebanon, said in an email.

“Sadly, this rise began in late July 2021 rather than in the fall.”

He noted that mask wearing also declined sharply this summer, and cellphone data showed Americans were moving around as much as they did pre-pandemic.

The latest state data do contain, perhaps, a sliver of good news — a jump in new vaccinations. An average of 574 people per day received first doses in the week ending Thursday, according to state data. That number had hovered below 200 for most of July.

Still, it’s well below the thousands per day who were getting vaccinated in the spring. And with state data showing just 53.9 percent of the population is fully inoculated, New Hampshire is far from herd immunity, according to Calderwood.

“We are not close to a statewide vaccination rate where those who are unvaccinated can feel safe,” he said.

Due to the “higher transmissibility and slightly lower vaccine efficacy with the delta variant,” he said, “it may be necessary to vaccinate 90% of the NH population to achieve herd immunity.”

Even against delta, people who are fully vaccinated have “significant protection” against COVID, Calderwood said. In particular, he said, it greatly reduces the risk of contracting severe illness or dying due to the virus.

Nationally, unvaccinated people seem to be 47 to 48 times more likely to be hospitalized with COVID-19 than vaccinated people, Calderwood said on Twitter recently.

Local transmission ‘high’

Both the CDC and state health department now rank Cheshire, Sullivan and Hillsborough counties in the highest category for transmission. For the CDC, that’s “high” transmission, the worst of four levels, while the state describes them as having “substantial” transmission, the highest of three tiers.

Both Cheshire County and Hillsborough County outside of Manchester and Nashua had about 200 new cases per 100,000 residents over the past two weeks. In Sullivan County, the rate was 232. The test positivity rates over the past seven days ranged from 5.5 percent in Cheshire to 6.7 percent in that portion of Hillsborough County.

Cheshire County had 117 active COVID-19 cases as of Friday. Most towns in The Sentinel’s coverage area had at least one known active case, led by Keene with 34. Rindge had 15, Jaffrey 12, Swanzey 10, Marlborough and Walpole six each, and Chesterfield, Fitzwilliam and Westmoreland five apiece. Various other communities had one to four cases.

The state announced two COVID-related deaths in the past week. Neither person was a local resident.

To date, more than 104,000 Granite Staters have tested positive for COVID-19, of whom 1,397 have died.

There were 2,191 known active cases statewide as of Friday.