SCARBOROUGH, Maine (AP) — Maine’s growing number of COVID-19 cases is disproportionately affecting smaller towns.

The 20 ZIP codes with the highest infection rate in the last month are concentrated in York, Androscoggin, Kennebec and Oxford counties, and only three — Lewiston, Waterville and Sanford — are major urban areas, the Bangor Daily News reported.

Health officials said factors could be growing weariness of masks and social distancing.

The growth in infections comes even as the state’s vaccine effort ramps up. About 36% of Maine residents are fully vaccinated, and the vaccine is now available to everyone 16 and older.

The Maine Center for Disease Control and Prevention Director Dr. Nirav Shah said the average age of patients with COVID-19 has also come down to 35. Earlier this year it was 43, he said.

“All of this shows how opportunistic the virus is,” Shah said.

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As of Thursday, Maine's seven-day rolling average is over 400 per day, the highest since late January. Aroostook County is reporting nine new cases today and 70 in the past seven days.

So far, over 404,000 Maine residents have received their final dose of the COVID-19 vaccine. That amounts to nearly 36% of the population age 16 and older. That's about halfway to herd immunity that experts say is needed to curb the community spread of the virus.

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