Maine reported 59 new cases of COVID-19 on Wednesday, the largest one-day spike since May.

York, Cumberland and Androscoggin counties reported the bulk of the new cases, with 16 in York, 15 in Cumberland and 14 in Androscoggin. No additional deaths were reported.

The seven-day daily average of new cases stood at 33, compared to 36.1 on Sept. 23.

Robert Long, spokesman for the Maine Center for Disease Control and Prevention, said there were no new outbreaks, and it’s unclear what caused the one-day spike. He said the CDC is investigating Wednesday’s cases for trends and will report its findings when Dr. Nirav Shah, the agency’s director, gives a media briefing Thursday.

“Testing volume continues to rise, so one hypothesis is that more tests yield more cases,” Long said. “But that would not fully account for (Wednesday’s) statewide case count.”

Maine’s testing capacity continues to expand, with 459 tests given per 100,000 population, compared to about 250 tests per 100,000 population in late August. Maine has ramped up its testing by adding 27 “swab and send” sites statewide, where people can obtain tests without a doctor’s note and usually have results within 48 hours.

Maine is one of only 13 states to meet testing targets established by the Harvard Global Health Institute. All of New England, plus New York and New Jersey, are doing enough testing, according to the institute, while 33 states fall far below the standards needed to potentially control the spread of the virus.

Pedestrians pass each other on Oak Street in downtown Portland on Monday. Brianna Soukup/Staff Photographer

The percent of tests that come back positive continues to be low in Maine, another metric watched closely by public health experts. On Wednesday, the positivity rate for Maine was 0.49 percent over a seven-day period, much lower than the national average of about 4.5 percent.

A low positivity rate gives public health workers a better chance to slow transmission by isolating most of those who are sick and their close contacts.

Despite Maine’s positive indicators, Wednesday’s surge “provides a clear reminder that the virus is still present throughout Maine,” Long said.

The last time Maine reported more than 50 cases in one day was 51 cases on Sept. 2, and the last time the daily case numbers surpassed Wednesday was on May 22, when 65 cases were reported. The largest one-day spike in daily new cases was 78, which occurred on May 19.

The net new cases on Wednesday – when taking into account probable cases from previous days that later were found to be negative – was 54, the Maine CDC said.

Currently, 13 people are hospitalized statewide, including seven in intensive care.

In other developments, Gov. Janet Mills extended the state of civil emergency for 30 days, through Oct. 29, as the public health crisis continues. The civil emergency permits the state to continue Mills’ executive orders that attempt to control the spread of the virus by mandating mask wearing in indoor public places and limiting the size of gatherings among other protocols. The civil emergency orders must be renewed every 30 days to remain in force.

“As Maine enters the colder months and more activities move indoors, it is more important than ever to maintain the critical public health measures that have kept us all safe,” Mills said in a statement. “We know how to mitigate the spread of the virus. In order to protect our health, keep schools safely open for as many students as possible, and ensure our economy can continue on the road to recovery, we must wear our face coverings, maintain physical distancing and wash our hands often.”

More schools are starting to report outbreaks, including 18 cases at Sanford High School and Regional Technical Center. Last week, the Maine CDC conducted on-site testing of all students and staff at the 1,000-student high school.

New outbreaks and individual COVID-19 cases have also been reported in greater numbers in several school districts over the past week. Recent schools that have reported cases include Freeport middle and high schools, Massabesic Middle School, Mountain Valley High School in Rumford, Yarmouth schools, Maranacook Community Middle School in Kennebec County and others.

The U.S. Centers for Disease Control and Prevention said Monday that the incidence of COVID-19 in school-age children began rising in early September as many youths returned to their classrooms. Children now make up 10 percent of all cases, up from 2 percent in April, according to the American Academy of Pediatrics.

Shah said at Tuesday’s media briefing that a shipment of 26,000 antigen tests from Abbott Laboratories will be arriving in Maine soon, possibly within the next week.

The U.S. CDC has directed that, for the Abbott tests, the “principal use be in cases in and among schools,” he said. The state is working on a plan to quickly distribute the tests to school districts.

The Illinois-based company, which has a lab and manufacturing plant in Scarborough and a manufacturing plant in Westbrook, landed a $750 million contract with the federal government in August to produce 150 million tests nationwide. The tests cost $5 each and can produce results from a less-invasive nasal swab within 15 minutes.