COLUMBIA, Mo. -- A pilot study released Friday suggests low COVID-19 transmission among students and staff at some Springfield and St. Louis schools, and Missourians now have until mid-May to file state taxes.

Gov. Mike Parson on Friday announced his administration pushed the deadline for individuals to file state tax returns from April 15 to May 17, days after the federal government did the same.

Parson said the delayed due date will help people still struggling financially because of the pandemic.

Also on Friday, researchers released results from a pilot study of COVID-19 transmission in some Springfield and St. Louis schools.

The study by the Washington University School of Medicine and the Centers for Disease Control and Prevention reviewed the virus' spread in 57 schools over a two-week period in mid-December, when virus cases in Missouri were high.

Masks were required at the schools. Other safety measures included deep cleaning, emphasis on good hand-washing and sanitizing, better ventilation, daily symptom screening and virtual class options.

Of 102 students in close contact with 37 people who tested positive for COVID-19 -- other students, staff and teachers -- the pilot study found only two contracted the virus.

"We were seeing levels of hospitalizations that were scary for a lot of people," said Jason Newland, a Washington University pediatrics professor and study leader, "And these schools were still operating at that time and transmissions were still low."

The pilot study also sheds light on differences in quarantine procedures among schools. Springfield schools allowed students who were close contacts of infected people at school but reliably wore face masks to continue in-person learning. The spread of the virus was still rare.

"That's some data that's suggesting maybe there's other strategies of quarantining of our students that will allow them to stay (at) in-person school and not have them be home with traditional quarantining," Newland said. "We have more to learn on that."

The pilot study results came the same day U.S. health officials relaxed social distancing recommendations for schools to allow students to sit as close as 3 feet to each other in classrooms.

The new CDC guidelines signal the agency's turn away from the 6-foot distancing recommendation that had forced some schools to remove desks, stagger scheduling and take other steps to keep kids apart.

In the Missouri study, desks were spaced 6 feet apart or closer in roughly 70% of classrooms studied.

Newland said researchers are close to finishing a more expansive study on the virus' spread at six urban, suburban and rural school districts.

He said that study will include data on how far apart students are, not just classroom desk spacing, which could answer questions about the role of social distancing in reducing spread of the virus in schools.

As of Sunday, Missouri's health department had reported 485,997 confirmed cases of the coronavirus since the pandemic began, and 8,374 deaths.