Rules requiring teachers to wear face masks in schools helped slow transmission of the coronavirus, a study examining German schools found.

Though the first identified COVID-19 cases in one German region’s schools were most often reported among children, outbreaks were more serious when adults were the first ones infected. This was especially true during one period earlier in the pandemic, when campuses held face-to-face classes but staff were only required to wear masks some of the time, according to the study published in the journal Frontiers in Public Health.

In preschools, where there was no mask mandate during in-person instruction unless workers had contact with a child showing symptoms, results showed more secondary cases when the first case of the virus — described as an “index” case — was identified in adults.

“The strongest predictor of the extent of the outbreak in our study was found to be the teacher/caregiver mask obligation,” researchers said. “Furthermore, requiring children to wear masks may also reduce the number of secondary cases.”

Yet while requiring children to wear face masks helped slow the spread of the coronavirus in schools, researchers said “mask-wearing is not recommended in young children under 6 years of age, as they cannot use it properly.”

In the United States, the American Academy of Pediatrics on its website calls face coverings tools that can help slow the spread of COVID-19 and “can be safely worn by all children 2 years of age and older, including the vast majority of children with underlying health conditions, with rare exception.”

The study results were publishedlast month, after the start of a school year that has seen mask-related protests and debates erupt at school board meetings across the U.S. In recent days, the rising threat of the omicron coronavirus variant has led some of the country’s schools to consider taking additional measures to protect students after the winter break, The News & Observer reported.

To help get answers about the spread of COVID-19 in schools, researchers examined data in the Mecklenburg-western Pomerania region of Germany. The group represented three German institutions: University Medical Center Rostock, University Medicine Greifswald and the State Office for Health and Social Affairs Mecklenburg-western Pomerania.

Researchers studied figures from four time periods between August 2020 and May 2021. During two of them, schools were holding face-toface classes, with each phase having different coronavirus-related precautions.

In all, researchers reported 475 coronavirus outbreaks — or “infection events” — in schools and 438 of them in preschools.

During the phases when school children were getting in-person instruction, 72 adults were the first to be infected, leading to 194 secondary infections. In comparison, 321 children were the first cases but only led to 95 secondary infections, according to data shared with Mcclatchy News.

The first round of in-person classes in late 2020 had fewer mask requirements than the second one in early 2021. While the number of index cases was similar during the two time periods, results showed a significant drop in the number of second cases when stricter masking requirements were in effect.

For example, when there were no masking requirements, an adult index case resulted in an average of 4.5 more infections. But an adult index case only led to 0.5 more infections when more masking requirements were in place. Children identified as the index case caused the same average number of secondary infections — 0.3 — regardless of mask requirements.

Researchers said the findings could have implications for keeping schools open as the coronavirus pandemic continues.

“To avoid school closures at high incidence levels, it is important to identify factors contributing to the spread of COVID-19 infections in schools and preschools,” Dr. Anika Kästner, from University Medicine Greifswald, said in a news release.

As the coronavirus spreads, health officials in the United States urge people to wear masks inside public buildings. As of November, the Centers for Disease Control and Prevention recommends workers and students ages 2 and older put on face coverings in schools, no matter their vaccination status.