A COVID-19 outbreak in an elementary school classroom in Lower Merion could have been fueled by a faulty air duct, according to the school district.

Eight students in a second-grade classroom at Penn Valley Elementary have tested positive for the virus in what officials say marked the district’s first instance of significant in-school transmission.

Due to the scope of the outbreak, district staff evaluated the classroom’s HVAC system and found that a part within the ductwork above the ceiling “was too far closed, allowing only (approximately) 30% of the maximum amount of fresh air it should have into this specific room,” Terry Quinlan, lead supervisor of school health and student safety for the district, said in an email to families Friday.

The district “cannot say definitively whether the diminished fresh airflow contributed to the outbreak; however, it could be a factor,” Quinlan said. She added that the Montgomery County Office of Public Health “also noted the possibility of a variant strain of COVID-19 being a factor, citing both the rapid spread within the class and the fact that two vaccinated family members of impacted students have also tested positive.”

It wasn’t clear whether those family members were fully vaccinated; a spokesperson said the county health department could not share that information.

Vaccines do not prevent all cases of COVID. Clinical trials have suggested that the Pfizer and Moderna vaccines, for example, work very well, preventing close to 95% of COVID cases. But in a group of, say, 1,000 vaccinated adults, that means 50 could still get sick if they are exposed to the virus.

And for a variety of reasons, the real-world effectiveness of the vaccines is likely to be a bit lower, as is true with any vaccine outside the confines of a formal study. Human behavior and the amount of virus circulating both play a big role, which is why public health experts recommend a certain level of precaution even for vaccinated people.

Like numerous area school districts, Lower Merion is spacing students at least three feet apart in classrooms — a downgrade from the six-foot span that the Centers for Disease Control and Prevention endorsed for elementary schools, as well as for older grades at certain levels of community spread.

Lower Merion spokesperson Amy Buckman said she couldn’t comment on whether spacing may have played any role in the outbreak, but noted that distancing was “pretty consistent across all elementary classrooms in the district and this is the only outbreak impacting this number of students that we’ve seen.”

Quinlan, who said the second-grade class members had been quarantined since April 16, added that the district’s operations staff would be conducting air quality tests at all buildings.

The district’s ventilation systems met or exceeded standards “even before the pandemic,” she said, and had been further upgraded as a result of COVID-19, including with the installation of MERV 13 filters throughout schools and bipolar ionization in large group spaces.

Staff writer Tom Avril contributed to this article.