Vaccinated and masked college students had virtually no chance of catching COVID-19 in the classroom last fall, according to a sweeping study of 33,000 Boston University students that bolsters standard prevention measures.

The researchers screened the college's health records to find nine sets of students who developed COVID at about the same time, were in class together without social distancing and had no known contact outside school, suggesting that they might have transmitted it in the classroom. However, genome analysis of coronavirus samples from the groups showed that all of them more likely were infected in other places.

"When we looked at the genomes and compared them to one another, they were cousins but not closer than that," said Boston University School of Medicine virologist John Connor, a co-author. He said the study in the journal JAMA Network Open provides an answer to a nervous question common last fall: "I just walked into a class with 80 people in it. How do I know I'm not going to catch disease from them?"

The university was able to perform the study because of its comprehensive, in-house testing program that includes DNA analysis of virus samples. The semester under study included 140,000 class meetings with a mean size of 31 students, virtually all of whom were vaccinated as required. Classrooms were well ventilated, the researchers said.

In-class masking was mandatory at the time the samples were taken, in contrast to this coming fall, when many colleges will have lifted requirements. Another difference between then and now: the delta variant dominated last fall, while more contagious omicron variants like BA.5 now reign.

Those differences surely matter, Connor said, but the study's finding that in-class transmission among masked and vaccinated students was negligible can still inform future decisions about measures to take during outbreaks.