Coronavirus inside a flight spreads as fast as in other situations, a study has found.

This study is one of two published Friday demonstrating how coronavirus can spread on airline flights, and suggesting that simply spacing people out a little will not fully protect them.

In another incident, passengers on a flight from Boston to Hong Kong appear to have infected two flight attendants.

Both cases involved long flights early in the pandemic, before airlines began requiring face masks.

A team from Vietnam also tracked down a cluster of cases linked to the flight that arrived in Hanoi from London on March 2.

“A 27-year-old businesswoman from Vietnam, whom we identified as the probable index case, had been based in London since early February,” Nguyen Cong Khanh of the National Institute of Hygiene and Epidemiology in Hanoi and colleagues wrote.

Investigators say there have been a few incidences where the virus has spread despite physical distancing guidelines.

“We conclude that the risk for on-board transmission of SARS-CoV-2 during long flights is real and has the potential to cause COVID-19 clusters of substantial size, even in business class–like settings with spacious seating arrangements well beyond the established distance used to define close contact on airplanes,” the team noted.

In one incident, a couple flew from Boston to Hong Kong in business class on March 9. They both exhibited symptoms after they arrived and were diagnosed with coronavirus.

Contact tracing found two flight attendants were also positive for the virus.