The risk of being exposed to the COVID-19 virus on an airline flight drop by as much as half when airlines keep middle seats open, a new study published by the U.S. government concludes, a safety practice the carriers have abandoned.

The study is the latest to roil the waters on a controversial topic: just how risky it is to travel during the pandemic. It did not attempt to estimate infection risks and was based on modeling done before the pandemic, so didn’t consider rules that now require face masks on flights.

The risk of coming in contact with the virus dropped by 23 per cent to 57 per cent if airlines limited passenger loads on both single-aisle and widebody jets, compared to full occupancy, according to research released by the Centers for Disease Control and Prevention on Wednesday.

“Based on a data-driven model, approaches to physical distancing, including keeping middle seats vacant, could reduce exposure to SARS-CoV-2 on aircraft,” the study said.

While airlines have touted research -- sometimes funded by the industry -- showing low risks from the novel coronavirus during travel, there have been studies showing transmission can occur on flights even when passengers were wearing masks.

For the CDC study released Wednesday, scientists at the agency and Kansas State University, a leading center for study of airline cabin air quality, used multiple models to predict how airborne virus particles would spread on planes. The models were based on a surrogate virus and data from a 2017 study.

The study’s results may be too late for U.S. travelers.

Delta Air Lines Inc. will resume selling middle seats on May 1, the last U.S. carrier to lift the social-distancing policy that has been in place more than a year. The carrier temporarily reopened the seats on some planes April 4 and 5 after a staffing shortage forced it to cancel about 100 flights.

Delta decided to end the policy amid rising evidence of a long-expected resurgence in travel demand as U.S. vaccinations increase and consumers book flights to visit family or take a vacation. American Airlines Group Inc. is adding 150 new or restored routes this summer as it plans to fly more than 90 per cent of its pre-pandemic domestic seat capacity.

While travel by air remains severely depressed since the pandemic erupted in March 2020, it has grown substantially this year.

An average of 1.4 million people a day have flown in the U.S. this month, about double the roughly 700,000 a day in early February, according to data from the Transportation Security Administration. By comparison, about 2.3 million people a day flew on average in early April in 2019.