New York doctors are seeing marked improvements in COVID-19 patients by utilizing one simple technique: flipping them on their stomachs.

Doctors treating coronavirus patients have begun laying them on their stomachs to help them breathe, a technique known as prone positioning, according to CNN. The practice is particularly effective on patients with COVID-19, caused by a virus that attacks the respiratory system, by increasing the amount of oxygen in their blood by more than 10%.

“We’re saving lives with this, one hundred percent,” said Dr. Mangala Narasimhan, Northwell Health’s regional director for critical care. “It’s such a simple thing to do, and we’ve seen remarkable improvement. We can see it for every single patient.”

“Once you see it work, you want to do it more, and you see it work almost immediately,” said Dr. Kathryn Hibbert, director of the medical ICU at Massachusetts General Hospital.

COVID-19 is caused by the virus known as SARS-CoV-2, short for severe acute respiratory syndrome coronavirus 2. The virus infects the upper respiratory tract from the nose to the back of the throat and can spread down through the trachea and lungs, causing symptoms such as coughing and shortness of breath.

In severe cases, the virus can cause pneumonia accompanied by chest pains, high fever, chills, and other symptoms. In some, the damage to the lungs becomes so great that the patient can no longer breathe on his own and needs a respirator to get oxygen.

Laying patients on their stomachs has proven to be an effective treatment no matter how far the virus has progressed, though it does come at a cost. Ventilated patients lying on their stomachs need more sedation than patients allowed to lie on their backs. For patients not in intensive care, the prone position is generally less comfortable and people have trouble keeping themselves occupied for long periods of time.

Hibbert usually asks patients off ventilators to lie on their stomachs for a just few hours at a time.

“Most are willing to give it a try,” Hibbert told CNN. “How long they stay in that position really varies from person to person, whether they’re comfortable falling asleep in that position, or if they get bored and want to turn over to their backs.”

Hospitals began mainstreaming the prone position to treat respiratory diseases after several French doctors published a study in the New England Journal of Medicine on the technique’s effectiveness at treating acute respiratory distress syndrome (ARDS) in June 2013. The doctors monitored 466 patients with severe ARDS cases and made 237 of them spend at least 16 consecutive hours on their stomachs. The rest remained on their backs, known as supine position.

“A total of 237 patients were assigned to the prone group, and 229 patients were assigned to the supine group. The 28-day mortality was 16.0% in the prone group and 32.8% in the supine group,” the doctors wrote, adding that the hazard ratio, or likelihood of death, from prone positioning was 0.39, a significant improvement.

“Unadjusted 90-day mortality was 23.6% in the prone group versus 41.0% in the supine group (P<0.001), with a hazard ratio of 0.44,” the doctors said.