Apparently, the people who dragged him off the plane caused serious injuries.
The first few months were "horrible," he said. He suffered a concussion, lacerations to his mouth and nose, and several of his teeth were knocked out, he said. He was put on suicide watch by hospital staff and later spent months learning to walk again, he said.
Dao still struggles with issues sleeping and with his concentration and balance, he said. While he'd run more than 20 marathons before the incident, now he can only do about 3 miles -- with at least one of them by walking, he added.
And people from all over the world recognize him because of his involvement in the famous incident.
Since then, he has helped residents in Texas displaced by Hurricane Harvey and traveled to Vietnam and Cambodia to help install solar power in villages with no electricity, he said. Even in the Far East, people knew his story, he said.
One elderly man approached him and asked, "You the one on airplane?" Dao said.
"That touched me," the doctor said, holding back tears.
UA reiterates they have learned from the incident.
United Airlines issued a statement to ABC News, saying the changes they've implemented since the incident "better serve out customers and further empower our employees."
"Flight 3411 was a defining moment for United Airlines and it is our responsibility to make sure we as a company and all of our 90,000 employees continue to learn from that experience. The changes we have implemented since that incident better serve our customers and further empower our employees," according to the statement from United Airlines. "This year, we are focused more than ever on our commitment to our customers, looking at every aspect of our business to ensure that we keep their best interests at the center of everything that we do. As our CEO Oscar Munoz has said, we at United never want anyone in the United family to forget the experience of Flight 3411. It makes us a better airline, a more caring company and a stronger team."