Dr. Craig Spencer wrote on Twitter recently that almost all of the patients in his care that had to be admitted to the hospital for COVID were unvaccinated. 

By comparison, every patient he’s seen who has received two Moderna or Pfizer vaccine shots as well as a booster shot has had the most mild symptoms, which include a sore throat, some fatigue, and muscle pain. And notably, Spencer said he was not seeing those patients have difficulty breathing. 

Although the ER doctor wrote that people with two doses of Pfizer or Moderna and a booster shot fared the best when it came to symptom severity, he also noted that people with two doses and no booster still had “mild” symptoms but experienced more fatigue, more fever, and more coughing. 

Spencer said that patients who had received a single Johnson & Johnson dose felt worse overall, but it appeared they had more protection than the unvaccinated.

They were “Weak, tired,” Spencer wrote. “Some shortness of breath and cough. But not one needing hospitalization. Not one needing oxygen. Not great. But not life-threatening.”

As for unvaccinated patients, those were the people most likely to have complications, need oxygen, and face severe illness or even death, according to Spencer. 

“[A]s an ER doctor you’d trust with your life if you rolled into my emergency room at 3am,” Spencer wrote. “I promise you that you’d rather face the oncoming Omicron wave vaccinated.”

Dr. Spencer did not immediately return Fortune’s request for comment.

The U.S. is averaging more than half a million daily cases of COVID, as the Omicron variant rages throughout the country. The daily average infection rate is about 2.5 times higher than it was two weeks ago, according to the New York Times. In December the Omicron variant surpassed the Delta variant as the dominant strain in the U.S.