A new study may explain why Dr. Romano and many others who have had Covid report these unexpectedly intense reactions to the first shot of a vaccine. In a study posted online on Monday, researchers found that people who had previously been infected with the virus reported fatigue, headache, chills, fever, and muscle and joint pain after the first shot compared more frequently than did those who had never been infected. Covid survivors also had far higher antibody levels after both the first and second doses of the vaccine.

Based on these results, the researchers say, people who have had Covid-19 may need only one shot.

“I think one vaccination should be sufficient,” said Florian Krammer, a virologist at the Icahn School of Medicine at Mount Sinai and an author on the study. “This would also spare individuals from unnecessary pain when getting the second dose and it would free up additional vaccine doses.”