Even leading experts in vaccine science have run up against disbelief and ambivalence.

Dr. Gregory Poland, 68, editor in chief of the journal Vaccine, said that a loud whooshing sound in his ears had accompanied every moment since his first shot, but that his entreaties to colleagues at the Centers for Disease Control and Prevention to explore the phenomenon, tinnitus, had led nowhere.

He received polite responses to his many emails, but “I just don’t get any sense of movement,” he said.

“If they have done studies, those studies should be published,” Dr. Poland added. In despair that he might “never hear silence again,” he has sought solace in meditation and his religious faith.

Dr. Buddy Creech, 50, who led several Covid vaccine trials at Vanderbilt University, said his tinnitus and racing heart lasted about a week after each shot. “It’s very similar to what I experienced during acute Covid, back in March of 2020,” Dr. Creech said.

Research may ultimately find that most reported side effects are unrelated to the vaccine, he acknowledged. Many can be caused by Covid itself.

“Regardless, when our patients experience a side effect that may or may not be related to the vaccine, we owe it to them to investigate that as completely as we can,” Dr. Creech said.

Federal health officials say they do not believe that the Covid vaccines caused the illnesses described by patients like Mr. Barcavage, Dr. Zimmerman and Ms. France. The vaccines may cause transient reactions, such as swelling, fatigue and fever, according to the C.D.C., but the agency has documented only four serious but rare side effects.

Two are associated with the Johnson & Johnson vaccine, which is no longer available in the United States: Guillain-Barré syndrome, a known side effect of other vaccines, including the flu shot; and a blood-clotting disorder.

The C.D.C. also links mRNA vaccines made by Pfizer-BioNTech and Moderna to heart inflammation, or myocarditis, especially in boys and young men. And the agency warns of anaphylaxis, or severe allergic reaction, which can occur after any vaccination.

Listening for Signals

Agency scientists are monitoring large databases containing medical information on millions of Americans for patterns that might suggest a hitherto unknown side effect of vaccination, said Dr. Demetre Daskalakis, director of the C.D.C.’s National Center for Immunization and Respiratory Diseases.

“We toe the line by reporting the signals that we think are real signals and reporting them as soon as we identify them as signals,” he said. The agency’s systems for monitoring vaccine safety are “pretty close” to ideal, he said.