The New York Times interviewed several dozen of the newly vaccinated in the days afterward. They recounted a wide spectrum of responses, from no reaction at all — “Can’t even tell I had the shot,” said a hospital worker in Iowa City — to symptoms like uncontrolled shivering and “brain fog.” A nurse assistant in Glendora, Calif., wondered whether the fever he ran was a side effect of the vaccine or a sign that he had been infected by one of his Covid patients.

And there was a dizzying variety of sore arms. Some likened the pain to that from a flu shot; for others, it was considerably worse.

Like nearly every vaccine recipient who spoke to The Times for this article, Dr. Wilson stressed that she had no regrets about getting the shot, despite the headache, which was gone within 36 hours. The Food and Drug Administration has found the vaccines to be safe and remarkably effective. And public health leaders say mass vaccination is the only hope for controlling the virus that is now claiming the lives of close to 3,000 Americans a day.

But in these first weeks of vaccination, there is an inescapable element of suspense.