The analysis concluded that nocebo responses accounted for 76 percent of systemic adverse reactions—like headache, fever, and chills—after the first vaccine dose and 52 percent of systemic reactions after the second vaccine dose.

Of course, not all side effects are nocebo responses; some are clearly real, particularly local reactions and side effects after the second dose of a COVID-19 vaccine.

In the meta-analysis, Haas and her colleagues found that about 35 percent of placebo recipients reported at least one systemic side effect after their first faux dose.

Meanwhile, 46 percent of vaccine recipients reported at least one systemic side effect after getting their first real dose.

When the researchers looked at the severity levels of all of those systemic side effects, they found similar proportions of severity grades between the placebo and vaccine groups.

Only 16 percent of placebo recipients reported local side effects, like pain or swelling at the injection site, while 67 percent of the vaccine group reported such effects.

About 32 percent of the placebo group reported at least one systemic effect, while 61 percent of the vaccine group reported systemic effects.

And in this case, the vaccine group tended to report more moderate to severe systemic effects than the placebo group.

As in the first shot, the vaccine group had more local side effects, with about 73 percent reporting local effects while only about 12 percent of people in the placebo group reported them.