Many COVID-19 vaccine side effects caused by placebo effect: Study

Many continue to worry about experiencing side effects from vaccines -- especially the COVID-19 vaccines -- but new data from a comprehensive meta-analysis suggests there is little to fear.

The study from Beth Israel Deaconess Medical Center found that a large number of side effects reported by patients after receiving their shot can be attributed to the placebo effect.

Researchers examined 12 vaccine safety trials, involving thousands of people, and compared rates of side effects reported between those who received a placebo shot and those who received a real shot. They found that after the first shot, two-thirds of people experienced side effects like headache and fatigue, which the researchers said were attributable to the placebo effect. Shockingly, nearly a quarter of the people -- some who received the placebo shot -- experienced side effects like a sore arm, also attributable to the placebo effect.

What is the placebo effect?

The placebo effect occurs when people anticipate a medical treatment will have certain effects, so much so that they perceive the outcomes they were expecting after the treatment.

It is a well-known phenomenon among scientists and is important to investigate when developing vaccines and medicines, according to Dr. William Schaffner, professor of preventive medicine and infectious disease at Vanderbilt University.

"After the injection, people are more aware now that they think they might have gotten a vaccine. They're more likely to tell their doctor about things," Schaffner said. "Never underestimate the power of the human mind."

Experts say the placebo effect is a powerful example of the connection between our minds, bodies and circumstances.

In the study, the amount of side effects attributable to the placebo effect decreased to about half after the people studied received a second shot. Frequency of side effects was lower among placebo recipients after the second shot, while the opposite was true for vaccine recipients. This helps reinforce the placebo effect phenomenon, experts said.

Researchers noted one caveat is that the studies examined included different phases of clinical trials, and results were not standardized throughout.

Experts address vaccine hesitancy

With the omicron surge still straining hospitals across America, addressing vaccine hesitancy remains a crucial discussion.

Experts interviewed by ABC News said that if more people knew that experiencing side effects from the COVID-19 vaccines is not as common as they think, more people may be encouraged to get vaccinated.

"When people are armed with information, they are better suited to identify and manage their symptoms," Dr. Simone Wildes, infectious disease physician at South Shore Health, said. "This might also help those who are reluctant to get vaccinated."

Aubrie Ford, D.O. is an emergency medicine resident at Northwell Health in New York and a contributor to the ABC News Medical Unit.