Placebo effect accounts for more than two-thirds of Covid-19 vaccine adverse events, a new study has found.
Researchers at Beth Israel Deaconess Medical Center (BIDMC) in Boston in the United States found that nearly a third of participants who received no vaccine also reported adverse reactions, with headache and fatigue being the most common.
The scientists, whose findings are published in JAMA Network Open, compared the rates of adverse reactions reported between participants who took the Covid jab and those who were given a placebo injection containing no vaccine.
They found that significantly more trial participants who received the vaccine reported adverse events.
The placebo effect is the term for when a person experiences an improvement in physical or mental health after taking a treatment with no pharmacological therapeutic benefit, such as a sugar pill, or a syringe full of saline.
The so-called “nocebo effect” is used to describe the unpleasant side effects someone has after taking a treatment with no pharmacological effects.
Scientists behind the study said the results were not totally surprising.
“Adverse events after placebo treatment are common in randomized controlled trials,” said lead author Julia W. Haas, PhD, an investigator in the Program in Placebo Studies at BIDMC.
“Collecting systematic evidence regarding these nocebo responses in vaccine trials is important for Covid-19 vaccination worldwide, especially because concern about side effects is reported to be a reason for vaccine hesitancy.”
Haas and colleagues analysed data from 12 clinical trials of Covid-19 vaccines, which included adverse effects reports from 22,578 placebo recipients and 22,802 vaccine recipients.
After the first injection, more than 35 percent of placebo recipients experienced symptoms affecting the entire body, such as fever. Headache and fatigue were most common at 19.6 per cent and 16.7 percent, respectively.
In addition, 16 per cent of placebo patients reported at least one local event, such as pain at site of injection, redness or swelling.
In comparison after the first injection, 46 per cent of vaccine recipients experienced at least one systemic adverse event and two-thirds of them reported at least one local event.
While this group received a pharmacologically active treatment, at least some of their adverse events will have been the nocebo effect, since many of those effects also occurred in the placebo group.
Haas and her colleagues’ analysis suggested that nocebo accounted for 76 percent of all adverse events in the vaccine group and nearly a quarter of all local effects reported.