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Most COVID vaccine side effects due to placebo effect

Christian Fernsby |
A meta-analysis looking at data from 12 vaccine trials has concluded up to three-quarters of adverse events reported following COVID-19 vaccination can be attributed to the placebo effect.

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The researchers call for improved public health communication to inform people that many vaccine side effects can be attributed to what is known as the "nocebo effect."

Most people are familiar with the idea of the placebo effect, that strange phenomenon where health benefits are seen after someone consumes an inert treatment, such as a sugar pill, thinking it is an active drug.

At the other end of the spectrum, some people experience negative side effects from taking a placebo. This is known as the nocebo effect.

“Adverse events after placebo treatment are common in randomized controlled trials,” said Julia Haas, lead author on the new study.

“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.”

The new research looked at data from 12 COVID-19 vaccine trials encompassing over 45,000 people.

Over 35 percent of those in the trials receiving a placebo reported systemic side effects after the first dose, including fever, headache and fatigue. This compared to around 46 percent of those in active vaccine groups reporting similar side effects.

Nocebo responses were seen to drop after the second vaccine dose, with only 32 percent of those receiving a placebo reporting systemic side effects compared to 61 percent in the active groups.

This suggests real adverse effects are slightly more common after a second vaccine dose.

However, the researchers still calculate around 50 percent of side effects following a second dose can be attributed to the nocebo effect.

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