Any incident that occurs following a drug, vaccine, surgery, procedure or other medical intervention. If the adverse effect was actually caused by the intervention, then it’s a side effect. But even though all side effects are types of adverse events, not all adverse events are side effects. For example, in a randomized controlled trial, adverse events that occur at roughly the same rate in the intervention group and the control group probably aren’t caused by the intervention.
Deeper dive
Adverse events and side effects are often conflated in news stories, blogs, social media, everyday discussion and even in conversations with medical professionals. However, when writing about research studies, there is a key difference that journalists must understand to avoid inadvertently misleading readers. An adverse event refers to any event that affects a person’s health that occurs after they have received a treatment, whether that treatment is a medication, a surgery, a therapy or another intervention. The adverse event may or may not be caused by the intervention.
A side effect is an adverse effect that has been determined as a direct result of the intervention. In other words, the person who took a certain medication experiences an adverse event, such as a dry mouth or an increased blood pressure, that the medication definitely caused. Side effects are determined by comparing adverse events in randomized controlled trials where one group receives the intervention and one group does not. If the proportion of a certain adverse event is much higher in the group receiving the drug or intervention than in the control group, then the drug or intervention is the cause of that adverse event, which then becomes a side effect.
For example, say 100 people receive the flu vaccine. Then 90 of them have sore arms, 10 have fevers, two have muscle cramps, two of them get into car accidents later that day, and one of them has a heart attack that night. All of those events are adverse effects — including the car accidents and heart attack — even though there is no biological way the flu vaccine could have caused the car accidents and there is no evidence that flu vaccines increase the risk of a heart attack. The sore arms, fevers and muscle cramps, however, very well could have been side effects. It’s not a guarantee that all the fevers were caused by the flu vaccine, but fever is a known possible side effect of the vaccine. The muscle cramps depend on where they occurred. If the cramps are in the arms where the person got the shot, it probably is a side effect. If it’s a Charley horse cramp in the leg, it’s an adverse event that’s probably unrelated to the flu vaccine. If it’s general achiness for a day that feels similar to what the flu feels like, then it likely was caused by the vaccine since that’s a known side effect. This is why reading the list of adverse events in a vaccine package insert tells the reader nothing about actual possible side effects of the vaccine.