Adverse events are any negative outcomes that occur following administration of a pharmaceutical product, such as a drug or vaccine, or after other medical treatment, procedure or intervention. It’s important to distinguish between adverse events and side effects (or, in the case of vaccines, “reactions”). Side effects (or reactions) are adverse events that are known to be caused by the treatment, such as a fever that occurs after a vaccination or feeling drowsy after taking a barbiturate. Adverse events in clinical trials, however, include any event that occurs after the treatment within a predetermined period of time, such as two weeks after a vaccine or two months after a surgery. Just because something is an adverse event does not mean it was caused by the treatment.
Deeper Dive
There are two categories of adverse events described in vaccine clinical trials: solicited and unsolicited adverse events. Solicited adverse events are those that the trial investigators specifically ask participants about because they are either expected or likely based on known reactions to other vaccines, such as headaches or nausea. Unsolicited adverse events are those that participants report that were not expected or previously known. An example is myocarditis, an unsolicited adverse event (no one expected it) that was reported frequently after the Covid vaccine in VAERS.
Further study revealed that myocarditis can indeed be caused by the mRNA Covid vaccines in some people, so it’s now considered a possible side effect or reaction to the vaccine. That discovery has also led vaccine study investigators to add myocarditis to the list of solicited adverse events in future vaccine trials: Now that it’s known a vaccine can cause it, investigators explicitly ask about it. Other unsolicited adverse events that were determined to be unrelated to the vaccine are not listed among solicited adverse events of future trials.