There are several coronavirus vaccines that are currently in Phase 3 clinical trials. For vaccines, this final experimental stage requires tens of thousands of people in order to properly assess efficacy and safety. Among other things, such a large sample size is important for detecting rare but dangerous side effects.
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Most vaccine side effects are familiar to anyone who has ever gotten a jab: Sore arm, swelling or redness at the injection site, fatigue, and maybe a fever. Collectively known as reactogenicity, these are all classic signs of an inflammatory response, which is good.
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Sometimes, the antibodies produced in response to the vaccine actually make an infection worse, a phenomenon known as vaccine-enhanced disease. In other cases, the antibodies elicited start attacking the patient’s own body, which is an autoimmune disorder. Most likely, the volunteer who is thought to be experiencing a serious adverse reaction to the coronavirus vaccine is suffering from one of these two conditions.
It should be kept in mind that adverse events should not necessarily rule out a mass vaccination campaign. Once all the data are collected, scientists will need to determine if the risk of COVID-19 is greater or less than the risk posed by the vaccine. Most likely, the risk of the disease will be greater than the risk of side effects. But we don’t know for sure. That’s why we do the clinical trials.
