In an ideal situation, an adverse event would be directly linked to a vaccine using a specific lab test. For example, an early version of the polio vaccine, which used a weakened form of the virus to generate immunity, caused roughly one person to develop the disease for every 2.4 million doses given. The virus strain used in the vaccine could be isolated from spinal fluid in these cases, says [vaccinologist Kathryn] Edwards, so it was clear that the vaccine had caused the disease.
But these kinds of tests aren’t possible for most [COVID vaccine] adverse events, either because there aren’t specific biomarkers to test for, or because such tests are impractical. At least initially, the events are only linked by their timing… [It is] particularly challenging to prove whether the adverse event was actually caused by the vaccine.
A more complete understanding of vaccine safety could be garnered from active surveillance systems that collect adverse event data — both background rates and after a vaccine — from electronic health records without relying on people reporting them directly.
[Researchers have] called for an international network of active surveillance systems, which would allow public-health agencies to share data more easily, and hopefully determine the causes of adverse reactions quickly and definitively.