tens of thousands of volunteers to evaluate both safety and effectiveness.

Even after approval, vaccine safety monitoring continued through national and international surveillance systems.

Why Vaccine Safety Monitoring Continues

No medical treatment is completely free of risk.

For that reason, scientists continue to monitor vaccines long after they are approved.

Large safety monitoring systems help researchers identify extremely rare side effects that may not appear during clinical trials because they occur in only a very small number of people.

This ongoing surveillance allows health authorities to update recommendations whenever new evidence emerges.

What Large International Studies Have Found

One of the largest vaccine safety investigations involved the Global Vaccine Data Network (GVDN), which analyzed health data from more than 99 million vaccinated individuals across several countries.

Researchers examined reports following vaccination with vaccines including Pfizer-BioNTech, Moderna, and AstraZeneca.

The study identified several rare adverse events that occurred more frequently than