Data Literacy

How to read Yellow Card reports

Yellow Card data are valuable for detecting potential medicine and vaccine safety signals. They are also easy to misinterpret if report counts are treated as proven side effects.

Reviewed: 22 August 2026 · Source checked: MHRA

The central point: the MHRA asks people to report suspected reactions even when they are uncertain whether the medicine or vaccine caused the event. A report therefore means “this happened after use and was suspected,” not “this product caused it.”

Why reports are collected

Spontaneous reports can reveal unusual patterns quickly. That makes them useful as an early-warning system, especially for very rare events.

Why raw totals cannot establish risk

Report counts do not automatically tell you how common an event is among vaccinated people, how often the same event occurs without vaccination, whether duplicate or incomplete reports exist, or whether another medical condition explains what happened.

What investigators need next

To test a possible safety signal, researchers may compare observed and expected event rates, examine timing and biological plausibility, use healthcare-record databases, conduct epidemiological studies and compare findings internationally.

Questions to ask when you see a viral Yellow Card claim

Is the number a report count or a confirmed diagnosis? What population received the vaccine? What is the normal background rate? Has the MHRA concluded that there is a causal association? Has the claim adjusted for age, sex or other relevant factors?

Good evidence practice: Yellow Card reports should neither be dismissed nor treated as proof. They are inputs into pharmacovigilance — the investigation comes next.

Primary source

MHRA — What is being reported
The MHRA explicitly explains that conclusions about vaccine or medicine safety cannot be made from these report listings alone.