Theoretically, all adverse events after vaccination should be reported, even those that have improved clinically or resolved spontaneously. However, adverse events that occur commonly, such as mild fever, tiredness or headache are often not reported. Uncommon and serious side effects should always be reported. Serious adverse effects are those that need medical attention or admission to hospitalisation. In general, health authorities watch out for adverse effects that have proven associations with immunisation, but also for adverse effects that may theoretically occur but which have not yet been observed.