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Vaccines must comply with a very demanding safety profile (frequency and types of adverse effects) before they can be approved for use in humans. This safety profile must be assessed by regulatory agencies (such as the European Medicines Agency). The initial clinical trials always assess safety and then, once the vaccine has passed the agencies' evaluation, there is exhaustive monitoring of potential adverse effects, which are very rare. This monitoring is known as pharmacovigilance.
Like any type of medicine, vaccines are never free from adverse effects, which in most cases are local, mild and temporary, for example a low-grade fever, a general feeling of unwellness or chills. A vaccine cannot be approved for use in humans unless it has demonstrated a clearly favourable risk-benefit balance. This means that any adverse effects must be much less significant than their effectiveness and protection against disease. The benefits must always be greater by a wide margin in order for them to be approved and administered to the general population.