Personnes vaccinées au Royaume-Uni (1st dose): 52,399,031

People Vaccinated in the UK (2nd dose): 48,520,906

Comment savons-nous que le vaccin est sûr?

How are vaccines created?

Like any new medicine, vaccines are developed and tested in clinical trials. These trials comprise different stages to find out if they work and if they are safe to use in people. The clinical development of a vaccine takes place in four stages called phases. Safety monitoring takes place in all phases, including after a vaccine has been approved for use in people.

The principle is to start small and only move to the next stage of testing if there are no outstanding safety concerns.

Assessing vaccine safety

La phase 1

Un petit groupe de personnes en bonne santé (<100) reçoit le vaccin pour s'assurer qu'il n'y a pas de problème de sécurité, pour voir dans quelle mesure il stimule une réponse immunitaire et pour déterminer une dose efficace.

Phase 2

The vaccine is tested in a larger group (several hundred people) to see whether the vaccine works consistently, to assess the immune response and to look for side effects and adverse events.

Phase 3

The vaccine is studied on a much larger scale (several thousand people) under natural disease conditions. This produces enough data to identify rare side effects and adverse events and to evaluate how well the vaccine works in the real world; does it generate enough immunity to prevent and reduce disease?


Between phases 3 and 4 manufacturers apply for a license from regulators so that their vaccine can be marketed for human use. Experts review all the data to see whether the vaccine works to the standards required for both safety and its efficacy in reducing disease.

Phase 4

Phase 4 clinical trials happen after regulatory approval has been achieved. This phase involves thousands of participants and can last for many years. Investigators use this phase to get more information about the medication’s long-term safety, effectiveness, and any other benefits. The phase 4 trial is also referred to as “post marketing surveillance” and as the name suggests, it is conducted after the drug is already marketed and available to the general public.

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