Personas vacunadas en el Reino Unido (1S t dosis): 52.399.031

Personas vacunadas en el Reino Unido (2da dosis): 48,520,906

¿Cómo sabemos que la vacuna es segura?

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

Fase 1

Se administra la vacuna a un pequeño grupo de personas sanas (<100) para asegurarse de que no haya problemas de seguridad, para ver qué tan bien estimula la respuesta inmunitaria y para determinar una dosis eficaz.

Fase 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.

Fase 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.

Fase 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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