Balancing efficacy and safety

Vaccine efficacy refers to the ability of a vaccine to bring about the intended beneficial effects on vaccinated individuals in a defined population under ideal conditions of use. The potential benefits of an effective vaccine – e.g. promotion of health and well-being, and protection from illness and its physical, psychological and socioeconomic consequences – must be weighed against the potential risk of an adverse event following immunization (AEFI) with that vaccine. Vaccine-associated risk is the probability of an adverse or unwanted outcome occurring, and the severity of the resulting harm to the health of vaccinated individuals in a defined population following immunization with a vaccine under ideal conditions of use.

Potential benefits of an effective vaccine must be weighed against potential risk of an AEFI.

Key point

Public confidence in vaccine safety is increased by clear communication of risk/benefit assessments, comparing the very low vaccine-associated risk with the very significant benefits of vaccination.

Weighing the risks and benefits of a vaccine

An important criterion of vaccine safety that regulatory authorities must establish is the risk/benefit assessment of immunization with a particular vaccine in a defined population. You will learn how to conduct a risk/benefit assessment in Module 4 'Surveillance' and about the actions that follow the identification of an increased or new vaccine risk. Here we introduce you to some basic principles and the issues that regulatory authorities consider when balancing vaccine efficacy and vaccine safety.

Risk evaluation for a specific vaccine requires the collection and analysis of reliable data on:

Summarizing the risk/benefit relationship of a vaccine in tables and diagrams is useful to:

Risk of acquiring illnesses following infection versus risk following vaccination

  Measles infectiona Measles vaccineb
Otitis 7 – 9% 0
Pneumonia 1 – 6% 0
Diarrhoea 6% 0
Post-infectious encephalomyelitis 0.5/1 000 1/100 000 – million
SSPE 1/100 000 0
Anaphylaxis 0 1/100 000 – million
Thrombocytopenia Not properly quantifiedc 1/30 000d
Death 0.1 – 1/1 000 (up to 5 – 15%) 0

a   Risks after natural measles are calculated in terms of events per number of cases.

b   Risks after vaccination are calculated in terms of events per number of doses.

c   Although there have been several reports of thrombocytopenia occuring after measles including bleeding, the risk has not been properly quantified.

d   This risk has been reported after MMR vaccination and cannot be only attributed to the measles component.

MMR = measles, mumps and rubella; SSPE = subacute sclerosing panencephalitis.

P. Duclos, BJ Ward. Measles Vaccines, A Review of Adverse Events, Drug Safety 1998; Dec 19 (6): 435—454

Key point

Risk/benefit assessments should be applied to most situations relating to the efficacy or safety of vaccines to ensure public safety and public health.