What is it?

Vaccine-associated enhanced disease (VAED) is a rare phenomenon in which a (usually) more severe clinical presentation of an infection that would normally be seen in an unvaccinated person occurs in someone who has been vaccinated. Antibodies that have been generated from vaccination bind to a pathogen and aide in a virus getting more easily into cells than it would on its own. Vaccines that have been found to cause VAED are no longer in use (see below).

Mechanisms for enhanced disease

VAED has previously been observed during clinical trials involving individuals receiving inactivated whole-virus vaccines against respiratory syncytial virus (RSV). Clinical trials showed that children who had received the vaccine and then were later diagnosed with RSV infection developed more serious RSV symptoms. As a result, this vaccine was never approved for general use.

The other vaccine which has been linked to VAED is one of the earlier versions of the measles vaccines in the 1960s. It was found that following vaccination, children were at higher risk of developing an atypical form of measles disease. This vaccine has since been withdrawn, and the current measles-containing vaccines are not associated with the same adverse effects.

In these cases of VAED, the underlying causes remain to be further understood, but possibly relates to an abnormal T-cell (Th2) response.

Assessment and evaluation

It can be difficult to distinguish between vaccine-failure (also known as breakthrough disease) and VAED. Identification of a case of VAED requires the recognition that a clinical presentation is different, atypical, modified or more severe in comparison to the natural disease presentation. Many factors need to be considered when making the assessment, including background rates, age, sex, time post vaccination, duration of disease, clinical course and progression and co-morbidities. Various laboratory and clinical diagnostic parameters can be used to help assess the possibility of VAED, including detailed review of all cases of possible vaccine failure.

Current National Immunisation Program (NIP) vaccines

The vaccines currently available through the NIP are not linked to VAED.

COVID-19 vaccines

There is no evidence to suggest VAED is associated with the current COVID-19 vaccines. On the contrary, more COVID-19 related morbidity and mortality has been observed in unvaccinated populations globally. Ongoing surveillance and long-term vaccine follow-up will continue.


Authors: Nigel Crawford (Director SAEFVIC, Murdoch Children’s Research Institute), Adele Harris (SAEFVIC Research Nurse, Murdoch Children’s Research Institute) and Georgina Lewis (SAEFVIC Clinical Manager, Murdoch Children’s Research Institute)

Reviewed by: Julia Smith (RCH Immunisation Fellow), Francesca Machingaifa (MVEC Education Nurse Coordinator) and Rachael McGuire (MVEC Education Nurse Coordinator)

Date: November 29, 2022

Materials in this section are updated as new information and vaccines become available. The Melbourne Vaccine Education Centre (MVEC) staff regularly reviews materials for accuracy.

You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family’s personal health. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult a healthcare professional.