What are 'adverse events' and 'emergency use authorisation' in relation to vaccination?

While vaccines are the safest way to prevent the spread of infectious disease, a tiny proportion of those vaccinated may experience an adverse event. Here, we explain how often this happens and why.

  • 9 February 2021
  • 5 min read
  • by Wellcome
Photo by Gabby K from Pexels
Photo by Gabby K from Pexels


What are 'Adverse Events Following Immunisation'?

Adverse events are unfavourable and unintended medical outcomes that occur after a vaccination. They may be related to – caused by – the vaccine or may be unrelated to – not caused by – the vaccine. 

It is normal for these events to occasionally occur, even when a vaccine has been approved as safe and effective. 

For example, the hepatitis B vaccine has been approved since the 1980s. It is considered extremely safe and is on the World Health Organization Model List of Essential Medicines. But mild adverse events (such as headache and soreness where injected) occur in 3 in 100 vaccine recipients, and last just 1-2 days. Severe adverse events associated with the vaccine are incredibly rare, at just 1.1 per 1 million vaccine recipients.

How are they monitored?

Adverse events are proactively monitored throughout vaccine development and rollout. Policies and systems are in place that require vaccine manufacturers and clinicians to report these events to keep regulatory authorities informed on the vaccine’s safety (for example the Vaccine Adverse Event Reporting System in the US, and the Yellow Card Scheme in the UK). 

Studies are done to determine whether adverse events are 'related' or 'unrelated' to administration of the vaccine:

  • Related event: when an unintended medical outcome happens to someone who’s taken a vaccine, as a result of taking that vaccine. For example if you were watching your favourite TV show, causing the sitting room light fuse to blow. These can range from very mild side effects (like headache or soreness where vaccinated) to more severe reactions (such as allergic reactions). They can, very rarely, be due to quality faults by the manufacturer, or errors when receiving the vaccine.
  • Unrelated event: when an unintended medical outcome happens to someone who’s taken a vaccine, but not because of the vaccine, it just happened coincidentally. For example if you were watching your favourite TV show and at the same time a pipe burst in your kitchen.

How do we know Covid-19 vaccines are safe and how are they authorised for use?

Extensive global collaboration and massive investment has allowed vaccine development timelines to be sped up without affecting safety

Due to the life-threatening nature of Covid-19, especially for vulnerable people like frontline workers and the elderly, Emergency Use Authorisation (EUA) of Covid-19 vaccines are being offered in some countries.

EUA is granted by national regulatory authorities like the Medicines and Health Regulatory Authority (MHRA) for the UK, and the Federal Drug Administration (FDA) for the US. It allows unlicensed vaccines to be used in an emergency to prevent serious disease, provided important safety, efficacy and manufacturing criteria are met. An EUA can be submitted based on interim phase 3 data if it meets the rigorous standards set by the regulatory authority, which determine whether the benefits of the vaccine outweigh the risks.

Monitoring of vaccine recipients, including follow-up of adverse events, continues after an EUA is approved to validate continuation of the EUA. Clinical trials and further evaluation also continue to collect additional safety and effectivity data. All of this information will be used to gain final (non-emergency use) regulatory approval. 

What adverse events are likely with the Covid-19 vaccines?

With so many needing protection from Covid-19, this will be the largest vaccine rollout in human history. 

It is therefore likely that we will see some adverse events as the rollout progresses. It’s also important to remember that it can take weeks to build immunity after vaccination, and that no vaccine is 100% effective. 

With clinical trials having tested for and identified common adverse events, the risk of severe adverse events is expected to be tiny compared to the benefits from vaccination.

So far, early data suggests that one potential adverse event we might see is an allergic reaction, though this is considered rare and is being investigated further. 

In very rare and extreme situations when there is a notable safety concern, a vaccine could be suspended, recalled, or replaced. This is unlikely to happen but processes already exist to do this quickly if necessary. 

These processes ensure that any adverse events following vaccination are monitored, that the patient receives the care they need, and that further vaccination is stopped if a significant risk is posed. For example, in September 2020 the Oxford-AstraZeneca phase 3 trials followed these processes and temporarily paused all trials while an independent committee reviewed a single adverse event in the UK trial. They resumed the trials after receiving official confirmation that it was safe to do so.

The extensive safety measures for dealing with adverse events mean that even if there are a few during the vaccination rollout, they will be identified and managed, while still preventing a colossal number of deaths and hospitalisations caused by Covid-19.


This article was originally published by Wellcome on 22 January 2021