• Safety monitoring continues once a vaccine is approved. This is to detect rare side effects and to monitor safety in groups of people who may not have been included in clinical trials that may be at a higher risk of experiencing side effects. This is often called Pharmacovigilance
• As with all medicines there is a risk that some people will experience side effects, such as minor expected things like very mild inflammation at the site of an injection, to more serious adverse events like severe allergic reactions. Data about adverse events or adverse drug reactions are collected and analysed. These can be directly related to the vaccine, or other ingredients in the vaccine formulation. Others may be related to the process of immunisation – for example incorrect preparation or poor vaccination technique. Others may occur but be purely coincidental and are unrelated to the vaccine.
• Vaccine safety is continuously monitored using information from a number of sources. This includes real-time reporting of reactions, called ‘adverse events’, in a UK national system called the Yellow Card Scheme.
• Analysis of reported adverse events determines why and how often they occur and what action is needed to reduce any risks. It also determines which are real side effects from those which are unrelated to the vaccine.
• The MHRA acted within 24 hours to revise guidance on the use of the Pfizer / BioNTech vaccine following reports of three people experiencing allergic reactions on 8 December 2020.

The process of pharmacovigilance begins in clinical trials using well-established safety protocols. It is not uncommon for a vaccine trial to be paused if there is a report of a serious adverse event. This allows investigators to investigate possible causes and then decide whether the trial should continue. Here are two examples of how this process worked for new COVID-19 vaccines in development:

• Janssen / Johnson & Johnson adenovirus-based vaccine. The phase 3 trial was paused for a few days following a serious medical event in one of the participants. There was no evidence that it was related to the vaccine and the trial resumed.
• Oxford University / AstraZeneca adenovirus-based vaccine. An unexplained illness in the UK arm of the phase 3 trial led to a pause of all the phase 3 trials in progress internationally. This was not considered to be linked to the vaccine and trials resumed a few days later.

Once a medicine is used in the general population more data can be collected about how it works in people. This expands on the evidence that is collected when a medicine is being developed and tested by a manufacturer in phases 1–3 of clinical trials.

For a vaccine, continued data collecting can tell us more about its ability to reduce the transmission of an infectious disease. It also tells us more about its impact on the number and severity of infections in people from different age groups, or groups that share other characteristics, such as ethnicity or underlying medical conditions.

Importantly, more data are collected about safety. This is to reveal any adverse events or side effects that may be so rare (for example 1 in 500,000 people may be affected) that they are unlikely to be detected in clinical trials that involve fewer people.

Monitoring also tells us about how the vaccine may affect people who have a greater risk of experiencing adverse events and who were less likely to be represented in clinical trials. This might include people who have underlying medical conditions or pregnant women.

It is expected that the number of reported adverse events increases once a vaccine is used in the general population, simply because it is being used in more people. Pharmacovigilance collects data about all these events to understand what reactions and side effects are occurring so that assessments of the risks and benefits of a medicine can be reviewed.

It is also a way to provide healthcare professionals and the public with more information about how to use vaccines safely. The safety monitoring process also collects information about the impacts of any action taken.

What happens when an adverse event is identified?

This will vary according to the type of adverse event that is being reported. 

• It may lead to changes in the patient information leaflet about possible side effects. 
• It may require issuing advice to immunisers about vaccine handling and immunisation protocols. 
• In some cases, the advice about who should receive a vaccine may change, based on an analysis of the risks and benefits. 
• New research studies to explore adverse events might be needed to answer specific question about why a reaction occurs.