Vaccine side effects and adverse reactions

Vaccine product leaflets will usually contain a detailed list of ‘undesirable effects’, or they may refer to ‘adverse reactions reported in clinical studies or from the postmarketing experience’. These are not always exactly the same as ‘side effects’, because reporting an adverse event after vaccination does not prove a link with the vaccine. When a vaccine is given to a very large number of people in a population, it is likely just by chance that a few of them will develop some kind of medical problem around the time of vaccination, but this does not prove ‘cause and effect’. This means that the reactions listed in the product information may not all be side effects of the vaccine itself.

Some events after immunisation are clearly caused by the vaccine (for example, a sore arm at the injection site). However, others may happen by coincidence around the time of vaccination. It can therefore be difficult to separate those which are clearly caused by a vaccine and those that were going to happen anyway. In clinical trials all serious adverse events are reported so that unexpected reactions to medicines or vaccines can be identified. This is very important for safety monitoring, but it also means that extreme examples of ‘adverse events’ may get listed when they have no connection to the vaccine (such as injuries caused to a passenger in a car accident, which are clearly not related to any vaccine they may have received). Also events such as fainting at the sight of a needle may be called an adverse event, even though the person has not actually received the vaccine.

However, experts consider it important that every single adverse event is collected and reported, so that we can identify any ‘signal’ of a possible vaccine-caused harm amongst all the chance events that happen to people every day. In the UK it is the job of the MHRA to collect all these reports and investigate further if necessary (see more information on Monitoring of vaccines). Scientific method is then used to determine if these events are a coincidence or a result of the vaccine.

Here are two examples of how collecting and analysing data on adverse events has helped us to understand more about the safety of vaccines.

Rotavirus vaccine

Some studies on the rotavirus vaccine identified that there was possibly a very slightly increased risk of a rare condition called intussusception following vaccination. This was discovered by comparing rates of intussusception in a large group of babies who had received the rotavirus vaccine with rates in a large group of babies who had not received the vaccine. It led to a change in the information in the product leaflet, and also to a change in advice about when the vaccine should be given (see the page on the Rotavirus vaccine for more information about this research).

MMR vaccine

A large number of studies on the MMR vaccine have found no increased risk of autism in very large groups of children who have received the vaccine. Again, this has been done by comparing rates of autism in large groups of children who have received the MMR vaccine with large groups who have not (see the page on the MMR vaccine for more information about this research).



As with any medicine or food, there is a very small chance of a severe allergic reaction (anaphylaxis) to the ingredients contained in vaccines. Anaphylaxis is different from less severe allergic reactions because it causes life-threatening breathing and/or circulation problems. It is always extremely serious but can be treated with adrenaline. Health care workers who give vaccines know how to do this. In the UK between 1997 and 2003 there were a total of 130 reports of anaphylaxis following ALL immunisations. Around 117 million doses of vaccines were given in the UK during this period. This means that the overall rate of anaphylaxis is around 1 in 900,000.

For information on side effects of individual vaccines, please see the page for the relevant vaccine under the "vaccines" menu at the top of this page.


Page last updated Thursday, August 15, 2019