What kind of side-effects can occur after having a vaccine?
Mild reactions such as headaches, fatigue, a slight fever and soreness at the site of injection are very common and go away quickly. The number of serious side-effects by comparison are very low. Many vaccines have now been in use for decades, with billions of doses given around the world every year, meaning there are a large amount of data on any potential harmful effects.
The possible side-effects of a vaccine can be described in several different ways, often relating to their severity.
- Minor side-effects: Mild reactions such as headaches, fatigue, a slight fever, a rash and soreness at the site of injection are very common and go away quickly, usually within a few days.
- Adverse events: Sometimes referred to as undesirable effects on vaccine information leaflets, these are reported to regulators on a regular basis but do not necessarily involve a direct link to the vaccine. A patient who faints during the vaccination or who develops another illness in the days that follow will be reported as having had an adverse event. Adverse event reports are collected by regulators to help them identify any patterns that might indicate a potential safety issue. These are then studied to determine if they are related or unrelated to the vaccine. Most health issues that arise in the days and weeks after vaccination are completely unrelated to the vaccine, but all reports are still carefully assessed.
- Severe adverse reactions: These are very rare but can cause serious harm or even death in some people.
In rare cases, vaccines can cause allergic reactions, which is why you might be asked if you have any allergies before being vaccinated and be asked to wait for a short period at the clinic after you have received your vaccine. One study of 25.2 million vaccinations in the US found just 33 cases of life-threatening allergic reactions triggered by vaccines. Similar research in the UK on 117 million doses of vaccine found just 130 reports of severe allergic reactions. This means just one to two people experience a severe allergic reaction for every million doses of vaccine administered. Although allergic reactions are potentially life-threatening, in the UK, these have not resulted in any deaths in recent decades because vaccination clinic staff are trained to treat these reactions if they occur.
People who have a severely weakened immune system may be at risk from some live vaccines where even the weakened virus or bacteria (which is a part of the vaccine) can cause an infection.
One to two people out of every 100,000 people who received the Oxford-AstraZeneca vaccine, used during the COVID-19 pandemic, went on to develop unusual blood clots. A small number of cases were fatal. Some recent research has provided some clues suggesting that people with certain genes may be at increased risk of this severe adverse reaction, but this observation has not yet been verified. The rare, but serious reaction meant policy was amended, so that use of the vaccine in young people was restricted as the benefit of vaccination in younger age groups was not in favour of vaccination as the risk of serious illness from COVID-19 was low in this age group. By contrast, in older adults the risk of COVID-19 was so high that it still made sense to be vaccinated to avoid severe outcomes from COVID-19, even with the risk of rare vaccine side effects. After more than 3 billion doses were distributed worldwide, the vaccine was withdrawn from the market in 2024 by the manufacturer for commercial reasons. Use of electronic health data, linked to vaccination status, has improved over the past decade and transformed the ability of health systems to monitor for very rare side effects of drugs and vaccines.
Close monitoring of the rare, serious side-effects of a vaccine can result in a change in how it is administered. For example, analysis of safety data from the vaccine against rotavirus – an infection that causes diarrhoea and vomiting and is very common in babies and young children – identified an increased risk of a rare complication, called intussusception, that causes an obstruction in the intestines. This led to a recommendation in the UK that the first dose should not be given to children older than 15 weeks of age and the second dose should be given before 24 weeks of age, as the risk of this complication is higher the older the child.