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How safe are vaccines?

Vaccines have a very strong safety record. Regulators assess them for side effects while they are being tested in clinical trials and continually monitor for any signs of risk when they are being used in the general population. They weigh up the risk of any side-effects with the risk of getting the disease and being harmed by it. 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. Occasionally, side-effects caused by a vaccine are found to be unacceptable and development of the vaccine is stopped or it is withdrawn from the market.

The balance between potential benefits and potential risks will depend on the disease the vaccine is for and who it is being given to. For example, the balance would be different for a vaccine to prevent a cancer in someone at high risk than for one to prevent flu.

In most cases side-effects tend to be mild. Redness and soreness around the site of injection is common, but also entirely expected due to the inflammation triggered by the vaccine as part of the immune response. Tiredness and a mild fever are also common side-effects of vaccinations against many diseases such as influenza, mainly because of the work your immune system is doing in response to the vaccine.

Severe adverse reactions are very rare but can cause serious harm or even death in some people. These can include rare allergic reactions, which are well-known but can be managed by the trained immunisation doctors or nurses, or other adverse reactions which could mean that the vaccine is avoided in certain groups (such as live vaccines in people who are immunocompromised or in pregnancy). If the adverse reaction is very serious, it could lead to the vaccine being withdrawn from use altogether. You can find out more at What kind of side-effects can occur after having a vaccine?

You can read more about the approval process for vaccines in How are vaccines made? and in How vaccines are tested?

Are there any side effects with the MMRV vaccine?

The measles, mumps, rubella and chickenpox (varicella) vaccine is one of the recommended childhood vaccines. MMRV has replaced the MMR vaccine in the UK. It is known to cause a mild rash in 10 of every 100 children who receive it while 27 in every 100 will develop a temporary fever.

Fewer than one child in every 1,000 who receive the MMRV vaccines experience short fits brought on by a fever – known as febrile seizures. Such febrile seizures occur in children who are susceptible to having these fits when they get a temperature for any reason (not just vaccination).

Answers to key questions

Vaccines train your body to recognise and fight off infections which may be caused by viruses, bacteria or parasites. They are a preventative medicine that protects you from infectious diseases by helping your immune system to prepare its defences before an infection strikes. In most cases, this means we are either completely protected from the disease or don’t fall seriously ill when we do catch the infection. Each vaccine is designed to target a particular microbe (virus, bacteria or parasite). By completely blocking or reducing the amount of infection in your system and the time it is there for, most vaccines also reduce the risk of passing on the illness to other people you are spending time with, including friends and family.

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The goal of vaccination programmes is to protect and improve the health of the entire population through disease prevention. Vaccines are estimated to prevent between 3.5 and 5 million deaths worldwide every year. The Smallpox vaccine was so successful that nobody has had the disease globally since 1978. Vaccines also serve wider purposes in public health as they prevent us passing infections from one person to another, help control disease outbreaks that can overwhelm health services and help protect the most vulnerable in society who can sometimes not get a vaccine themselves. The effectiveness of individual vaccines can vary depending on different factors, including the disease being targeted, the population being vaccinated and the type of vaccine.

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Vaccines are an effective way of protecting yourself and those around you from infectious diseases that can make you seriously ill or threaten your life. Whether you are recommended or offered a vaccine will depend on whether it is deemed to be beneficial for you, given your health, age, vulnerability or other personal circumstances and cost-effectiveness to the NHS. Other factors such as risks to the health of those around you and the wider population may also be taken into account. As with any medical intervention, great care is given to ensuring that the potential benefits and any potential harms of vaccines are carefully considered.  

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Vaccines have a very strong safety record. Regulators assess them for side effects while they are being tested in clinical trials and continually monitor for any signs of risk when they are being used in the general population. They weigh up the risk of any side-effects with the risk of getting the disease and being harmed by it. 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.   Occasionally, side-effects caused by a vaccine are found to be unacceptable and development of the vaccine is stopped or it is withdrawn from the market.

Read the full answer

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.

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The development of a new vaccine follows on from extensive scientific research on the disease and the virus, bacteria or parasite (microbe) that cause it. This know-how allows researchers to understand how the body’s defences respond to an infection and how to help it prepare. Vaccines are designed, tested and manufactured according to strict regulations. All vaccines undergo rigorous testing to demonstrate they are sufficiently safe and effective before they can be used more widely.

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Vaccines can contain other ingredients that are added to help stimulate a stronger immune response or to help the biological material remain stable for longer. These can include preservatives, stabilisers and adjuvants. There can also be trace materials left over from the manufacturing process. All ingredients added to a vaccine must be approved for use by regulators after safety testing. 

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Vaccines are developed and manufactured through a collaborative process involving academic researchers, governments, pharmaceutical companies and biotechnology firms. They are usually the result of decades of research, development and testing made possible thanks to funding from public bodies, non-profits and private companies. Vaccines are manufactured by pharmaceutical and biotechnology companies who aim to make a profit on the investment in the vaccines they develop and produce. They will work with market experts to determine whether a vaccine is worth producing based on the number of people it might help to protect from a disease.  Global health authorities independently assess the potential importance of a vaccine for protecting their population from a disease and place orders for vaccines based on this assessment. Ultimately, there needs to be sufficient demand for the vaccine to make it worth producing.

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Most of the vaccinations you will be offered in the UK are offered as part of national vaccination programmes that are decided and funded at a national level through the NHS. This means you can get these vaccines for free from the NHS, usually through your local primary health provider such as your GP. In some cases, vaccines are administered at pharmacies or at hospitals.

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There are currently two specific cancers caused by viruses that can be prevented by vaccines, cervical cancer and liver cancer. The vaccines work by training our immune systems to fight off these viruses, which can otherwise stay in our bodies for a long time and can cause cancer later in life. Separately, there are also new vaccines starting to emerge that can be used to treat patients already with cancer. These teach a patient's immune system to detect and destroy cancer cells in their body.

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All vaccines undergo rigorous safety testing during their development and use.  There are three phases of clinical trials and the data from these trials is reviewed by regulators in each country before being approved. The vaccines are then rigorously monitored for safety.

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Vaccines give crucial protection to children during the first months of their lives while their immune systems are still developing. This is when babies are at their most vulnerable to infections that can kill them or make them seriously ill. Decades of safety data indicate that giving children multiple vaccines is safe and there is no evidence it overwhelms their immune systems. 

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Certain vaccines are offered during pregnancy to protect against diseases that can pose a risk in pregnancy. Vaccines recommended for use in pregnancy undergo thorough testing in clinical trials. Many years of research show that vaccines against influenza; diphtheria, tetanus and whooping cough (DTaP); respiratory syncytial virus (RSV); and COVID-19 are all safe when given during pregnancy. They are also continually monitored once in use, through regular reporting of any side-effects, which allows regulators and manufacturers to detect any potential safety issues during pregnancy.

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Decades of detailed research has found no evidence that childhood vaccines can cause autism spectrum disorders.  In December 2025 experts from around the world analysed the evidence from the past 15 years of studies in multiple countries. They found no evidence of a link to autism spectrum disorders. The same conclusion was reached by previous analyses conducted in 2002, 2004 and 2012.

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Misinformation about COVID-19 vaccines began circulating on social media during the pandemic, claiming that the vaccines could harm fertility or lead to miscarriage. No evidence that COVID-19 vaccines have caused a reduction in fertility in men or women or an increase in miscarriages has been found. False claims that certain vaccines including those against polio, tetanus, human papilloma virus cause infertility date back decades.

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The protection offered by a vaccine can wane with time. This is because your immune system can forget the training it was given. A booster helps to maintain your immune system’s memory. Some vaccines are also better than others at teaching your immune system to recognise and fight off a disease. This is why sometimes multiple doses of a vaccine might be required. 

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Herd immunity, which is also sometimes known as herd protection or community immunity, is what occurs when an infectious disease is no longer able to spread in a community because enough people are immune. This can help to indirectly protect people who cannot have a vaccine. If enough people take a vaccine, then herd immunity can be achieved. 

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If you choose not to have a vaccine, you could risk your own health by leaving yourself vulnerable to some harmful diseases. You may also be putting those around you at risk as many vaccines also help to prevent you from transmitting these diseases to others. Getting a vaccine may mean you can help to protect your loved ones and other people around you, particularly those who may be more vulnerable to an infection than you.

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There are more than 250 vaccines currently in various phases of development. These are intended to target a wide range of infectious diseases, cancers, allergies and diseases such as Alzheimer's. While not all of these potential vaccines will make it through the extensive testing and clinical trials successfully, those that do will bring further improvements to the health and life expectancy of people around the world. 

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Immune responses can differ between individuals in terms of the strength of a response, how good their immune system is at recognising a particular virus or bacterium and what diseases they have been exposed to in the past. Age and other health conditions can also play a role. 

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Vaccines are given to the people who need them most – usually those who are most vulnerable to the disease the vaccine works against. The age you receive a vaccine can also be important to ensure you get protection at the right time. The cost-effectiveness of vaccination can also play a role in decisions about who it is offered to.

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There are several types of vaccine that use different approaches to train your immune system to fight off germs, such as viruses, bacteria or parasites that cause disease. They use three basic approaches – using a weakened version of the virus, bacteria or parasite (microbe), a dead or inactive version of it or fragments of it that your immune system can recognise.  During vaccine development, scientists carefully weigh up what type to use for a given germ based on our understanding of the germ and the nature of immunity, what is found to work best, and the most cost-efficient way to provide protection.

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See the main contributors to and reviewers of the answers about vaccines.

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