This site addresses some of the key questions we are asked about vaccines. We will continue to develop this site over time with more questions and answers. To find out more about the COVID-19 vaccine please see our vaccine tracker tool which contains summaries of latest trial results.
How do we know vaccines are safe?
- What is a clinical trial?
Clinical trials are research studies that are used to determine if a new vaccine is safe and effective.
In our animated video on clinical trials below, we describe the four phases of vaccine clinical trials and the steps it takes to license a vaccine.
- Phase I is an initial trial with a small group of healthy volunteers and usually takes a few months.
- Phase II examines consistency of the vaccine, any potential side effects and the presence of immune response expected. This phase can last several months.
- Phase III gathers robust data on safety and efficacy and can last several years as it usually involves thousands of volunteers.
- Phase IV occurs after the vaccine is licenced and used in the public. This phase continues for as long as the vaccine is being used in the community and monitors the vaccine’s benefits and any side-effects.
- What’s in a vaccine?
Each ingredient in every vaccine is present for a very specific purpose and our animated video on vaccines below explores what goes into a vaccine and why.
The main ingredient in any vaccine is the antigen, which is a small part of the virus or bacterium being targeted. The antigen is the ingredient in the vaccine that challenges our immune system to generate the right defences.
Some vaccines add an adjuvant to the antigen to help strengthen and lengthen your body’s immune response. Stabilisers are used to help the active ingredients to remain effective while the vaccine is made, stored, and moved. Antibiotics and preservatives are sometimes used in the manufacturing process of some vaccines however these elements aren’t in the final vaccine. The patient information leaflet that comes with every vaccine tells you exactly what was used in making the vaccine, what is still in it and how much is in the final product.
- How do we know that vaccines are safe?
How safe are vaccines? How do we know that vaccines are safe? Our animated video on vaccine safety below explores the safety measures in place throughout the entire vaccine development process, from the four clinical trial phases and through to the use of vaccines in the community.
What is immunity?
- How do vaccines work?
Vaccines help protect you against infectious diseases by training your immune system to recognise and attack invading pathogens. To better understand how vaccines work, it’s important to understand how your immune system works because vaccines employ the natural defences of your immune system to give you protection against diseases.
When pathogens, such as bacteria or viruses, enter your body, they invade and multiply - this is called an infection. Your body recognises these pathogens as attackers and initiates a complex defence response that uses lots of different pathogen-fighting tools, namely many types of white blood cells.
One type of white blood cell, called B cells, create antibodies that attach to the antigens of the pathogen. However, making these antibodies can take a few days as the right antibody needs to fit perfectly into the antigen, much like a key fitting a lock. Once the right antibody has been found, your body produces them in masses. The antibodies set to work with phagocytes, another type of white blood cell, to destroy the pathogens. Meanwhile, your body uses Killer T cells to recognise and destroy cells in your body that have already been infected.
Once your body has defeated the infection, your body remembers the tools that were needed so that if you encounter the same pathogen again, your immune system can react quicker. Some B and T cells are left behind in your immune memory bank so that they can quickly go into action again with B cells at the ready to make the right antibodies if the same pathogen comes up again.
Vaccines work in the same way. They train your immune system to recognise and fight certain pathogens. A vaccine provides you with parts of an antigen that cannot make you sick or a deadened antigen, like a decoy, so that your body will still recognise the antigen as dangerous and launch its fighting tools. Your body will remember the infection and be able to fight the infection faster meaning that you shouldn’t get sick or show any symptoms of the illness. Some vaccines have more than one dose to increase the “memory bank” and make sure the protection can last for a very long time.
For more information about how vaccines work, check out the animations on the Nature website.
- What is community immunity (or herd immunity) and how does it work?
Community immunity, or often called 'herd immunity' or 'herd protection', occurs when a high percentage of a population is vaccinated making it difficult for an infectious disease to spread. For example, if you have received two doses of the Measles, Mumps, and Rubella (MMR) vaccine, and there is measles in your community, you won’t become infected by the disease and therefore won’t pass it on to anyone else. This break in the chain of infection can effectively stop diseases circulating.
Community immunity protects those who are particularly vulnerable to infectious diseases, such as newborn babies, the elderly and people who have compromised immune systems. But community protection only works when enough people in the community are vaccinated.
The percentage of the population that needs to be vaccinated changes for each infectious disease, depending on how infectious the disease is. For example, the measles is a highly infectious disease meaning that 95% of the population needs to be vaccinated to attain herd immunity within a community.
For more information on community immunity, you can watch a video produced by the Microbiology Society.
Why do we need vaccines?
- What diseases do vaccines prevent?
Vaccines provide protection against more than 20 potentially life-threatening diseases and currently prevent 2-3 million deaths every year. Vaccines can help to prevent:
- Whooping cough (pertussis)
- Haemophilus influenzae type b (Hib)
- Pneumococcal pneumonia and meningitis
- Meningococcal sepsis and meningitis
- Cancers caused by human papillomavirus
- A and B
- Chicken pox and shingles
- Yellow fever
- Japanese encephalitis
This extensive list of already available vaccines shows why so many deaths can be averted each year. Not all of these vaccines may be required in the country where you live, and some vaccines are only given prior to travel or to people in high-risk categories.
Even though this list is long, scientific developments in vaccines have led to some vaccines being combined with others, meaning you receive less injections.
Some of these diseases, like measles (a highly contagious and serious respiratory disease), used to be common in countries like the United Kingdom and the United States of America. However due to good vaccination coverage, measles outbreaks are uncommon and between 2011 and 2020, it’s estimated that measles vaccination has prevented 14 million deaths alone. Because diseases like measles are still common in some countries, it’s important that we continue to vaccinate, even if a certain disease isn’t common in our community at the moment, because it could easily come back if the community immunity drops. In fact, we have recently seen this with measles in many countries.
- Why should I get vaccinated?
Vaccines are a safe and effective way to protect yourself and your community from a whole series of harmful infections. And they not only protect you and the people around you, but vaccination can also help eradicate certain diseases.
When you are vaccinated, your body develops immunity, meaning that you’ll be protected from getting sick with that particular infection targeted by the vaccine, even if the bug is still circulating in your community.
Not only have you put a protective shield around yourself, but when you are vaccinated, you’re also helping to protect people you meet in your community. Because if you are not infected and you are vaccinated, you cannot pass the bug on to anyone else.
When more and more people in a community are immunised against a disease, it is increasingly difficult for that disease to spread. Ultimately, this also means that by getting vaccinated, we can help to protect people who cannot be vaccinated due to age or ill-health. This is called community immunity, or often also called herd immunity.
Diseases can die out altogether if enough people in a community are vaccinated because that disease can no longer spread from person to person. This happened in 1980 when the World Health Organisation declared the world smallpox free after mass vaccination led to the eradication of the small pox virus as it could no longer multiply anywhere.
What about vaccine side effects and adverse reactions?
- What are the side effects of vaccines?
All vaccines recommended in national immunisation programs are proven to be safe and effective. However, no vaccine is 100% risk-free and adverse events can occasionally occur after vaccination.
An adverse event following immunisation (AEFI) is an undesirable reaction that occurs after a vaccination which does not necessarily mean that they are caused by the vaccine itself. These adverse reactions and potential undesirable effects are described in detail in the product information leaflet that comes with each vaccine and which everyone can access. This information on reactions to a particular vaccine is gathered throughout the clinical trials that led to the vaccine being licensed in the first place and post-licensing surveillance, which refers to the observations and monitoring that is completed for any vaccine given to people.
Vaccine adverse reactions can be classified as:
Local reactions (e.g. redness, swelling at the injection site) are usually the least severe and most frequent. Systemic reactions (e.g. fever, headache, muscle ache) occur less frequently than local reactions, and severe allergic reactions (e.g. anaphylaxis) are the least frequent reactions.
Adverse reactions can be classified as mild, moderate or severe. Severe adverse reactions are very rare.
Adverse events aren’t arbitrarily defined by the person who administers the vaccine or by any one individual but follow agreed criteria.
The World Health Organisation (WHO) classifies the causes of reactions into five categories.
Adverse events following immunisation are further classified as:
- Vaccine product related (for example, swelling near the injection site due to properties of the vaccine)
- Vaccine quality-related (a reaction resulting from a quality defect of the vaccine product)
- Immunisation error-related (an adverse reaction caused by inappropriate handling or administration)
- Immunisation anxiety-related (a reaction from anxiety surrounding immunisation, such as an adolescent fainting when seeing a needle)
- Coincidental events (an event that occurs at the time of vaccination that is unrelated)
To learn more about the WHO classifications of adverse events following immunisation, you can download the WHO document on adverse events (pdf).