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Updated: 13 Jun 2022

Covid vaccines explained: protection, safety, side effects and how to get yours

Find out about the different types and how they work, potential side effects, boosters and the latest on the UK vaccine roll-out
Anna Studman

Vaccination is one of our best tools in the fight against Covid, but there's a lot of confusing information about their safety and effectiveness. 

Here, we answer key questions about how the vaccines work, vaccine safety and side effects, who is eligible for what, and how to get your vaccine or booster.

UK Covid vaccine roll-out latest:

  • Due to the emergence of the Omicron variant, people aged over 18 will now be eligible for a booster dose.
  • Priority for booster doses will still be given to older adults and those most at risk of serious disease from Covid.
  • Vaccination is now available to everyone aged 12 and over, and 12-15 year olds will now also be eligible for a second dose.
  • Pregnant people are being encouraged to take up the Covid vaccine. 

Full information on current eligibility and how to book is available on the NHS Covid vaccine info site.

Flu vaccine 2021 - it's also important to get your flu vaccine. Find out who is eligible for a free one and how much they cost.

How vaccines work

Vaccines train your body to fight disease by introducing a modified version of a virus, which trains our immune system to attack and inactivate the real thing if it enters the body. 

Most vaccines do this by using a weakened, harmless form of the virus that our immune cells can learn how to defeat. Others introduce a specific part of the virus that’s particularly important for our systems to recognise (for example, the coronavirus ‘spike’ protein) to do the same thing. 

Some Covid vaccines use a newer approach, which uses mRNA (messenger ribonucleic acid) instead. This is essentially a set of instructions, akin to those our body makes on its own, which teaches our cells how to make the key piece of the virus themselves. This then triggers an immune response that shows our system how to defeat the real virus. 

Some vaccines only require one dose, while others require two or even three. The number of recommended doses will depend on what clinical trials show to offer the best protection.

Covid vaccines types explained

mRNA vaccines

The Pfizer/BioNTech and Moderna vaccines use mRNA. This contains the code for cells to create a part of the coronavirus – the ‘spike’ protein that our immune systems need to recognise and fight. 

You might have heard that mRNA vaccines are a totally new technology, which excites some people and makes others anxious. 

It’s true that the use of mRNA vaccines is new, but the technology hasn’t popped up overnight. The World Health Organization (WHO) points out that mRNA vaccines have been studied for more than a decade. 

There's been renewed interest in this type of vaccine because of the pandemic. It's quicker to develop in laboratories and easier to scale up, which is a big advantage when we're in a race against time to vaccinate large numbers of people. This technology is also being studied for a wide range of potential therapeutic uses, including in the fight against cancer.

Adenovirus vaccines

The Oxford/AstraZeneca and Johnson & Johnson/Janssen vaccines are adenovirus vaccines, a type that's been well studied in previous clinical trials. It uses a harmless modified virus to deliver the genetic code of the spike protein to the body. 

As with mRNA vaccines, it has the advantage of being quick and easy to develop on a large scale. 

None of the vaccines have the ability to alter your DNA (a concern that's been doing the rounds). 

Oxford University’s Vaccine Knowledge Project explains that mRNA vaccines can't reach the part of the cell that contains the DNA, mRNA can't be turned into DNA, and neither mRNA nor adenovirus vaccines have the ‘tools’ needed to copy or edit DNA. 

The vaccine also can't replicate inside your body and only stays there for a few days.

What's in the Covid vaccines?

Another common concern is that vaccines contain harmful ingredients, or ones that don’t line up with people’s values or beliefs. So it's worth knowing that:

  • none of the Covid vaccines contain any animal products
  • all the vaccines being offered in the UK are halal and kosher.

Aside from the active ingredient, the main ingredient in vaccines is usually water. Other ingredients are found in tiny amounts and usually for stability purposes to ensure maximum efficacy. 

The vaccines are not made from human embryos, as some social media reports have claimed. The Vaccine Knowledge Project explains that the Oxford/AstraZeneca vaccine uses a harmless virus that was grown in human embryonic stem cells, which are copies of stem cells that originally date from the 1970s and would otherwise have been destroyed. 

So the vaccine doesn’t contain any embryonic cells and the stem cells weren’t taken directly from a human embryo. 

If you want a full list of ingredients of the vaccine you’re getting, you can ask for a patient information leaflet or look up this information online. The Vaccine Knowledge Project also has a helpful guide to common vaccine ingredients and what they do.

Which vaccines are being given in the UK?

The UK is currently administering the Pfizer/BioNtech, Oxford/AstraZeneca and Moderna vaccines. 

The Johnson & Johnson/Janssen vaccine has also been approved. 

There are multiple other vaccines still going through trials and regulatory approval that will be used in the UK if they’re approved. 

You might not know in advance which vaccine you’re getting, but all vaccines being offered are shown to work well at preventing you from getting sick from coronavirus. 

The vaccine you get will have been approved by the Medicines and Healthcare products Regulatory Agency (MHRA) and the Joint Committee on Vaccination and Immunisation (JCVI) for your age and risk group.  This is being constantly monitored and updated and information is publicly availably on these organisations' websites.

The only demographic-based recommendation at this stage is that under-40s should be offered an alternative to the AstraZeneca vaccine if there’s one available, because of extremely rare reports of specific types of blood clots occurring after a dose of this vaccine.

How well do the vaccines work against variants?

The Delta variant is slightly different from the original Covid virus, and much easier to transmit. We now know that fully vaccinated people can still get infected and in some cases pass on Covid - but the risk of this is far lower compared to unvaccinated people. 

Vaccines still provide high protection against serious disease and hospitalisation from Covid, and do lower your chances of catching it.

Imperial College London’s React study (August 2021) found that people who said they'd received two vaccine doses were half as likely to test positive for Covid, and it estimated that the risk of infection from the Delta variant is 50-60% lower in fully vaccinated people.

Public Health England's vaccine surveillance report (September 2021) estimates that full vaccination still gives more than 90% protection against hospitalisation and death from variants including Delta. 

Can you still infect others after being vaccinated?

As above, it's possible for vaccinated people to spread the virus, due to the virulence of the Delta strain. 

But it's still less likely to happen in vaccinated populations than unvaccinated, because the risk of getting Covid in the first place is still much lower for those who are fully vaccinated.

Covid booster shots roll-out: who's eligible and how to book

We don't know exactly how long the protection lasts after being vaccinated, but there's evidence to suggests it wanes over time.

A booster vaccination programme is in progress across the UK and has been expanded to include all adults in the UK, due to the emergence of the Omicron variant.

Priority will still be given to older and more vulnerable people.

Booster doses will be offered to people no sooner than three months after their previous dose. If you think you're eligible but haven't heard anything yet, you may be able to book your appointment online without an invitation. See where and how to book on the NHS Covid vaccine mini site.

You'll get either the Pfizer or Moderna vaccine as a booster shot, regardless of which vaccine you got initially. This decision is based on the results of the COV-boost trial that showed this was the safest and most effective choice.

Do we need Covid booster jabs?

The decision to administer boosters has come from the JCVI and is based on a 'precautionary position' to maintain a high level of protection in vulnerable adults throughout the winter. 

However, concerns have been raised - including by the WHO - that richer countries such as the UK offering booster shots relatively soon after the initial vaccination programme could exacerbate global health inequality, as people in developing countries might still be waiting for their first jab.

If you are offered a jab, you should take it, as refusing it doesn't necessarily mean it will be put to use elsewhere. With high transmission rates going into winter and pressure on the NHS increasing, it's sensible to take it up.

Can children have the Covid vaccine?

Due to the emergence of the Omicron variant, children over 12 are now eligible for both a first and second dose of the Pfizer vaccine. 

Previously, 12-15 year olds were only being offered one dose.

The decision was made to help reduce the educational disruption that has been caused by Covid in schools, coupled with a marginal health benefit for children on an individual level. 

Jabs are being administered in schools.

Children aged 12-15 are already eligible for a Covid vaccination if they have an underlying health condition that puts them at higher risk of severe Covid.

This includes children with severe neurodisabilities, Down’s syndrome, immunosuppression, and severe learning disabilities or who are on the learning disability register. It also includes children who live with someone who is immunosuppressed.

If your child has tested positive for Covid recently, they need to wait twelve weeks before they can have the vaccine. Find out more on the NHS Covid vaccines for children guide.

Can pregnant people have the vaccine?

The Royal College of Obstetricians and Gynaecologists says that Covid vaccines are recommended in pregnancy, following advice from the JCVI. 

It says vaccination is the best way to protect against the known risks of Covid in pregnancy for both women and babies, including admission of the pregnant person to intensive care and premature birth of the baby. 

This is based on real-world data from the US, where around 148,000 pregnant people have been vaccinated with Pfizer or Moderna. The data hasn't raised any safety concerns.

What are the rules around Covid vaccines and travel?

Your vaccination status will affect the rules you have to follow if you want to travel abroad. For example, unvaccinated people will still have to quarantine for 10 days when returning from certain countries, and will have to do additional Covid tests upon return.

Many countries also now require proof of full vaccination as a condition of entry.

For up-to-date Covid travel advice, see our Covid holiday rules advice: where you can go now and what tests or vaccines you need

Covid vaccine side effects

The data from clinical trials show that the risk of severe side effects from the Covid vaccines is tiny, and real-world data we have from most of the adult UK population reflects this.

This doesn’t mean you won’t get side effects, though. In fact, experiencing some temporary side effects after vaccination is pretty common, but they aren’t serious. 

The MHRA, which is in charge of monitoring vaccine data in the UK, says the most common Covid vaccine side effects seen so far from both vaccines available are: 

  • sore arm 
  • flu-like symptoms: headache, chills, fatigue (tiredness), nausea (feeling sick), fever, dizziness, weakness, aching muscles and rapid heartbeat.

While described as ‘mild’ (in clinical terms), these symptoms can nevertheless be unpleasant - like a taste of having flu for a day or so. They can be treated with normal painkillers, rest and fluids, and should be gone within a few days. 

These types of reactions come from the immune response in our bodies triggered by the vaccine. You’ll know if you’ve ever felt a little under the weather after a flu vaccine that they often happen with other types of vaccines, too. 

Some people might not get any side effects at all, while others are hit harder – this is down to the fact that our immune systems all work slightly differently. 

Older people are less likely to get severe side effects. The MHRA says that side effects like the above tend to be more frequently reported by younger adults. 

While side effects might make you feel rotten for a short amount of time, it’s important to know that this isn’t unexpected and is part of your body’s normal process.

Covid vaccines and changes to the menstrual cycle

The MHRA has received nearly 35,000 reports of menstrual irregularity - commonly heavier or earlier periods - following administration of the Covid vaccine, mostly via its Yellow Card reporting scheme. This doesn't necessarily mean there's a link, but it's worthy of investigation.

There is, however, no evidence that the Covid vaccines affect fertility, and periods seem to return to normal following the first cycle after vaccination. 

Dr Victoria Male, a lecturer in reproductive immunology at Imperial College London, wrote in the BMJ that any connection is likely to be as a result of the body's immune response, and not because of any specific vaccine. Menstrual irregularities have been observed after all available Covid vaccines, as well as vaccines for other diseases, and in people who have Covid. 

Still, Dr Male notes that proper research into a possible link is important in maintaining vaccine confidence and countering misinformation around false claims that Covid vaccination affects fertility. 

Covid vaccines and heart inflammation

There have been extremely rare reports of myocarditis and pericarditis (inflammation of the heart muscle or surrounding tissue) following a second dose of the Pfizer or Moderna vaccine - more commonly in men under 40. 

The MHRA says these cases are usually mild and that the benefits of vaccination still outweigh any risks. 

People who've had their second vaccination should seek medical attention if they experience a new onset of chest pain, shortness of breath, palpitations or arrhythmia (irregular heartbeat).

Serious adverse reactions to the Covid vaccine 

The risk of severe effects from the vaccine is tiny compared with the risk of getting ill from Covid-19 and potential complications of having the virus. 

However, it's worth being aware of what these are so you know what to look out for and are fully informed.

Allergic reactions 

There have been a few reports of severe allergic reactions to the Pfizer/BioNTech vaccine, in people who had a history of severe anaphylaxis. Wider use of the vaccine now suggests that this reaction is very rare. 

People who have previously had severe allergic reactions to vaccines are advised to consult with their doctor before being vaccinated. Vaccination stations are prepped to deal with any allergic reactions, which tend to come on swiftly and can be quickly remedied.

The MHRA’s guidance is that those with a previous history of allergic reactions to the ingredients of the vaccine should not receive it.

Blood clots

There have been extremely rare reports of specific types of blood clots occurring following a dose of the AstraZeneca vaccine. The MHRA estimates that the overall incidence of these after a first or unknown dose was 15 per million doses, and two per million doses after the second shot. 

In light of this information, the MHRA has advised that: 

  • people who are at higher risk of blood clots because of their medical condition should only be considered for the AstraZeneca vaccine if the benefits outweigh potential risks 
  • anyone who experienced a blood clot following their first dose of the vaccine should not have a second dose; those who did not experience these side effects (most people) should still have their second dose 
  • people under 40 should be offered an alternative vaccine if there's one available in their area and if their risk of contracting Covid-19 is low.

It's important to remember that Covid-19 itself can cause serious blood clotting, and the risk associated with this is much higher for most people.

The patient information leaflet for the AstraZeneca vaccine includes some advice on side effects to watch for once you've been vaccinated. These include a severe or unusual headache that isn't helped by painkillers and gets worse when you bend over or lie down, a rash at the injection site that looks like bruising, shortness of breath, chest pain, leg swelling, leg pain or persistent abdominal pain.

Heart inflammation

There have been extremely rare reports of myocarditis and pericarditis (inflammation of the heart muscle or surrounding tissue) following a second dose of the Pfizer or Moderna vaccine, most commonly in men under 40. 

The MHRA says these cases are usually mild and treatable, and that the benefits of vaccination still outweigh any risks. 

The patient information leaflet for the mRNA vaccines says people who've had their second vaccination should seek medical attention if they experience new onset of chest pain, shortness of breath, palpitations or arrhythmia (irregular heartbeat).

How vaccines are monitored for safety

Vaccine safety monitoring is ongoing, and investigates any serious illness somebody might experience after having a vaccine (although this doesn’t mean the vaccine caused it). 

Researchers will gather data and look at how common illnesses are in the wider population to gauge how likely it is that it’s down to the vaccine, and update safety information or do further investigation as appropriate.

The MHRA has a proactive plan for continuous monitoring of vaccine safety, which involves four main strands: 

  • reports submitted by the public and healthcare professionals (through the Yellow Card Scheme) and also by drug companies as part of legal requirements
  • analysing anonymised vaccination records and patient data from GP practices 
  • targeted monitoring of certain groups to help build on the safety profile detected in clinical trials
  • formal epidemiological studies designed to further explore possible side effects detected through the above three methods.

Other countries have similar systems in place. 

How to report vaccine side effects

You can report any suspected adverse reactions to the vaccine by using the MHRA Yellow Card Scheme

This is a way of collecting data to help the MHRA to monitor the safety of medicines and medical devices, and there’s a dedicated site for the Covid vaccines. 

A report of a suspected adverse reaction to the Yellow Card Scheme doesn't necessarily mean that it was caused by the vaccine, only that the person reporting it suspects it might have. The MHRA then analyses the reports to see if there are any patterns that point to causal links.

How to fact-check vaccine information

We asked Full Fact – an independent factchecking organisation – for some tips on how to identify coronavirus misinformation. 

It recommends asking yourself three key questions when you’re faced with information: 

  1. Where’s it from? A trusted source is your best bet – this includes official organisations such as the NHS and WHO. If you don’t recognise the source, try to find out more about it and why it’s sharing the information. Clues to suggest a dodgy website include phony URLs, bad spelling or awkward layouts. On Twitter, check their handle to verify that the name matches. On Facebook, dig into the source to see what other content it shares and how it describes itself. 
  2. What is missing? Don’t just rely on a headline, image, quote or or statistic that might have been taken out of context. Over the past year, we’ve all become armchair epidemiologists, but it’s important to corroborate facts about the vaccine – especially if they sound too definitive or simple. Check to see whether multiple sources are reporting the same thing, and use a fact-checking database. 
  3. How do you feel? Full Fact says people making fake news know how to manipulate our emotions – look out for content that makes sweeping or shocking statements that are intended to discourage people from getting vaccinated. Nuance is key.