If you or someone you know is hesitant about getting a Covid jab, or worried about any part of the process, read our round up to get the facts and feel more confident about vaccination.

Mass vaccination has a strong track record of preventing, and in some cases eradicating, infectious diseases.

Indeed, it is thanks to mass vaccination programmes that deadly diseases such as smallpox and polio – once rife – have been virtually consigned to history in the Western hemisphere.

But news stories about possible adverse effects, and misinformation shared on social media or messaging apps, can cause anxiety about getting vaccinated.

We’ve pulled together insights on how the different Covid vaccines work, possible side effects, and how safety is measured and monitored throughout the process, to help you feel better informed.


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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 RNA) 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 (such as the rabies vaccine). How many doses are recommended will depend what the clinical trials showed offered the best possible protection.

How the different Covid-19 vaccines work

The mRNA Covid vaccines

The Pfizer/BioNTech and Moderna vaccines use mRNA (messenger ribonucleic acid). 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 Organisation (WHO) points out that mRNA vaccines have been studied for more than a decade.

There has been renewed interest in this type of vaccine due to the pandemic. This is because it can be developed more quickly in laboratories and development scaled up more easily, which is a big advantage when we are in a race against time to vaccinate large numbers of people.

mRNA technology is also being studied for a wide range of potential therapeutic uses, including in the fight against cancer.

The adenovirus Covid vaccines

The Oxford/AstraZeneca vaccine is an adenovirus vaccine, a type that has 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 able to be developed quickly and easily on a large scale.

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

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

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


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What else is in the Covid vaccines?

Another common concern is that vaccines contain harmful ingredients, or ones which don’t line up with people’s values or beliefs.

None of the Covid vaccines contain any animal products, and all of 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 this information up online.

The Vaccine Knowledge Project also has a helpful guide to common vaccine ingredients and what they do if you want to know more.

How are vaccines developed?

Vaccine development consists of a few main stages; laboratory research, clinical trials involving humans, regulatory approval, manufacturing and delivery to the public and finally ongoing monitoring.

The main phases of the clinical trials stage are:

  • Phase I – an initial trial involving a small group of adult participants (up to 100 people). This is carried out to make sure that the vaccine does not have major safety concerns in humans, and also to work out the most effective dose.
  • Phase II – a trial in a larger group of participants (several hundred people). Phase II trials check that the vaccine works consistently, and look at whether it generates an immune response. Researchers also start looking for potential side effects.
  • Phase III – a trial in a much larger group of people (usually several thousand – in the case of the Covid vaccines, tens of thousands). Phase III trials gather statistically significant data on the vaccine’s safety and efficacy (how well it works). This means looking at whether the vaccine generates a level of immunity that would prevent disease, and provides evidence that the vaccine can actually reduce the number of cases. It also gives a better chance of identifying rarer side effects not seen in the phase II study.

Vaccines then go through licensing, where the Medicines and Healthcare products Regulatory Agency (MHRA) reviews the trial data and checks that it shows that the vaccine is safe and effective. It also makes sure that overall, the benefits of using the vaccine far outweigh the risks.

The next stage is manufacturing and delivery of the vaccine to the wider population.

Vaccine production and rollout safety checks

Rigorous quality assurance processes take place during manufacturing and production. For example, AstraZeneca’s vaccine undergoes more than 40 different quality control tests in its manufacturing process.

This includes quality audits of manufacturing sites prior to starting work, and quality tests of raw materials, bulk batches and finished products throughout the process.

Phase IV studies also monitor the vaccine as it’s rolled out to the population, to watch out for any more potential issues or side effects.

This is why we’ve been seeing reports of possible side effects being flagged and checked. It may seem alarming when you see news reports, but it’s ultimately a good sign that vaccine safety monitoring systems are working as they should.

Speed of vaccine development

Vaccines normally take around 10 years to develop, and the speed with which Covid-19 vaccines have been produced has worried some people.

But there are some key differences with the development of the Covid vaccines that have allowed scientists to produce them quickly without compromising on safety.

Usually the clinical trial stages above have to happen in sequence, with one stage only beginning as the previous one is completed.

For the Covid vaccines these have run in tandem – so vaccines have been developed much quicker but without rushing through or skipping any important steps.

The lengthy period of time we’ve had to wait for other vaccines isn’t because they’ve had 10 years of safety monitoring: it’s actually largely down to logistical and funding issues which slow the process down.

With Covid vaccines, there’s worldwide backing for quick results, with funding readily available.

What’s more, we aren’t starting from scratch. Scientists have been developing coronavirus vaccines for years, as this family of viruses is known to be a potential risk to humans.

Get more information about the Covid vaccination roll out and different types of vaccine

Could vaccination make Covid worse?

This is another concern that did the rounds on social media.

One of the things that vaccine developers look out for during the research stages is something called ADE – antibody-dependent enhancement.

This is where an initial infection – or vaccine – produces antibodies that don’t protect you from a virus, and instead actually bind to them, making a second or post-vaccine infection more severe.

ADE is why getting Dengue fever twice can be very dangerous, as the antibodies produced in reaction to the first infection may actually allow a second infection to flourish. But it’s not common with other diseases.

However, it’s something researchers are aware of and check for when developing vaccines. In clinical trials for the approved Covid vaccines, there wasn’t any evidence of ADE.

Covid-19 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 so far from nearly 20 million first doses given in the UK reflects this.

This doesn’t mean you won’t get side effects. 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 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’, these symptoms can nevertheless be unpleasant. 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.

More serious adverse reactions

The risk of severe effects from the vaccine is tiny when compared with the risk of getting ill from Covid-19.

It’s also important to realise that vaccine safety monitoring picks up any serious illness somebody might experience after having a vaccine, but 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.

Allergic reactions

There were two early 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, and vaccination stations are prepped to deal with any allergic reactions, which can be quickly remedied.

Blood clots

The Oxford/AstraZeneca vaccine was briefly paused in multiple EU countries following reports in Denmark and Norway of blood clots in adults who had received the vaccine.

It’s important to note that this was a precautionary measure and that there is no proof that the vaccine caused the clots.

These events are extremely rare: the European Medicines Agency (EMA) says that of around 20 million people given the AstraZeneca vaccine so far in the UK and EU, there have been 25 cases of thrombosis – and no proven link between those cases and the vaccine.

After reviewing the evidence the EMA said: ‘the benefits of the vaccine in combating the still widespread threat of Covid-19 (which itself results in clotting problems and may be fatal) continue to outweigh the risk of side effects, and the vaccine is not associated with an increase in the overall risk of blood clots in those who receive it.’

The MHRA also reviewed the evidence and stated that: ‘reports of blood clots received so far are not greater than the number that would have occurred naturally in the vaccinated population.’ Thrombosis occurs naturally in about 1 in 1,000 people per year.

However, it is conducting a further, detailed review into five UK reports of a very rare and specific type of blood clot in the cerebral veins (CSVT) occurring together with lowered platelets. This has been reported in less than one in a million people vaccinated so far in the UK, and can also occur naturally – a causal association with the vaccine has not been established.

The MHRA has said that as a precautionary measure, it advises that anyone with a headache that lasts more than four days after getting vaccinated, or bruising beyond the site of vaccination after a few days, should consult their doctor.

It’s obviously alarming to see reports such as these in the news, but it’s important to remember a link hasn’t been proven and the numbers involved are absolutely tiny.

The EMA, MHRA and WHO have all investigated the issue and declared the vaccine is still safe and effective to use, while these investigations continue.

Putting side effects in perspective

All medicines and medical procedures have potential side effects. Reading the leaflet accompanying any common medicines you have at home will attest to this.

But the Covid vaccines have good data to back their safety and efficacy, and are being highly scrutinised and monitored worldwide as they are rolled out.

By contract, the risk of contracting Covid-19, and the potential complications of the disease, is very high.

The Office for National Statistics  says about 1 in 270 people in the UK currently have Covid, and there have been 758 deaths in the past seven days.

The ONS also estimates that one in 10 people may develop long Covid – with symptoms lingering for 12 weeks or more.

Is the vaccination programme making a difference?

The Covid vaccines being deployed in the UK are already having a positive effect – reducing the risk of serious illness and bringing down the number of hospitalisations.

Data from a real world study by Public Health England show that since January, protection against symptomatic Covid, four weeks after the first dose, ranged between 57 and 61% for one dose of Pfizer/BioNtech and between 60 and 73% for the Oxford/AstraZeneca vaccine.

Public Health Scotland published data in February that linked vaccination to a substantial decrease in hospital admissions for Covid in Scotland.

The study shows that, four weeks after getting a first jab, the Pfizer/BioNTech and Oxford-AstraZeneca vaccines reduced the risk of hospitalisation from Covid-19 in up to 85% and 94%, respectively.

Monitoring vaccine safety

The MHRA has in place 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

Other countries have similar systems in place.

How to report 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 does not necessarily mean that it was caused by the vaccine, only that the reporter has a suspicion it may have.

The reports are then analysed by the MHRA to see if there are any patterns that point to causal links.


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How to fact-check vaccine information

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

They recommend 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 like the NHS and WHO. If you don’t recognise the source, try and 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 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 stat 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 it sounds too definitive or simple. Check to see whether multiple sources are reporting the same thing, and you can also 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.

Useful sources:


Do you have other questions about the vaccine? Let us know at [email protected]



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