COVID-19 vaccines: all you need to know

26th October, 2022 • 15 min read

Most countries in the world are now vaccinating people against COVID-19. “All the vaccines that have been approved and used have gone through a thorough review of safety, quality and efficacy from clinical trials, and often a review from a government body as well,” says

Dr Adiele Hoffman
, GP and Healthily expert.

For example, in the US vaccines have been approved by the US Food and Drug Administration (FDA) and in the UK by the Medicines and Healthcare products Regulatory Agency (MHRA).

“Since they were introduced in December 2020, we have learned that vaccines dramatically reduce your risk of getting very sick, needing hospital treatment or dying with COVID-19; but also that protection does reduce over time. Boosters targeting the new variants have now been approved, too,” says Dr Adiele Hoffman.

So here is what you need to know about how well vaccines have worked, what new vaccines are available and who needs a booster to get the best protection.

Who can have a COVID-19 vaccine?

Different groups of people have been offered a COVID-19 vaccine at different times. When you can get a vaccine depends on several things, including which country you live in, your government’s plans, your age and how at risk you are from the virus:

  • in the US it’s recommended that everyone aged 6 months and over gets the COVID-19 vaccine and everyone 5 years and over also gets a booster, if eligible

  • in the UK everyone aged 5 or over can now have a first and second dose of the vaccine, if eligible. People aged 16 and over can get a booster, as can some children aged 12 to 15

Do I need a COVID-19 vaccine if I’ve already had the virus?

You’ll be offered the vaccine whether or not you’ve had COVID-19. But if you currently have COVID-19, you should wait until you’ve recovered to have your vaccine.

  • in the US the CDC suggests you consider waiting 3 months after a COVID-19 infection before you have the vaccine

  • in the UK the guidance is that you should wait until at least 4 weeks after your symptoms started – or after your positive test if you didn’t have any symptoms. People under 18 who aren’t in high-risk groups should ideally wait 12 weeks

How many people have had COVID-19 vaccines in the US and UK?

In the US and UK vaccination programmes were introduced in December 2020. Worldwide 4.95 billion people have now been fully vaccinated, including 226 million people in the US (68% of the population) and 55.7 million in the UK (77.5% of the population). In the UA almost 111 million fully vaccinated people have also received an additional booster dose.

Which vaccines are available in the US and UK?

In total, 4 COVID-19 vaccines are recommended in the US.

  • Pfizer/BioNTech
  • Moderna
  • Novavax
  • Johnson and Johnson Janssen

Pfizer/BioNTech have both produced an updated bivalent vaccine – these target the original virus and the newer BA.4 and BA.5 variants, and they are being used for the

booster program
from Autumn 2022 in the US and UK.

In the UK, the 2 vaccines currently available are:

  • Moderna
  • Pfizer/BioNTech vaccine

As in the US, the newer bivalent Moderna and Pfizer vaccines are being used for the seasonal booster campaign.

There are 4 other vaccines approved for use in the UK but not currently available. These are:

  • Johnson and Johnson Janssen
  • Novavax
  • Valneva
  • AstraZeneca

While most people in the UK can have any of the COVID-19 vaccines that are currently being used:

  • if you’re under 40 or pregnant, you’ll usually be offered the Moderna or Pfizer/BioNTech vaccine
  • if you’re under 18, you’ll only be offered the Pfizer/BioNTech vaccine

Check for updates to the COVID-19 vaccination program in the

and in the

How do COVID-19 vaccines work?

A vaccine works by training your body’s defences (immune system) to spot and fight off the virus or bacteria it’s designed for. If you’re then exposed to those germs, your immune system will fight back and stop you from getting seriously sick, or even stop you getting sick at all.

There are several types of vaccine being made for COVID-19, including:

  • viral vector vaccines – these use a different virus (not the one that causes COVID-19) to carry bits of the COVID-19 virus into your body – this is called a ‘viral vector’. It doesn’t cause disease, but it makes your immune system respond to the COVID-19 elements in the vaccine. The Oxford/AstraZeneca vaccine is a viral vector vaccine
  • mRNA vaccines – these contain instructions that teach your body to make and then recognise a ‘spike protein’ from the COVID-19 virus. This trains your immune system, so if you’re exposed to the COVID-19 virus, you’re prepared to fight it. The Pfizer/BioNTech and Moderna vaccines are mRNA vaccines
  • protein-subunit vaccines – these use harmless pieces of protein (not from the COVID-19 virus), which imitate the virus. They’re close enough to make your immune system respond, so that if you come into contact with the real COVID-19 virus you’ll be protected

How effective is a COVID-19 vaccine?

How well the COVID-19 vaccine works varies very slightly depending on which vaccine you have and you need to complete the course to be fully vaccinated. You will have some protection after the first dose but, for most COVID vaccines, you will need 2 doses (although 1 dose is sufficient for some vaccines). If you have a weakened immune system you may need extra doses to become fully vaccinated.

We also now know that the

vaccine’s effectiveness wears off – or ‘wanes’ – over time
. How quickly will depend on factors such as which vaccine you had and your age. Protection will usually last around 4-6 months and having booster doses can improve your protection and keep vaccine effectiveness as high as possible.

Variants can also evade the vaccine but

newer boosters
are being developed to protect against these. So you may be offered ‘boosters’ to top up your protection.

A vaccine’s effectiveness (known as efficacy) is worked out by comparing the risk of disease for people who have been vaccinated with people who haven’t. So, 90% effectiveness means that a group of vaccinated people would have 90% fewer cases of the virus than they would if they hadn’t been vaccinated.

How quickly does the COVID-19 vaccine start working?

After having a COVID-19 vaccination, it usually takes your body a few weeks to make the white blood cells that fight the virus. This means it’s possible to get infected by the virus just before or after having the vaccine and then become sick, because the vaccine didn’t have enough time to work.

After your first dose, it takes 3 to 4 weeks before your body has built up enough antibodies to properly provide immune protection.

Study results showed that booster vaccines provide more than 90% protection against symptomatic COVID-19 infection in adults aged 50 years and over, 2 weeks after receiving the vaccine.

Is the COVID-19 vaccine safe?

Approved COVID-19 vaccines have gone through the clinical trials and safety checks that all licensed medicines have to go through before they are brought to market.

The World Health Organisation and regulatory authorities in individual countries continuously monitor the ongoing safety of the COVID-19 vaccines. Watch this WHO video to understand how vaccines are assessed for safety.

In the US all the COVID-19 vaccines have met the Food and Drug Administration standards for safety, effectiveness and quality and are being stringently monitored for safety.

As with any medicine, some people get side effects.These are mostly mild and don’t last more than a week. But in some rare cases, the vaccines can have more serious side effects. Find out more about

side effects.

Getting the COVID-19 vaccine if you’re pregnant

If you’re pregnant, you’re at higher risk of getting very sick if you get COVID-19, so it’s important to stay up to date with the vaccine and any boosters available where you live.

If you haven’t had a vaccine yet, you’ll usually be offered the Moderna or Pfizer/BioNTech vaccine. We now have a lot of evidence – from millions of pregnant people around the world that have been vaccinated – that these vaccines are safe for you and your baby.

The vaccine can’t give you or your baby COVID-19. And there’s no evidence that the vaccines used in the US and UK can increase your risk of miscarriage, birth complications or defects, or stillbirth.

Can you have the COVID-19 vaccine if you’re breastfeeding ?

There’s no evidence that getting a COVID-19 vaccine while breastfeeding causes any harm to your child or problems with breastfeeding.

If you're breastfeeding, the vaccines you can have depend on your age. If you're 40 or over, you can have any of the COVID-19 vaccines. If you’re under 40 and don’t have a health condition that increases your risk of getting seriously ill from COVID-19, it's preferable for you to have the Pfizer/BioNTech or Moderna vaccine.

You can’t catch COVID-19 from the vaccines and you can’t pass it to your baby through your breast milk.

Can you have the COVID-19 vaccine if you have an underlying medical condition?

People with certain underlying medical conditions are at high risk of severe illness from COVID-19. They may have the vaccine, as long as they haven’t had a serious allergic reaction to any of the vaccine’s ingredients, and this includes people who are immunosuppressed.

If you’re immunosuppressed, it’s very important to keep taking measures to protect yourself from catching COVID-19, even once you’ve had the vaccine.

The vaccine is safe for people with lung conditions and there are very few people who shouldn’t get the vaccine.

  • you shouldn’t get the vaccine if you've had a confirmed anaphylactic reaction to any of the ingredients in the vaccine, or a previous dose of the vaccine
  • if you have an underlying medical condition, take immunosuppressive medication or are in any way unsure, speak to a healthcare professional before having the vaccine

If you’re unwell on the day you’re supposed to have your vaccine, talk to your doctor – they may recommend postponing until you have recovered.

How is a COVID-19 vaccine given?

Most of the COVID-19 vaccines are given by an injection into your arm. You’ll usually need 2 doses, given at different times, plus at least 1 booster dose at a later date.

Here’s what you need to know:

  • the 1st dose of the vaccine provides some protection in the short term, starting from 3 to 4 weeks after you’ve had it
  • you should have your second dose from at least 3-8 weeks after your 1st dose depending on which vaccine you’ve had, you’re age and what the guidance is where you live
  • children are given smaller doses of the vaccine, depending on their age

What you need to know about COVID-19 booster shots

As the effects of the COVID-19 vaccine wear off over time and different strains of the virus emerge, booster jabs that have been tweaked to target new variants of COVID-19 as well as the original SARS-CoV-2 virus, are now being offered in some countries.

This is because they can restore protection that has decreased and provide broader protection against newer variants.

The updated vaccines (called bivalent vaccines) target the most common sub variants of the Omicron strain of COVID-19 including BA.4 and BA.5, which are both more contagious and more resistant than earlier strains of Omicron.

When will I get a COVID-19 booster jab?

The timing of booster dose varies according to which country you live in, but the World Health Organisation recommends booster doses be given between 4 and 6 months after completing the primary course of 2 (sometimes 3) COVID-19 injections.

How and when to get your COVID-19 booster shot in the US

The CDC in the US has recommended booster shots for everyone aged 12 and over who has had their primary vaccinations (1,2 or 3 shots of the vaccine depending on which vaccine was used and whether you are immunocompromised) and are at least 2 months on from a previous booster shot. You can check your eligibility for a booster shot on the

CDC site.

  • for Fall 2022, the CDC has recommended booster shots for all over 12s even if you have previously had a booster. This is because the new shots target the latest BA.4 and BA.5 sub variants
  • to arrange your COVID-19 vaccine booster at a location near you in the US go to the
    CDC Vaccines site

How and when to get your COVID-19 booster shot in the UK

  • in the UK, you can have a first booster dose if it’s 3 months since your 2nd dose and you’re either over 16 or aged 12 to 15 and have a condition that means you’re at high risk

In autumn 2022, as well as the 1st booster dose available to everyone over 16, new seasonal bivalent boosters that have been updated for the newer variants are also being offered to certain groups who are at higher risk

These include:

  • anyone aged 50 and over
  • pregnant women
  • frontline social or health care workers
  • aged 5 and over with a health condition that puts you at high risk
  • aged 5 or over with a weakened immune system
  • aged 16 and over and a carer

The NHS website
can help you find out if you’re eligible for a seasonal booster in the UK.

3 reasons why you need to get your booster shots

What are the side effects of the COVID-19 vaccine?

Like all medicines, vaccines can cause side effects. However, side effects of the COVID-19 vaccine are usually mild and shouldn’t last longer than a week. They can include:

  • a sore arm at the injection site
  • feeling tired
  • a headache
  • feeling achy
  • feeling or being sick
  • fever – you may get a high temperature or feel hot or shivery 1 or 2 days after your vaccination. You can take painkillers such as paracetamol if you need to. If your symptoms get worse or you're worried, call a doctor
  • side effects from new bivalent boosters are expected to be similar

If you have a high temperature that lasts longer than 2 days, a new, continuous cough or a loss of or change to your sense of smell or taste, you may have COVID-19. You can’t catch COVID-19 from the vaccine, but you may have caught it just before or after your vaccination.

Very rare side effects


In a very small number of cases, people have had an allergic reaction to the vaccine. This is most likely to happen within a few minutes of being vaccinated, and vaccination staff will be trained in how to look after you if this happens.

If you think you’re having an allergic reaction, call for urgent medical help.

If you’ve had a serious allergic reaction (such as

) in the past, tell a healthcare professional before you have a COVID-19 vaccine. They can advise if it’s safe for you to have the vaccine.

Blood clots

There have been a very small number of reports of blood clotting problems in people who have had the Oxford/AstraZeneca vaccine. It’s not yet clear why this happens, and it’s extremely rare.

Get urgent medical advice if you have any of these symptoms from 4 days to 4 weeks after having a vaccine:

a really bad headache that doesn’t go away with painkillers, or is getting worse
a headache that’s different from headaches you usually have, plus blurred vision, feeling or being sick, problems speaking, weakness, sleepiness or fits (seizures)
a headache that feels worse when you lie down or bend over
shortness of breath, leg swelling, chest pain or tummy pain that doesn’t go away
a rash that looks like small bruises or bleeding under your skin

Heart inflammation

Very rarely, people get heart inflammation (

) after having the Moderna or Pfizer/BioNTech vaccine.

It’s reported less often after the Pfizer/BioNTech vaccine, which is 1 of the reasons this is the preferred vaccine for children and young people. It’s reported more often in men under 25, and after the 2nd dose.

Symptoms usually appear within a week of vaccination, and most cases are mild and get better without treatment.

However, you should get urgent medical advice if you have any of these symptoms within 10 days of having a COVID-19 vaccine or booster:

  • chest pain
  • dizziness
  • shortness of breath
  • a fast-beating, fluttering or pounding heart (

Guillain-Barré syndrome

A very small number of cases of

Guillain-Barré Syndrome (GBS)
have been reported after having the Pfizer-BioNTech vaccine, but the number of cases is still around the number we would see normally in the population.

Guillain-Barre Syndrome requires treatment in hospital so get an urgent review if you have these symptoms.

Important: Our website provides useful information but is not a substitute for medical advice. You should always seek the advice of your doctor when making decisions about your health.