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COVID-19 mRNA Vaccine

Comirnaty - Pfizer BioNTech, Spikevax - Moderna

Updated versions of Comirnaty® and Spikevax® vaccine are available to protect against two coronavirus variants; the original 2020 strain and the Omicron variant. These are known as bivalent vaccines.

Use of all coronavirus vaccines will be closely monitored to allow quick identification of new safety information. If you develop any side-effects, you can help this important process by reporting these to the dedicated COVID-19 vaccine monitoring scheme.

Please be aware that updates to safety information may happen frequently as new data become available. Always refer to the manufacturer's printed information given to you at the time of your vaccination.

This vaccine may be offered to you as part of the UK's COVID-19 vaccination programme. People in other countries may receive this vaccine through their local programmes.

A healthcare professional will assess whether this vaccine is suitable for you before you are vaccinated. Ask your vaccinator if you have any questions or concerns.

The most common side-effects are pain at the injection site, tiredness, headache, muscle or joint pain, and chills or fever.

Ask a pharmacist to recommend a suitable painkiller in case you feel unwell. Drink plenty of fluids and rest if possible.

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About COVID-19 mRNA vaccine (Comirnaty®, Spikevax®)

Type of medicine

COVID-19 mRNA vaccine

Used for

Protection against infection caused by SARS-CoV-2 virus, original and Omicron variants

Also called

Comirnaty® 'Pfizer BioNTech' vaccine; 'Moderna' Spikevax® vaccine

Available as

Multi-dose injection vials

SARS-CoV-2 is a strain of coronavirus that first emerged in Wuhan, China, in late 2019. It has led to the worldwide COVID-19 pandemic. Social distancing measures, travel restrictions and use of personal protective equipment have been used to help control the spread of the virus. A number of vaccines are now available as part of national vaccination programmes.

The coronavirus has a distinctive 'spike' protein on its outer coat that it uses to infect host cells. The COVID-19 mRNA vaccine contains single strands of messenger RNA that encode the spike protein. They are produced in test tubes and embedded within tiny fat particles (lipid nanoparticles). Once inside the body the mRNA strands are used as templates by your own cells to mass produce the coronavirus spike protein.

These spike proteins trigger your immune system to create antibodies that will attack the SARS-CoV-2 virus if it enters your body. They also prime other parts of your immune system - your T cells - which provide longer-lasting protection against the virus. The vaccine does not contain coronavirus and cannot cause COVID-19 disease.

In clinical trials the Pfizer BioNTech vaccine was around 95% effective at preventing COVID-19 disease from seven days after the second dose. The Moderna Spikevax° vaccine was shown to be 94.1% effective from two weeks after the second dose.

The UK COVID-19 vaccination programme aims to protect the most vulnerable people from infection, such as the elderly and those with underlying health conditions, as well as front-line healthcare workers. The remaining population will be offered the vaccination based on age-related cohorts. You will receive one of a number of different vaccines available. Wherever possible, your second dose should be with the same vaccine as your first dose.

For information on other COVID19 vaccines, see our leaflet for COVID-19 Viral-vector Vaccine (AstraZeneca, Janssen).

Before having COVID-19 mRNA vaccine (Comirnaty®, Spikevax®)

Some vaccines are not suitable for people with certain conditions, and sometimes a vaccine may only be used if extra care is taken. For these reasons, before you are given COVID-19 mRNA vaccine it is important that your vaccinator knows:

  • If you are allergic to any of the ingredients in the vaccine as listed in the manufacturer's information.

  • If you have ever had a severe allergic reaction or breathing problems after any previous vaccination.

  • If you currently have a severe illness with a high temperature (fever).

  • If you are pregnant, trying for a baby or breastfeeding (see below).

  • If you have a weakened immune system. This may be a result of an illness or taking medicines. You should still get vaccinated, but your immune response may not be as high as the general population's.

  • If you have a condition that makes you bleed more than is normal, such as haemophilia, or are taking anticoagulant medicines such as warfarin.

  • If you have recently been given any other vaccine.

COVID-19 vaccination if you are pregnant or breastfeeding

Initial trials of the vaccine did not include pregnant or breastfeeding women. As a result, in the early stages of the UK vaccination programme, it was recommended that women in these categories should discuss their personal risks with a healthcare professional.

However, it is now recommended that women who are pregnant, trying for a baby, or breastfeeding should be vaccinated in line with other people in their age or clinical risk group. This is because:

  • There are now data from over 90,000 women vaccinated against COVID-19 with no safety concerns identified.

  • Pregnant women, particularly in the last trimester of pregnancy, may be at higher risk of complications of COVID-19 infection.

Because most of the women in these groups who have been vaccinated have received Pfizer BioNTech or Moderna vaccines, the UK recommendation is that they should be given one of these vaccines.

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How COVID-19 mRNA vaccine (Comirnaty®, Spikevax®) is given

  • Before you are given this vaccine, ask to read the manufacturer's printed information leaflet from inside the pack. The leaflet will give you more information about the vaccine and a full list of side-effects which you may experience from having it.

  • You will be given the vaccine by a trained vaccinator or healthcare professional.

  • The vaccine is given by injection into a muscle, usually in your upper arm. If you have a condition or take medication that makes you bleed more easily than normal, let your vaccinator know.

  • You will need two doses of the Comirnaty® vaccine given between 3 and 12 weeks apart.

  • You will need two doses of the Spikevax® vaccine, given at least 4 weeks apart.

  • The second dose of the vaccine is important to boost your immunity and prolong its duration. It is not yet known if or when further booster doses may be needed.

  • Before receiving your second dose check to make sure it is the same vaccine as your first dose. If you have any questions, ask your vaccinator for advice.

  • If you have a high temperature (fever) or are acutely unwell at the time of a scheduled vaccination, you are advised to delay having the vaccine. You should arrange for an alternative appointment for the vaccination to be given. A minor illness (such as a cough or cold) will not interfere with the vaccine.

Getting the most from your treatment

  • Try to ensure you attend both appointments for your vaccine doses. If you are unable to attend an appointment, cancel your unwanted appointment and book an alternative.

  • As a safety measure you will be observed for 15 minutes after your dose for any signs of an allergic reaction. Let someone know if you begin to feel unwell during this time.

  • It will take at least seven days after your second dose of Comirnaty® vaccine before you get maximum protection against COVID-19 infection.

  • Spikevax® vaccine may provide around 94% protection from 14 days after your second dose.

  • After you have been vaccinated continue to follow any local restrictions or guidelines to help prevent the spread of COVID-19 infection.

  • Maintain hygiene measures such as routinely washing or sanitising your hands.

  • If you are having an operation or dental treatment, tell the person carrying out the treatment that you are planning to have, or have had, the COVID-19 mRNA vaccine.

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Can COVID-19 mRNA vaccine cause problems?

Along with their health benefits, most vaccines can cause unwanted side-effects although not everyone experiences them. The table below contains some of the most common ones associated with COVID-19 mRNA Vaccine. You will find a full list in the manufacturer's information leaflet supplied at the time of vaccination. The unwanted effects often only last for a few hours or days, but speak with your doctor or pharmacist if any of the following continue or become troublesome.

Very common COVID-19 mRNA vaccine side-effects (these affect more than 1 in 10 people)

What can I do if I experience this?

Pain at the injection site, headache, muscle or joint pain

Drink plenty of water and ask a pharmacist to recommend a suitable painkiller

Tiredness (fatigue), chills or fever

Drink plenty of water and rest if possible. Ask your pharmacist to recommend a suitable painkiller

(Moderna vaccine only) Swelling of the underarm glands on the side you were vaccinated

This should soon pass. If it persists, let your doctor know

(Moderna vaccine only) Feeling sick (nausea) or being sick (vomiting)

Drink plenty of water to replace any fluids lost. Stick to simple or bland foods - avoid rich or spicy foods

Common COVID-19 mRNA vaccine side-effects (these affect fewer than 1 in 10 people)

What can I do if I experience this?

Redness or swelling at the injection site

Ask a pharmacist to recommend a suitable painkiller. Using a cold-pack on the area may help relieve symptoms

(Moderna vaccine only) Rash or hives at the injection site

Ask a pharmacist to recommend a suitable antihistamine. Seek urgent medical attention if a rash develops quickly along with other signs of allergic reaction (see below)

Feeling sick (nausea)

Stick to simple or bland foods - avoid rich or spicy foods

Important: be aware of signs of serious allergic reaction after you have been vaccinated. Seek medical attention if you experience the following:

  • Wheezing or breathing difficulties.

  • Swelling of the face, lips or tongue.

  • Hives or rash.

  • A fast or racing heartbeat.

  • Feeling faint or light-headed.

  • Feeling or being sick (nausea or vomiting) or stomach pain

Clinical author's note Michael Stewart 22/7/21: the MHRA has reviewed reports of extremely rare cases of inflammation of the heart muscle (myocarditis) and surrounding tissues (pericarditis) after vaccination with Pfizer BioNTech and Moderna vaccines. Cases were mild and and occurred most frequently in young men following their second vaccination. Individuals recovered quickly following standard treatment and rest. Seek medical attention if you experience any chest pain, shortness of breath, palpitations or an irregular heartbeat after your vaccination.

Some people have reported a sudden feeling of cold with shivering/shaking accompanied by a rise in body temperature, possibly with sweating, headache, muscle aches and feeling sick (nausea) or unwell, starting within a day of having the vaccine and usually lasting for a day or two.

If your fever is high and lasts longer than three days, or you have other persistent symptoms, speak with your doctor or a pharmacist.

Use of the vaccine will be closely monitored to allow quick identification of new safety information. If you develop any side-effects, you can help this important process by reporting these to the dedicated COVID-19 vaccine monitoring scheme.

How to store COVID-19 mRNA vaccine

  • You will not be asked to store this vaccine. The vaccination clinic must store the vaccine according to strict guidelines.

  • Undiluted Comirnaty® vaccine can be stored in a monitored medicines refrigerator for up to 31 days. Once diluted the vaccine must be used within six hours. The time of dilution should be written on the vial.

  • Unopened Spikevax® vaccine can be stored in a monitored medicines refrigerator for up to 30 days. Once pierced, the vial can be stored for up to 12 hours at room temperature.

Important information about all medicines

Important information about all medicines

If you have any questions about this vaccine ask a pharmacist or your vaccinator.

Report side effects to a medicine or vaccine

If you experience side effects, you can report them online through the Yellow Card website.

Further reading and references

Article history

The information on this page is written and peer reviewed by qualified clinicians.

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