Covid booster vaccines: what you need to know
Get the latest Covid-19 booster updates, from what the side effects are, to how long boosters take to be effective.
Updated 18 April 2023
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Top questions
Covid-19 booster side effects
Top questions
Who is being offered a Covid-19 booster?
The following people are being offered a seasonal Covid-19 booster:
- adults aged 75 years and over
- residents in a care home for older adults
- individuals aged five years and over who are immunosuppressed.
If you're eligible for a booster dose you should be contacted by the NHS.
How long does the booster last, and how effective is it?
The UK Health Security Agency (UKHSA) published research in February 2023 which suggests that three to five months after a monovalent booster, most people will have around 65% protection against serious illness from Covid-19, and one year later protection was still at around 52%. These data were collected in July 2022, when the Omicron BA.5 variant was the dominant strain.
The UKHSA research also looked at the effectiveness of the autumn 2022 bivalent booster. Having this booster meant that at ten weeks, people aged 50 years and older were half as likely to need a hospital visit due to Covid-19 compared to those who didn’t have it.
Additionally, an Israeli study published in The Lancet in April 2023 found that people over 65 who had the bivalent booster were 72% less likely to need a hospital visit and 68% less likely to die from Covid-19, compared to people over 65 who did not receive a bivalent booster.
How long does it take for the booster to be effective?
Studies have shown very high protection begins one or two weeks after receiving the booster jab.
A trial by Pfizer suggests that from seven days onwards, a booster dose of Pfizer is extremely effective at preventing illness from Covid-19.
I am immunosuppressed or immunocompromised, can I have a booster?
The Joint Committee on Vaccination and Immunisation (JCVI) has recommended that everyone over five with a severely weakened immune system has a booster.
This includes people whose immune systems do not work properly or who are taking immune suppressant medication, including people who have had a heart transplant or who are taking immune suppressant medication such as steroids.
What is a dual vaccine, or Omicron booster?
Updated versions of the Pfizer and Moderna vaccines have been approved for use in the UK.
These updated vaccines are 'bivalent' vaccines, which means they offer protection against both the original coronavirus, as well as the Omicron variant BA.1. You might also see them referred to as a ‘dual vaccine’, ‘dual variant vaccine’ or ‘Omicron booster’.
Moderna's research found that trial participants for its bivalent vaccine had antibody levels against Omicron subvariants (mutations of the Omicron BA.1 variant) that were 1.7 times higher than those who received the original booster.
It's unclear at this point what this increase in antibodies means in terms of protection against the virus. The Joint Committee on Vaccination and Immunisation (JCVI)’s guidance is that getting any Covid jab is more important than receiving a particular vaccine.
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Booster side effects
What are the side effects of the booster, and how long do they last?
Side effects of the booster are likely to be mild and short-lived. They often include things like pain or tenderness where you had the injection, fatigue, and headache.
- For more information about the side effects of Covid-19 vaccines, please visit the NHS website.
What are the side effects of the Moderna booster?
Side effects for the Moderna vaccine when used as a booster include pain or tenderness where you had the injection, swelling of the lymph nodes in the arm where you had the injection, tiredness, headache, muscle or joint pain, chills, nausea, vomiting, fever, or rash.
These side effects are similar whether you have the Moderna bivalent vaccine (also known as Omicron booster) or the original Moderna vaccine.
If you do experience side effects, these are likely to be mild and last no more than a few days.
What are the side effects of the Pfizer booster?
Symptoms for the Pfizer booster may include pain at the injection site, fatigue, headache, muscle and joint pain, fever and chills. The side effects may be milder after a booster than after previous doses.
Research from the UK COV-Boost trial suggests that Pfizer as a booster could have fewer side effects than Moderna, although there were some differences between different groups (see what are the side effects of the Moderna booster?)
Is myocarditis more common after the booster vaccine?
Myocarditis and pericarditis are very rare side effects of the Pfizer and Moderna vaccines.
Data published by the UK Health Security Agency (UKHSA) shows that myocarditis and pericarditis are less common after a booster jab, than after the first two doses of the vaccine, and this risk is already very small.
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