COVID-19 vaccination: third dose or booster?

What is the difference between a third primary dose and a booster?

Person giving injection

We’ve been hearing from a lot of people who are not sure whether they should have a third primary dose or a booster dose of the COVID-19 vaccine. Here we summarise the main differences between the two. People who have had a third primary dose are also eligible for a booster (fourth dose) no sooner than 3 months after their third dose.


Third primary dose

  • Aim of the jab: To improve the immune response to vaccination in people who might not have responded well to their first two doses.
  • Who can have it: People who had a severely weakened immune system at the time of their first two doses. This includes many people with lymphoma. The JCVI guidance has full details on who is eligible.
  • When you can have it: 8 weeks or more after your second dose. If possible, it should also be at least 2 weeks after your immune system has recovered, or when it is least weakened. Your specialist can advise you on the best time to have it.
  • What you have: A full dose of the Pfizer/BioNTech vaccine (Comirnaty®) or the Moderna vaccine (Spikevax®). Some people might have a full dose of the Oxford/AstraZeneca vaccine (Vaxzevria®) instead, depending on their individual circumstances.
  • How to get it: The NHS will contact you to let you know when and where you can have your dose. If you think you are eligible for a third primary dose but you have not been contacted yet, get speak to your GP or specialist.
    • In England, people aged 18 or over who have a letter from their GP or hospital consultant confirming that they are eligible can book a third dose online. People aged 12 or over can get their third dose at a walk-in vaccination site if they have a letter confirming they are eligible and it has been 8 weeks or more since their second dose.
    • In Northern Ireland, people aged 12 or over who have a letter from their GP or hospital consultant confirming that they are eligible can book a third dose online.
    • Eligible people in Scotland and Wales should be contacted by the NHS to invite them for a third primary dose.
  • More information: The government has produced a guide for people who are eligible for a third primary vaccine dose. You might also find it helpful to read the letter the NHS sent out to GPs and vaccination centres in England, telling them about the third dose.

Booster dose

  • Aim of jab: To make the immune response to vaccination last as long as possible in people who are likely to have responded to their first two doses.
  • Who can have it: All over 16s, and over 12s who are either at higher risk of becoming seriously ill from COVID-19 (previously known as ‘clinically extremely vulnerable’) or live with someone who is.
  • When you can have it: 3 months or more after your second dose (or third primary dose, if you were eligible to have one).
  • What you have: A full dose of the Pfizer/BioNTech vaccine (Comirnaty®) or a half-dose of the Moderna vaccine (Spikevax®). If you can’t have these, your medical team might consider offering you a full dose of the Oxford/AstraZeneca vaccine (Vaxzevria®) instead.
  • How to get it: The NHS should let you know when it’s your turn to have a booster.
    • In England, you can pre-book your booster online or by calling 119 if it is more than 2 months since your second vaccination. This will give you the opportunity to book an appointment that is at least 3 months after your second dose. You can also get a booster at some walk-in vaccination clinics. If you have had a third primary dose, your GP or hospital specialist should invite you for your booster (fourth dose) when it's due, or you can get it at a walk-in vaccination site if you have a letter confirming you were eligible for the third primary dose.
    • In Northern Ireland, you can book a booster appointment online, at a Health Trust vaccination clinic, a participating pharmacy or wait for your GP to contact you. Over 18s can get a booster dose at walk-in vaccination hubs. People who have had a third primary dose should be contacted with an invitation for a booster (fourth dose).
    • In Scotland, you can book a booster appointment online if it has been more than 3 months since your second dose, or call 0800 030 8013. Drop-in booster doses are available in some areas. If you had your third primary dose at least 3 months ago, your health board should invite you for a booster (fourth dose).
    • In Wales, your health board will contact you to arrange your booster appointment. If you think you are eligible and you haven't been contacted yet, get in touch with your local health board.
  • More information: The government has produced a guide to booster vaccination.

What if I’m offered a booster instead of a third dose?

Many people are having difficulty accessing a third primary dose of COVID-19 vaccine. If you are eligible for a third dose but you are invited for a booster instead, a consensus of UK lymphoma specialists recommends that you have the booster dose. This helps you get additional protection against COVID-19 as soon as possible.
If you book a booster dose, try to find out which vaccine you are having.

  • Most boosters will be the Pfizer/BioNTech vaccine (Comirnaty®). The booster dose for this vaccine is exactly the same as the third primary dose.
  • Some centres might be offering the Moderna vaccine (Spikevax®). The booster for this vaccine is a half dose but the third primary dose is a full dose. If possible, try to arrange your appointment to a clinic that is offering the Pfizer/BioNTech vaccine instead.
  • The Oxford/AstraZeneca vaccine (Vaxzevria®) is not routinely being offered as a booster dose. When it is used, it is exactly the same as the third primary dose.

Your dose will be recorded as a booster dose initially, while the NHS system is being updated. Your record should be updated at a later date to show that you had a third primary dose. This is important so you can be invited for your booster 3 months later.

Published 30 September 2021

Updated 1 November 2021

Updated 30 November 2021

Updated 8 December 2021