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.


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 by 11 October, get in touch with your GP for advice.
  • 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: People over 50, over 16s with health conditions that put them at higher risk of severe COVID-19 (previously known as ‘clinically extremely vulnerable’), health and social care workers, adult carers, adults who live with someone with a weakened immune system, and homeless people.
  • When you can have it: 6 months or more after your second dose.
  • 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 will let you know when it’s your turn to have a booster. After you’ve been contacted, you might be asked to book an appointment at a local vaccination service or through an online portal. Online booking is not yet open for most people.
  • 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 any booster you might be eligible for in the future.

30 September 2021

Updated 8 October 2021