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Cancer and COVID-19 vaccination : 3rd dose and booster explained.

Sep 15, 2021

Third dose of vaccine being offered to those who were severely immunocompromised at the time of their first or second COVID vaccination dose.

In brief :

  • Third dose – is a 3rd COVID vaccination, given no sooner than 8 weeks after the 2nd COVID vaccination, for those who are severely immunocompromised or were severely immunocompromised at the time of their initial vaccination (doses 1 and/or 2).
  • Booster – is just that, a booster or ‘top-up’ vaccination, to be given at a specific point in time after completion of initial vaccination.

Third dose

The UK’s Joint Committee on Vaccination and Immunisation (JCVI) has recommended that people with severely weakened immune systems, including some (but not all) people with cancer, should have a third vaccine dose as part of their primary vaccination schedule against covid-19.

The third dose of either the Moderna or the Pfizer-BioNTech vaccine should be offered to people over age 12 who were severely immunosuppressed at the time of their first or second dose, including those with leukaemia, advanced HIV, or recent organ transplants. 

For 12-17 year olds the Pfizer-BioNTech vaccine is preferred.”

Full announcement here

The committee has based its new advice on preliminary data from the Octave trial, which has published preliminary data today showing that most people who are immunocompromised or immunosuppressed do generate antibodies following vaccination, with 60% generating a strong antibody response (following two doses of a vaccine).

However, that leaves 40% with a low, or undetectable, response – the actual level of antibody response varies within the groups studied : 1in 10 being those with solid organ cancers (the figure is higher in blood cancers).

“A significant number of people in the UK were advised to shield because they have conditions or long term illnesses which place them at greater risk of severe illness and death from COVID-19.

The rapid development of vaccines for COVID-19 has been a major step forward in the battle against this global pandemic, and the most clinically at-risk people were among the first in the UK to be offered one. However, while we know COVID-19 vaccines are highly effective in healthy individuals, questions have remained as to how effective they are in protecting the chronically ill.

These preliminary results of OCTAVE and the results of our continuing and forthcoming research will be instrumental in helping inform how best to vaccinate patients with chronic conditions and protect them from COVID-19 infection in the future.” 

Professor Pam Kearns – University of Birmingham UK

The third dose is to help protect those with severely weakened immune systems from the virus, to bring response levels up nearer to those achieved in other groups.

The third dose will not be offered to everyone on the original list of clinically extremely vulnerable people – it will be offered  to those over 12 years old who were severely immunosuppressed when they had their first or second vaccine. These include people with cancer who have had immunosuppressive chemotherapy or radiotherapy in the last six months.

The third dose – should be administered at least eight weeks after the second and at a time when patients are least likely to be immunosuppressed. The JCVI have suggested that the individual’s oncologist or specialist team is best placed to decide on the timing of this for each person.

The NHS will contact people as soon as possible to discuss their needs and arrange an appointment for a third dose.


This is not the start of the booster programme which is due to get underway this month. 

A booster dose is one that is given much further down the line, separately from completion of initial vaccination, to extend the duration of protection that has been given.

Completion of initial COVID vaccination is following 2 vaccinations for most people – and following 3 vaccinations for those mentioned above.