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COVID-19 vaccination: A Guide for people with a severely weakened immune system

This leaflet explains why the third primary dose of COVID-19 vaccination is needed for people who have a severely weakened immune system. 

Published 16 September 2021


― Why do some people with a severely weakened immune system need a third dose?
Who is considered to have a severely weakened immune system?
― How many doses will I need?
― When will the third dose of the COVID-19 vaccine be given?
― How will I get my vaccination?
― Who shouldn’t have the third vaccination?
― Will I experience any side effects?
― What if I miss a vaccination?
― More information


Why do some people with a severely weakened immune system need a third dose of the COVID-19 vaccine? 

COVID-19 is a very infectious respiratory disease which can cause serious illness in people who have a severely weakened immune system (severe immunosuppression). If you are an individual who has a severely weakened immune system due to an underlying health condition or medical treatment you may not have made a good immune response to the first two doses of COVID-19 vaccination. You are being offered a third primary dose of the vaccine to improve your levels of immunity to give you better protection.  


Who is considered to have a severely weakened immune system?  

This third dose of vaccine is being offered to individuals aged 12 years and over who had severe immunosuppression at the time of their first or second dose of COVID-19 vaccination. This includes those who had or have: 

  • blood cancers (such as leukaemia or lymphoma) 

  • lowered immunity due to treatment (such as high dose steroid medication, biological therapy, chemotherapy, radiotherapy) 

  • lowered immunity due to inherited disorders of the immune system 

  • an organ or bone marrow transplant 

  • diseases  and treatments that affect the immune system (such as poorly controlled HIV) 

If you are unsure if you require a third dose of the vaccine, speak to the specialist involved in your care.  


How many doses will I need? 

At this time you will need this 1 extra dose to help improve your protection. Like your previous doses, the vaccine will be given in your upper arm. 

A booster vaccination may also be needed at a later date. 


When will the third dose of the COVID-19 vaccine be given?  

The third dose should be given at least eight weeks after the second dose, but timing will depend on any treatment you may be having.  

Where possible, the third dose should be delayed until at least two weeks after the period of immunosuppression or if you are receiving a series of treatments at a point between treatments that your immune system is strongest. The timing of your third dose can be discussed with your specialist.  


How will I get my vaccination?  

The NHS will be in contact with you to let you know when and where to have the vaccine.  


Who shouldn’t have the third vaccination? 

If you’ve had a very severe reaction to the previous dose of the vaccine, you should discuss this with your doctor. 


Will I experience any side effects? 

Very common side effects include: 

  • having a painful, heavy feeling and tenderness in the arm where you had your injection for several days after the vaccine 

  • feeling tired 

  • headache 

  • general aches, or mild flu like symptoms 

A mild fever may occur for two to three days but a high temperature is unusual and may indicate you have COVID-19 or another infection. You can take the normal dose of paracetamol (follow the advice in the packaging) and rest to help you feel better. Do not exceed the normal dose. A less common side effect is swelling of the local glands in the neck and armpit, this starts a few days after the vaccine and may last for up to two weeks. 

These symptoms normally last less than a week. If your symptoms seem to get worse or if you are concerned, call NHS 111 Wales on 111 or your GP surgery. If 111 is not available in your area, call 0845 46 47. Calls to NHS 111 Wales are free from landlines and mobiles. Calls to 0845 46 47 cost 2p per minute plus your telephone provider’s usual access charge.  

You can report any side effects online at: or via the Yellow Card app. 

Further information and patient leaflets can be found at:   


What if I miss a vaccination? 

If you have not received a COVID-19 vaccine yet or have missed your second dose you should have them as soon as possible. 

You will still need the third dose but the timing of it will depend on any treatment you may be having. You can discuss this with your specialist. 


More information  

You can find out more information about COVID-19 vaccines, including their contents and possible side effects at  and