Jeers the first casualty wards giving extra vaccinations (8)

'extra vaccinations' is the definition.
Although both the answer and definition are plural nouns, I can't see how one could define the other.

'jeers the first casualty wards' is the wordplay.
'jeers' becomes 'boos' (I've seen this in another clue).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'first casualty wards' becomes 'ers' (I can't explain this - if you can you should give a lot more credence to this answer).
'boos'+'t'+'ers'='BOOSTERS'

'giving' is the link.

Can you help me to learn more?