The EU has confirmed that, from 1st February 2022, a coronavirus vaccination certificate will not be valid if it is more than nine months since the second dose and no booster has been received.
The European Centre for Disease Prevention and Control recommends a booster jab after six months. The nine month deadline has been chosen to give some flexibility to states which are struggling with booster take-up.
Your NHS vaccination certificate in the NHS app will not register as valid in the EU if you are travelling after 1st February, had your second vaccination more than nine months ago and have not received a booster shot.
This does not mean that you will be refused entry to an EU country – each can set their own rules – but you would be refused entry to restaurants, museums and any other facility which required you to prove your status.
You can see details on the EU website here.