COVID booster jab

You would need to register at your local health centre (Centro de Salud).  However, if you´re not a resident and don´t have residencia, an NIE number, and a Spanish mobile phone number, I´m not sure if you can.  Take your passport along, it´s worth a try.  An alternative is The Health Centre in Arrecife - Centro de Salud Valterra, Calle Pérez Galdós, s/n, 35500 Arrecife, Palmas, Spain.  They are doing ´walk in´ boosters on Saturdays and Sundays 8am to 9pm until 19th December (limited to 726 doses per day) so you could try there.  However, they do check that you´re on their system first.  Hope this helps.