Vaccination is the noun for the act of giving a vaccine, and it works two ways. It can mean the whole practice — "vaccination has almost ended polio" — and there it takes no a and no plural. Or it can mean one particular time someone gets one — "I have a vaccination on Tuesday", "the baby needs three vaccinations this year". Both are common; the sentence tells you which. It is the everyday word in health writing, official notices, and school letters, a step more formal than shot or jab. Keep it apart from its neighbours. The vaccine is the substance itself, the liquid in the bottle. The vaccination is the event of giving it. So you can say the vaccine works but vaccination rates are low, and those are two different facts. Immunisation is close in meaning and often swapped in, though strictly it points at the result — the protection — rather than the act. The word carries a small piece of history in it. It comes from the Latin word for cow, because in 1798 an English doctor found that giving people cowpox, a mild illness from cows, protected them from smallpox, which killed. The name of that cow disease became the name for the whole idea.