I don't know if you're familiar with it, but I thought I'd introduce the phrase "pleasantly surprised" today.
It means that you are expecting that something will not be good, but you are surprised to find that it is good, or better than you had expected.
For example, one of my friends is a nurse. She has received one dose of the COVID-19 vaccine. She will get her second dose soon and is worried because she has heard it has worse side effects than the first dose. I also have a student who is a doctor. She has had both doses and said the second dose was not as bad as the first (she felt no side effects). Should I tell my friend the nurse not to worry? Maybe not. If she expects that it will be bad, but it is not bad, she will be pleasantly surprised.
Other examples of when you might be pleasantly surprised:
You go to restaurant you don't know. It doesn't look very special, but the food is actually good. You are pleasantly surprised.
You watch a movie that was not well-rated, but it's better than you had thought it would be. You are pleasantly surprised.
When you are pleasantly surprised, it probably means you had low expectations. This can be nice because you are not disappointed.
dose = the amount of medicine you take at one time, in this case, one injection of the vaccine
side effects = some (often unpleasant) effects of a medicine, such as headache, fatigue, etc.
well-rated = rated as good; people say it is good
low expectations = not expecting much
Hope that was useful for you.