If someone is vaccinated against the measles virus, they likely won’t get measles.
If someone is vaccinated against SARS-CoV-2, there’s still a good chance they’ll get COVID-19-maybe multiple times.
A quick glance at the above example and one might think the measles vaccine “works,” and the COVID-19 vaccine doesn’t-with “works” implying complete prevention of disease. But what does it mean for a vaccine to be effective? That depends on the virus.
And how much it’s spreading.
And the type of vaccine.
And the immune response stimulated by the vaccine.
In short, it’s complicated.