According to Center for Disease Prevention and Control (CDC), vaccines do prevent infection, however, it is not guaranteed, while the preventive protection dies down after a few months. They certainly assist in preventing severe illness/symptoms or even death.
I was vaccinated with two doses of Pfizer, the second one was received approximately 6 months ago. I got infected but only developed mild symptoms, some fever (~38 C), sore throat, a little cough and that's about it.
https://www.cdc.gov/coronavirus/2019-ncov/vaccines/effectiveness/why-measure-effectiveness/breakthrough-cases.htmlThe chart below demonstrates the extent to which vaccines have prevented infection in the UK, and also demonstrates quite clearly how protection wanes over time. As with a lot of Covid charts, there are many complicating factors, and arguably the dimensions on the second chart should be flipped vertically to retain the time trend from the top chart... but it's all quite clear in any case.
It's interesting that people who had a first vaccine more than 6 months ago, but never followed it up with a second vaccine are more likely to contract Covid than someone who is unvaccinated. This is interesting, and I'd like to see more detail. I suspect this might be a very small group, of whom a disproportionate percentage have since moved into the 'too vulnerable to vaccinate' category, and so would be at higher risk regardless of vaccination status... but this is supposition, and I have no data on it.
One thing - perhaps the most important thing - is not covered on these charts, and that's hospitalisation by vaccination status, the data for which isn't available from the ONS. But data from elsewhere, as I've presented previously, have certainly indicated that the risk of severe symptoms is much lower if you've been vaccinated.
Another factor, again supposition, might be that people are more likely to be tested if they are symptomatic. If vaccines protect against symptoms, then they would also reduce the likelihood of you taking a test, in which case the vaccine protection would actually be greater than that presented on those charts.
@Ultegra134 - your second Pfizer jab ~6 months ago, with no third jab, would seem to protect you slightly against infection... but as I say, the protection from severe symptoms is perhaps more important.
https://www.ons.gov.uk/peoplepopulationandcommunity/healthandsocialcare/conditionsanddiseases/articles/coronaviruscovid19/latestinsights