He wasn't "fully vaccinated". He got a shot, but clearly it wasn't a vaccine.
"Fully vaccinated" is nothing more than marketing and propaganda term. One is either vaccinated or they are not, just like being pregnant.
If you have had a full series of shots for one of those diseases and then get a full blast of viruses from and infected person, how does the body respond?
Do the virus start reproducing but then your antibodies kick in and you shake it off?
Do you get any symptoms?
Are you asymptomaticly contagious?
What's the likelihood that you do progress to being symptomatic despite being vaccinated?
I would be willing to bet that the old "working" vaccines aren't nearly as perfect as advertised and they mainly work by breaking the pool of infectious people around, so few are infected which means you have few chances to catch it and spread it. Also, no one is going around testing everyone to see if you have been exposed and fighting against the disease, unlike the widespread testing for COVID.