And if none of them have any visible symptoms?
At least one of the six (or more) had symptoms or they wouldn’t have started testing in the first place. There are likely many people who have been vaccinated who get the virus, but unless they have symptoms and unless they then get tested, they would never know. Two of my health care providers told me they tested positive several months after having gotten the vaccine. They had no symptoms, and would not have known but for the fact that their job requires them to get weekly testing. But even though they had no symptoms, they had to isolate from their unvaccinated children.