Can vaccinated individuals still carry/spread disease? Say they come into contact with someone who has measles. Even if the individual is vaccinated and doesn't get sick, is it still possible to spread it to other unvaccinated people?
There are 2 basic ways diseases are spread: direct contact and fomites. Direct contact would be like a person sneezing in your face or a cut coming in contact with someone else’s body fluids. Fomites are objects that a virus or bacteria will live on for a certain period of time (anywhere from minutes to days) that when we touch them we pick up the big and then ingest it and become infected. Fomites are why you should clean your cell phone (I see you texting on the toilet) and doorknobs.
If a person has been vaccinated against a disease and has an adequate immune response to the vaccine, the chance of them carrying the disease (aka being infected but not showing signs of illness) and spreading it to someone directly is extremely low. Now a vaccinated person CAN act as a fomite though. So say a vaccinated person shakes the snotty hand (not usually visibly snotty- microscopically) of a person with a virus and then shakes another person’s hand. The vaccinated person can pass that virus on from the sick person if the well person, say, eats an apple with that snotty hand.
There are many diseases that can be spread by people who aren’t showing any symptoms. For example, the flu is still contagious for several days after symptoms have resolved. People pass on HIV and hepatitis all the time without knowing they’re infected. Some bacterial infections—salmonella and typhoid come to mind—are also still shed by the carrier long after they get well. Remember Typhoid Mary?