With a bumper crop of flu vaccine expected this year and the clock ticking to get it into people before influenza strikes, disease fighters are no longer content to rely on doctors to dispense shots in their offices. Caritas Norwood is believed to be the first hospital in Massachusetts to implement a drive-through flu-shot clinic, joining medical centers and health departments stretching from Seattle to rural Virginia. Last year was the first flu season that drive-through vaccinations were offered at the hospital.