A vaccine is a biological preparation that improves immunity to a particular disease.
immunity [i'mju:nəti] ：免疫力
Traditional vaccines contain microbes that have been killed or weakened so that they don't cause disease.
When an individual is vaccinated against a disease or an infection his or her immune system is prepared to fight the infection.
When a person is inoculated with these preparations, the immune system confronts these harmless versions of the germs. The immune system quickly clears them from the body.
In turn the body remembers the germs so that later in life when it encounters the real live virulent germs it may be able to fight it off with the retained memory against the particular germ.
Before vaccines, the only way to become immune to a disease was to actually get it and, with luck, survive it.
Group one: residents are required to be vaccinated and they are free of charge.
Group two: the vaccines are not free and residents can choose whether they want to be inoculated.