They help prevent the spread of infectious diseases like measles, mumps, and flu, and protect those who can’t be vaccinated, such as very young children or people with weakened immune systems.
Maen nhw'n helpu i atal lledaeniad clefydau heintus fel y frech goch, clwy'r pennau, a'r ffliw, ac yn amddiffyn y rhai na ellir eu brechu, fel plant ifanc iawn neu bobl â systemau imiwnedd gwan.