“For most people having a full course of vaccination and a booster offers the most protection against severe illness, however a small number of people do not have a full response to the vaccine.
“I’r rhan fwyaf o bobl, cael cwrs llawn o’r brechlyn, ac yna ddos atgyfnerthu, sy’n rhoi’r amddiffyniad mwyaf effeithiol yn erbyn salwch difrifol.