Protection against hospitalisations remains at around 90% for people aged 65 and over even three months after boosting, according to data from the UK Health Security Agency.  So, UK experts consider a fourth Covid jab unnecessary, yet. This happens with severe disease. With mild infection it is shorter, dropping around 30% by about 90 days.  The agency also said that with two vaccine doses, protection against serious illness decreases to 70% after three months, and to 50% after six. Hence the importance of the booster shot.  So far, over 35 million boosters have been administered in the UK.