The chance of testing positive for Covid after being double jabbed rises by nearly 60 per cent in the months following vaccination, with experts saying boosters are needed to keep immunity high.

The latest data from Imperial College’s React survey, which monitors community infection levels, found prevalence among fully vaccinated people in the three months after their second jab was around one in 285 – 0.35 per cent.

However, between three and six months it rose to one in 181, a 57 per cent increase. The risk is still far less than for unvaccinated people, among  1.76 per cent, or one in 56, were found to have the virus.

Prof Paul Elliott, the director of the React programme, said: "The positivity rate for two doses within six months is higher than that for less than three months.

"So there’s some suggestion that maybe there is a higher rate of breakthrough infection after three months and we can’t really say what happens after that. It definitely shows incentive for people to get their booster when it becomes available."

Booster appetite

The study also showed that overall Covid prevalence increased in September to 0.83 per cent, or one in 120. The previous survey by Imperial in July found that one in 169 people tested positive for the virus.

Combined vaccine effectiveness against any infection was estimated to be 62.8 per cent and 66.4 per cent for those with symptoms. The study also showed a higher prevalence in school aged children and households with children.

Sajid Javid, the Health Secretary, said: "The phenomenal progress of our vaccination programme has built a strong wall of defence across the country, allowing us to live safely with this virus.

"These findings show how important it is for young people to get the jab to protect them from Covid-19, and for those eligible to get their booster vaccine to prolong their existing protection. I urge anyone who needs one to get a jab as soon as possible – it’s vital to keep you and your family safe this winter."