Almost half of hospital patients with Covid are “incidental” cases, which means they are more likely to reflect infection rates in the community without the time lag associated with severe illness.

But there is also concern that the patterns could reflect waning immunity among those who were the first to receive third doses.

The NHS is drawing up plans to offer fourth jabs but the rollout is not expected to start until early April, with most over-75s unlikely to be offered the vaccinations before Easter.

Prof Penny Ward, a visiting professor in pharmaceutical medicine at King’s College London, told the Telegraph: “I was surprised that they didn’t get on with it immediately, to be honest.”

She said Britain should follow other countries such as Israel, which  “swung into action” with its fourth vaccinations.

Scotland has already begun its rollout, with all over-75s being offered jabs 24 weeks after their last booster, and the JCVI is understood to be keen to see England start as soon as is operationally possible.

Prof Ward urged the committee to consider recommending shorter intervals between jabs. It met on Thursday to discuss this but is waiting for further data on vaccine effectiveness, due to be published next week.

The latest data show vaccine efficacy after a Pfizer booster drops from around 90 per cent to 75 per cent at 10 to 14 weeks. The next set of data is expected to show how well this holds up at around five months.

A third of those aged 75 and over have not had a booster in the last five months, figures show.

Earlier this week Meaghan Kall, an epidemiologist at the UK Health Security Agency said waning was her “leading theory” to explain the recent rises in hospital admissions.

Amid debate with fellow scientists, she came to the conclusion that the rise in overall infections was driving high levels of incidental Covid.