It has been around six months since the oldest and most vulnerable members of society last received a jab, as the booster campaign started in mid-September 2021.
But waning protection is one of the reasons thought to be behind the recent uptick in infections, hospitalisations and deaths, along with the abandonment of all Covid restrictions and BA.2, the slightly more transmissible form of omicron, which is now dominant.
Data from the Office for National Statistics show that more over-70s have the virus right now (one in 23) than at any other point during the pandemic.
One in 20 people in England, around 2.6 million people, are currently infected, the ONS says. The figure is one in 25 for Wales and one in 14 in both Scotland and Northern Ireland.
Commenting on the high level of infections, Professor James Naismith of Oxford University said: “For the UK, my main concern is for the vulnerable, for whom this disease is serious, and for those whose lives will be blighted by long Covid.