Boris Johnson has warned double-vaccinated Britons against potential complacency over Covid, saying that waning immunity over time meant booster jabs were vital for continued protection.The prime minister insisted, however, that there was no need yet for the government’s so-called plan B, which would reimpose restrictions such as mask wearing and home working, despite data showing infections in England at levels last seen at the height of the second wave in January.“We’re watching the numbers every day,” Johnson told reporters before the G20 summit in Rome, which he is attending in the run-up to next week’s Cop26 climate summit in Glasgow.“Yes it’s true that cases are high, but they do not currently constitute any reason to go to plan B. I think it’s agreed among absolutely everybody, apart from possibly the Labour party. So we’re sticking with the plan.”Figures published by the Office for National Statistics on Friday estimated that around one in 50 people in England had Covid in the week ending 22 October, the highest level since late December and early January.Johnson said booster vaccinations, currently offered to people aged 50 or over and those with clinical vulnerabilities who had their second jab at least six months ago, were vital to avoiding a difficult winter.“I think rather than thinking new restrictions the best thing everybody can do is get that booster jab as soon as you’re offered it. It’s a very important message,” he said.“I think people don’t quite realise that the first two jabs do start to wane. There is a waning effect on the first two. So people who are double-vaccinated can experience a waning effect in their immunity.“How sad, how tragic it would be if people who had other complications, other compromises in their health, got seriously ill because they were overconfident about their level of immunity and didn’t get their booster when they needed it.”Asked if he could guarantee a good Christmas, Johnson replied: “I see no evidence whatever to think that any kind of lockdown is on the cards.”In a separate announcement on Friday, ministers said care home residents and some vulnerable people would be able to get their boosters a month before the usual six-month minimum.For people who are about to receive immunosuppressive treatment that would hinder their immune system, the wait for a booster will be cut even further, to four months.