* SAGE advises government to work to prepare new measures
* ONS Survey: COVID prevalence highest since January
* Delta subvariant designated as under investigation (Recasts with PM Johnson's remarks)
By Alistair Smout and James Davey
LONDON, Oct 22 (Reuters) - British Prime Minister Boris Johnson said on Friday a new COVID-19 lockdown is not on the cards, as advisers warned that acting early with lighter measures to cut rising case numbers would reduce the need for tougher restrictions later.
Johnson's government has said there is no need yet to introduce a "Plan B" involving mask mandates, work-from-home orders and vaccine passports, though such measures could be introduced if more people do not come forward for COVID-19 vaccines and booster shots.
Johnson scrapped England's COVID restrictions in the summer, and he said that, while case numbers were rising, the trend was in line with what was expected.
He said earlier that the widespread deployment of vaccines this year means that the link between cases and deaths has been disrupted, in contrast with earlier stages of the pandemic.
Asked on Friday about the possibility of another lockdown over winter, the Conservative prime minister said: "I've got to tell you at the moment that we see absolutely nothing to indicate that that is on the cards at all."
While Johnson has scrapped the legal requirement to wear masks, he said they should still be worn in confined spaces, especially when meeting strangers.
COVID-19 infections in Britain are up 17.9% over the last seven days, with 52,009 reported on Thursday, and the reproduction "R" number is estimated between 1.0 and 1.2. Any figure above one suggests an exponential growth in cases.
"In the event of increasing case rates, earlier intervention would reduce the need for more stringent, disruptive, and longer-lasting measures," the Scientific Advisory Group for Emergencies (SAGE) said in minutes from a Oct. 14 meeting, published on Friday.
"SAGE advises that policy work on the potential reintroduction of measures should be undertaken now so that it can be ready for rapid deployment if required."
SAGE added that among Plan B measures, reintroduction of work-from-home guidance would have the biggest impact to cut transmission, and warned that "presenteeism" might become an increasing reason for contagion in the workplace.
The Office for National Statistics (ONS) on Friday found that COVID-19 prevalence had risen again. One of the areas with highest prevalence was southwest England, which was impacted by false negative test results from a lab that was suspended last week.
The ONS said COVID prevalence was at its highest level since January, when England had just entered a third national lockdown, although deaths are much lower.
The current rise in cases has been fuelled by high levels of infections in secondary schools, where 7.8% of children were infected in the latest week, according to ONS statistics.
Though young people are less likely to fall seriously ill, ONS figures show the rise in cases there is now feeding through into older age groups.
Johnson has said that the government will rely on vaccines rather than lockdowns to navigate a difficult winter, and SAGE said that things were unlikely to be as bad as last winter.
"Scenarios modelled for the coming winter and into 2022 suggest COVID-19 hospital admissions above the level seen in January 2021 are increasingly unlikely, but there are uncertainties around behaviour change and waning immunity," the minutes said.
The UK Health Security Agency (UKHSA) said it was investigating a Delta subvariant of the coronavirus called AY.4.2 as it was possibly more transmissible than Delta, though there was no evidence that it caused more severe disease or rendered vaccines ineffective.
"It is not unexpected that new variants will continue to arise as the pandemic goes on, particularly while the case rate remains high," UKHSA chief executive Jenny Harries said.
"However, it should serve as objective evidence that this pandemic is not over." (Reporting by Alistair Smout and James Davey with additional reporting by Paul Sandle and William James; editing by Mark Heinrich)