Locking down entire streets could be an important way of keeping outbreaks of new Covid variants under control, an expert has suggested. Dr Jeffrey Barrett, director of the Covid-19 Genomics Initiative at the Wellcome Sanger Institute, said using interventions to minimise asymptomatic transmission could be crucial. Speaking on BBC Radio 4’s Today programme, Dr Barrett said it was important to deploy “the most effective measures possible” to contain these outbreaks. When questioned if shutting down entire streets would have a medical impact on clusters of cases, he responded: “Well it certainly could, because one of the trickiest parts of this virus overall is, of course, some individuals who are infected don't have symptoms and so they can transmit. “Trying to use interventions that might stop asymptomatic transmission may well be an important part of keeping outbreaks of these new variants to be as absolutely small as possible." He added there could be a “chance” new variants will be “less well neutralised” by vaccines, so “it's really important to be able to try to keep that number as close to zero as possible”. Restrictions have so far kept the number of new variants “very small”, he said, adding: “And as the restrictions are lifted the key thing to watch will be, does that number ever go up sort of week by week, and if so it's really important to deploy the most effective measures possible to contain those outbreaks." One effective measure to contain outbreaks is deploying surge testing, he added. Surge testing has now been expanded in south London boroughs after cases of the South African variant, B.1.351, were discovered. Three boroughs have set up additional testing facilities to process thousands of residents who are eligible for tests. More than half a million adults living in south London boroughs have been offered tests, including 264,000 in Lambeth, 265,000 in Wandsworth, and 14,800 in the Rotherhithe ward of Southwark. A case of the variant was also detected in Barnet, north London, and home testing kits were delivered door-to-door in the N3 postcode on Thursday. The Department of Health and Social Care (DHSC) confirmed the case in Barnet was unrelated to other clusters, but it had been isolated and the person's contacts traced. The extra testing comes as new analysis revealed that Covid-19 rates dropped below 100 cases per 100,000 people in all local areas of the UK for the first time since September. A total of 19,196 people tested positive for Covid-19 in England at least once in the week to April 7, according to the latest Test and Trace figures. This is down 34 per cent on the previous week and is the lowest number since the week to September 2, 2020.