Coronavirus outbreaks are up to 20 times more likely in large care homes, according to a major study seen by the Guardian, prompting calls to divide them into “bubbles” before any second wave hits. In research that will increase scrutiny of private care chains, which often operate the largest facilities, NHS Lothian and Edinburgh University found the likelihood of infection getting into a home tripled with every additional 20 beds. It comes amid pressure on ministers to draw up stricter guidelines to prevent further outbreaks in care homes. Of more than 55,500 people to have died with Covid-19 in the UK according to their death certificates, nearly 40% (21,600) were care home residents, despite ministers’ claims to have “thrown a protective ring” around them. Last week Boris Johnson came under fire for accusing many homes of failing to follow proper procedures during the pandemic. Care leaders have condemned the government for not providing clear procedures. At the height of the crisis, 25,000 people were discharged from hospitals into care homes without being tested for coronavirus. In the newly published study, researchers examined infection patterns at 189 care homes in the NHS Lothian area where a total of more than 400 people died from coronavirus. In homes with fewer than 20 residents, the chance of an outbreak was 5%, but in homes with 60 to 80 residents the likelihood soared to between 83% and 100%. The study is believed to be the widest analysis yet of coronavirus care home test results from anywhere in the UK. High footfall of staff – including agency workers, cooks and maintenance engineers, going in and out of the largest homes – is thought to be a key factor for infecting elderly residents. The study led to calls for operators to establish infection control “bubbles” in big facilities before any second wave emerges, but ministers have been warned it will be costly. “More footfall will give you more risk of infection,” said the report’s author, Prof Bruce Guthrie, the director of the Advanced Care Research Centre at Edinburgh University and associate researcher at Health Data Research UK. “Although care home size cannot be altered without losing places for existing residents, there may be potential to create discrete units within care homes where smaller numbers of staff and residents are effectively cohorted to create self-contained units.” While the average UK care home has around 36 beds, HC-One, the largest provider of private care homes, averages 50 beds. It has lost more than 1,000 residents to the virus. Care UK, another major chain, averages 66 beds. Twenty-six people died at the 87-bed Melbury Court in Durham and 22 died at the 80-bed Highgate care home outside Glasgow, both run by HC-One. Fifteen died at the independent Castletroy residential home in Luton, which has more than 60 beds. Sally Warren, the director of policy at the King’s Fund health thinktank, said the relationship between the size of care homes and the likelihood of infection was “a difficulty not just for providers but for the government”. She said ministers provided funding levels for social care that encouraged private operators to seek economies of scale. The creation of “bubbles” would increase the cost of staffing and providing facilities which have previously been shared across large homes, she said. “If we want to move to a model based on smaller units we are going to have to pay more as taxpayers.” Separate research into how the pandemic affected 248 care homes in Norfolk has shown outbreaks were more likely in locations that employed higher numbers of support staff, such as cooks and cleaners. The research by the University of East Anglia found they were less likely to wear PPE and more likely to work across multiple locations than care workers. Another study, published last week by the Office for National Statistics, of 5,000 care homes in England found that regular use of temporary “bank” staff who worked across several homes – a common practice among larger operators – increased the risk of infection more than one and a half times. In 2018, the Care Quality Commission reported that “in both nursing and residential homes, there is a trend that smaller homes are rated better than larger homes”. HC-One said it recognised that the high numbers of staff required in large homes increased infection risk, but that this could be mitigated. “In a large service you can create smaller services, cohorting your staff and having a safe way of working with shared kitchen and laundry staff,” said Liz Whyte, the director of standards at HC-One. “That is now in place. Until there is a cure, we have to work as if we are in an outbreak.” The ability of care operators to adapt buildings is also likely to be limited by the strain Covid-19 has put on their finances. Four Seasons Health Care, which went into administration in April, said it was on course to spend £6.5m on PPE this year. HC-One said occupancy levels had fallen so sharply it was no longer generating cash. The need to better isolate infected care home residents was recognised by Public Health England (PHE) in April when it recommended that the government set up separate quarantine units for infected care home residents using spare capacity in NHS Nightingale hospitals. The idea was not taken up. A limited version of the proposal has been introduced in Ealing, west London: the NHS last month opened a special unit to accept infected care home residents as well as infected hospital patients before they are discharged into care homes. The Lothian study also says the intensity of outbreaks in a few locations has left a large pool of residents untouched by the virus so far. It found no outbreaks in 63% of the Lothian homes – close to the 57% of care homes in England that have not reported outbreaks, according to PHE. “There is considerable risk of further outbreaks with large number of deaths in care homes if community Covid-19 incidence increases again,” Guthrie said. Katie Dee, the deputy director of public health and health policy at NHS Lothian, said: “It is important to analyse the risk factors for Covid-19 infection in care homes in order to prevent future outbreaks.” The Department of Health and Social Care has said it “worked tirelessly with care homes to reduce transmission and save lives”, and has announced £600m to help tackle the spread of coronavirus, including limiting staff movement between care homes.