Outbreaks of coronavirus are up to 20 times more likely in large care homes, according to research.

The NHS Lothian and Edinburgh University study looked at 189 care homes in the health board area and found 70 (37%) of them had experienced a Covid-19 outbreak.

In homes with fewer than 20 residents, the chance of an outbreak was 5%.

But in facilities with 60 to 80 residents, the likelihood rose to between 83% and 100%, according to analysis first reported in The Guardian.

The report suggests creating “bubbles” within homes could help with infection control.

It states: “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.

“Such efforts will be complicated by individual care home built environment, and will be difficult to sustain without rapid outside support during any large Covid-19 outbreak when staff illness and absence may risk compromising safe care.

“Additional measures to respond to new outbreaks of Covid-19 will also be required, including maintaining high provision of adequate personal protective equipment (PPE), better support for infection control, ensuring self-isolation and active surveillance of residents and staff to ensure early detection of outbreaks and ongoing transmission, and staffing support for care homes with many staff absent.”

There have been more than 55,000 deaths in the UK where Covid-19 has been mentioned on the death certificate, including suspected cases, with nearly 40% of these (21,678) care home residents.

Professor Bruce Guthrie, director of the Advanced Care Research Centre at the University of Edinburgh who was involved in the research, said higher footfall might explain the increased likelihood of outbreaks in larger homes.

He told the PA news agency: “The key reason is almost certainly footfall. Larger homes have more staff and more admissions of people from the community and hospital, so more movement in and out of the home. So there are more opportunities for the virus to come into the care home.

“To what extent does it then spread, that’s driven more by infection control practices.”

The new report says the impact of Covid-19 is concentrated in a small number of care homes with “repeated or sustained” outbreaks.

It warns that as many care homes for older people and virtually all other facilities do not yet appear to have had an outbreak, there is “considerable risk” of further outbreaks and a large number of deaths in care homes if the incidence of Covid-19 in the community increases again.

It states: “Allowing families and friends to visit residents again is important for quality of life, but needs to be balanced against the need to shield residents of care homes in areas where community incidence is high or increasing.

“Early detection of outbreaks through regular testing, reliable PPE supply, support for infection control, and measures to ensure safe staffing are all likely to be needed to contain the size of established outbreaks.”

Asked about the findings, First Minister Nicola Sturgeon said: “We’re looking carefully at that research and we’re looking at our own information we’ve been publishing to make sure that we are understanding – just as we are in how the virus is affecting different populations – how it is spreading in particular settings and whether there are disparities, as this research suggests, between larger care homes and smaller care homes.

“That will, as necessary and appropriate, be reflected in the advice we give.

“But the advice to care homes right now – whether that’s on isolation, infection control and testing – is very clear and I know care homes are working hard to ensure that is implemented.”

if you have relatives or loved ones in a large care home, you shouldn't be specifically worried because of this research

National clinical director Jason Leitch added: “Their best estimate of why [there is disparity] is about footfall and number of people, it’s not about quality of care in one or the other.

“So if you have relatives or loved ones in a [large] care home, you shouldn’t be specifically worried because of this research.

“The guidance applies to all care homes, all settings, and is being managed by the Director of Public Health, the care home owners and everybody who cares deeply about your loved ones in those care homes.”