An air filtration machine made all traces of Covid-19 “disappear” from the air in a hospital ward, a study has revealed, which scientists say could signal the end of in-hospital transmission.
The machines, which contain a high-efficiency particulate absorbing filter and ultraviolet light, were placed in one Covid-19 ward and one intensive care unit (ICU) at Addenbrookes Hospital, Cambridge.
During a three-week trial at the height of the second wave in January, air samples were taken when the machines were switched both on and off.
In the first week, when the filter was off, Covid-19 was detected in the air on all five days of sampling in the Covid ward.
But during the second week, when the device ran continuously, no Covid-19 samples were identified.
The results were “really quite striking”, said lead author Dr Andrew Conway-Morris, a critical care doctor at Addenbrookes.
“I have to confess there was a little bit of scepticism when we set up the tests … but when we got the results they were really quite striking.
“If we look at the ward particularly, the week before we turn the device on there was lots of detectable [Covid-19] in the air and then you turn the filter on and it disappears.
“It is unusual, I would say, in science to have results so binary.”
Crucially, in the third week when the device was switched off again, the virus came back, Dr Conway-Morris said, which proved the effectiveness of the machine, which cost around £2,000 per unit, and are made in the UK and Switzerland.
Cabinet Office takes an interest in filters
In the ICU ward, the air samples picked up “limited evidence” of Covid-19 in weeks one and three, but detected a single sample of medium-sized particulates in week two.
The authors said this is likely due to the higher levels of PPE worn by the staff and because patients were more likely to be on ventilators and wearing oxygen masks, which would limit the spread of the virus.
It is estimated thousands of patients caught Covid-19 while in hospital, raising concerns over infection control measures and the quality of ventilation in some NHS buildings.
One study showed up to 11,800 people caught Covid-19 in British hospitals in the first virus wave in 2020.
The study, which is due to be published in the journal Clinical Infectious Diseases later this month, is understood to have been discussed by the Cabinet Office, which was “enthusiastic” about the findings.
During the pandemic, ventilation and air filtration devices have been central to discussions on reducing the spread of the virus, but this study is believed to be the first in the world to prove the effectiveness of these devices in a real-world hospital setting.
Fifteen patients were admitted to the Covid-19 ward and 14 to the ICU over the study period. All patients were symptomatic and tested positive for Covid-19 before being admitted to hospital.
More research planned
Further research will now be conducted at Addenbrookes Hospital to test if the devices can prevent patients becoming infected while staying on wards.
Beginning in January, the machines will be placed on two identical geriatric wards.
Researchers will measure the prevalence of the virus in the air and also the number of acquired infections in the patients on the ward.
Dr Stephen Baker, co-author and professor of molecular microbiology at the University of Cambridge, said the findings open up the possibility of putting the devices in schools, shopping centres or transport hubs, “anywhere really that you have a high number of people coming through with poor air circulation”.
Dr Julian W Tang, a clinical virologist at Leicester University, who was not involved in the research, said the study design made the results “more convincing”.
“They also demonstrated that with the filters turned on, not just viruses, but most other environmental bacteria and fungi were also removed, some of which are known human pathogens, so the air filtration benefits patients and staff not just from a virus cleansing viewpoint,” he said.
“The wider use of such filters can be a stop-gap in older buildings where upgrading their existing ventilation systems is difficult and expensive.”
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