The research from Cambridge University Hospitals Trust (CUH) and Cambridge University provides a new insight into how infections might spread in hospitals.
Out of 22 cases where patients were infected while in Addenbrooke's in the first wave of the pandemic, 20 were the result of the virus spreading from patient to patient.
The study also found that a few individuals caused most of the transmission.
Dr Chris Illingworth, a lead author on the study, who carried out his research while at Cambridge’s MRC Biostatistics Unit, said: “The fact that the vast majority of infections were between patients suggests that measures taken by the hospital to prevent staff transmitting the virus to patients, such as the wearing of masks, were likely to have been effective.
“But it also highlights why it is important that patients themselves are screened for Covid-19 regularly, even if asymptomatic, and wear face masks where possible.”
The study analysed data from the first wave of the pandemic at Addenbrooke's, between March and June 2020.
Researchers identified five wards where multiple individuals, including patients and healthcare workers, tested positive for Covid-19 within a short space of time, suggesting a local outbreak.
Using new statistical methods that combines viral genome sequence with clinical information about the locations of individuals, researchers identified cases where the data was consistent with transmission between people in the hospital. Looking in detail at these transmission events revealed patterns in the data.
The results of the study showed that patients who were infected in the hospital were mostly infected by other patients, rather than by hospital staff. Out of 22 cases where patients were infected in hospital, 20 of these were the result of the virus spreading from patients to other patients.
Dr William Hamilton, an infectious diseases clinician at CUH and co-lead author on the study said: “Preventing new cases of hospital-based infection is a critical part of our work. Here we have shown that analysing clinical and viral genome sequence data can produce insights that inform infection control measures, which are so important for protecting patients and healthcare workers alike.”
The study found contrasting results among healthcare workers, who were almost as likely to be infected by patients as they were by other hospital staff.
Early findings from this research contributed to an upgrade in PPE masks for CUH staff working on Covid-19 wards. Another recent Cambridge study indicated that this resulted in staff being better protected against catching Covid-19.
The researchers also found a trend towards individuals either infecting no one else, or infecting multiple other people – just over a fifth of patients (21%) caused 80% of the infections.
This phenomenon is sometimes called ‘superspreading’ and can make infection control very challenging. Whether or not an individual can be identified in advance as being more or less likely to pass on the virus is an ongoing topic of research.
The study is published today in eLife.
The research was funded by COG-UK, Wellcome, the Academy of Medical Sciences, the Health Foundation and the NIHR Cambridge Biomedical Research Centre.