Just over one in 10 of the coronavirus patients who were in hospital during the UK's first wave caught the infection after being admitted, research suggests.
Hospital-acquired infections are common during outbreaks, like seasonal flu.
With patient numbers reaching "very high numbers" amid the pandemic, a team of scientists analysed the medical records of around two-thirds of those with a confirmed or suspected coronavirus infection while in hospital up to August 2020.
Results – published in The Lancet – suggest that, on average, at least 11.1% of the coronavirus patients in hospital caught the infection after admission, rising to 15.8% in mid-May specifically, "long after the peak" of hospitalisations.
The scientists were not particularly surprised by the results, noting around one in 10 flu patients in hospital during the winter months catch the infection while being treated for something else.
Throughout the pandemic, changing coronavirus hospitalisation rates have influenced the number of cases caught after admission.
At present, 2% to 5% of people with the infection in hospital are expected to have caught the coronavirus while being treated for another medical issue.
"The underlying reasons for these high rates of transmission in hospitals at the peak of the first wave must be investigated, so we can improve safety and outcomes for our patients," said study author Dr Annemarie Doherty, from the University of Edinburgh.
"Rates are considerably lower a year on and people should not be deterred from attending hospital if they are unwell."
COVID hospitalisations are thought to have peaked in the UK on 18 January, 2021, when 39,254 people were being treated. This had reduced to 5,912 on 10 August, the most recent data available.
The scientists analysed patients in 314 UK hospitals that were enrolled in the "International Severe Acute Respiratory and emerging Infections Consortium (ISARIC) Clinical Characterisation Protocol UK" study.
A hospital-acquired infection was determined according to the patient's admission date and symptom onset, up to 1 August. The date of their infection was estimated according to the coronavirus' incubation period, the time between infection and symptoms emerging.
"We estimated that of 82,624 patients admitted before Aug 1, 2020, 5,699–11,862 patients were infected during their hospital stay," wrote the scientists.
This is likely an underestimate due to the data not identifying "patients infected during admission but discharged before manifesting symptoms or patients infected during another healthcare visit before admission".
"Controlling viruses like [the coronavirus] has been difficult in the past, so the situation could have been much worse, however, infection control should remain a priority in hospitals and care facilities," said lead author Dr Jonathan Read, from Lancaster University.
Hospital-acquired infections may have occurred due to "the large numbers of patients admitted to hospitals with limited facilities for case isolation, and limited access to rapid and reliable diagnostic testing in the early stages of the outbreak", according to co-author Dr Chris Green, from the University of Birmingham.
Poor access to personal protective equipment (PPE), initial misunderstanding around when patients are most infectious, "misclassification of cases due atypical symptoms" and an "under-appreciation of airborne transmission" were likely also to blame.
Post-admission cases were lower in the hospitals that provided acute or general care, where one in 10 (9.7%) of the coronavirus patients caught the infection while being treated for another complaint.
This is compared to around two-thirds of the total coronavirus patients in residential community care (61.9%) or mental health (67.5%) hospitals, "reflecting outbreaks seen in care homes".
The reasons behind this "marked heterogeneity" require "urgent investigation" to "promote best infection control practice for future treatment of COVID-19 [the disease caused by the coronavirus] patients", according to the scientists.
Limited testing early in the pandemic, false-negative swabs and uncommon gut-related symptoms may have "led to some patients with COVID-19 being misclassified and placed in non-COVID-19 areas with different infection prevention control processes".
The coronavirus primarily spreads face to face via inhaled droplets that have been expelled in an infected person's cough or sneeze.
To a lesser extent, the virus can be transmitted via faeces or picked up from surfaces. This "raises the possibility of faecal-oral transmission in care settings under severe pressure", wrote the scientists.
"Going forward, it is critical to minimise the HAI [hospital-acquired infections] burden using the lessons learned this past year", they added.
"Unlike at the beginning of the pandemic, there are opportunities to pre-empt HAI and break chains of transmission".
This could be achieved via regular testing, "robust hospital infection prevention", "control policies" like staff vaccination, "environmental disinfection" and case isolation.
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