Most hospital patients diagnosed as having COVID-19 contracted the virus from other patients rather than healthcare workers (HCWs), with 21% of patients causing 80% of cases, finds a UK study today in eLife.
A team led by researchers at the University of Cambridge analyzed data from five wards at the university's Addenbrooke's Hospital during a COVID-19 outbreak among patients and HCWs from March to June 2020. The team applied a new network reconstruction algorithm to infer patterns of virus spread among patients and HCWs.
'Superspreaders' behind most transmission
Of 38 transmission events studied, 20 were patient-to-patient, 8 were from patient to HCW, 8 were HCW to HCW, and 2 were HCW to patient. "These results suggest that patients were significantly more likely to be infected by other patients than by health care workers (p-value 6.1 x 10−5, one-tailed binomial test)," the study authors wrote. "By contrast, HCWs were at approximately equal risk of being infected by patients and other HCWs."
In addition, 21% of patients were "superspreaders" and caused 80% of the infections, a result that the authors said was similar to that reported among the general population. In one ward, for example, two patients were the sources of four and five secondary COVID-19 infections, respectively.
Five people, including four patients and one HCW, infected at least three other people. All four patients had suspected hospital-acquired COVID-19 and significant underlying illnesses, including chronic liver disease and blood cancer, the latter of which one person was receiving immunosuppressive therapy for. Immunosuppression has been tied to prolonged viral shedding.
One superspreader was confused and wandered the ward, while another had a fever for a few days before testing positive for COVID-19, which was likely acquired in the community. The sole HCW superspreader infected only other staff sharing the same accommodations.
Viral loads were similar among superspreaders and others in the study. "We note that, while prolonged or increased viral shedding would increase the chance of an individual becoming a superspreader, behavioural and environmental factors may also be influential," the researchers wrote.
The inferred times of transmission from superspreaders followed different patterns. In one ward, three initial HCW infections by one person were likely to have occurred within 4 days after the introduction of the virus, which the researchers said suggested that outbreaks involving a superspreader source may spread quickly.
Preventing hospital virus spread
"Preventing new cases of hospital-based infection is a critical part of our work," co-lead author William Hamilton, MBBChir, PhD, said in a University of Cambridge press release. "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 researchers noted that effectively quarantining COVID-19 patients from uninfected patients is difficult when hospitals are overwhelmed with cases, a problem compounded by the high transmissibility of the virus and asymptomatic infections.
"The fact that the vast majority of infections were between patients suggests that measures taken by hospital staff to prevent staff transmitting the virus to patients, such as the wearing of masks, were likely to have been effective," co-lead author Christopher Illingworth, PhD, said in the release. "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 authors called for strict adherence to infection-control protocols such as the appropriate use of personal protective equipment at all times, regardless of clinical area; universal masking, as tolerated; screening of asymptomatic patients and HCWs; isolation of ill patients; vigilance to the potential for hospital-acquired COVID-19 infections; and a low threshold for testing—even in cases with other diagnostic possibilities.
"Finally, our identification of transmission from patients to HCWs highlights the use of higher grade respiratory precaution to protect HCWs (such as FFP3 respirators) as an important topic for future research," the researchers wrote.
They also recommend future research into factors with possible links to increased transmission risk, including SARS-CoV-2 variants, patient characteristics (eg, immunosuppression), and environmental factors such as patient location and room ventilation.