Behind the mask: Study shows COVID-19 exposure and viral carriage in health care workers

Healthcare workers walking down hospital corridor
The study demonstrated asymptomatic carriage of SARS-CoV-2 in 2.4% of health care workers highlighting the importance of transmission within hospitals

2.4% of asymptomatic health care workers at work in Birmingham were carriers of the SARS-CoV-2 virus and over a third of those individuals subsequently became unwell with symptoms of COVID-19, a new cross-sectional study by researchers at the University of Birmingham and University Hospitals Birmingham NHS Foundation Trust (UHB) has found. 

Led by immunology experts at the University of Birmingham and UHB, both founding members of Birmingham Health Partners, the study was undertaken to determine current infection by SARS-CoV-2 using nasopharyngeal swabs for viral carriage and to determine previous infection by SARS-CoV-2 by testing blood for anti-SARS-CoV-2 antibodies. 554 asymptomatic health care workers at UHB were recruited over a 24 hour period between 24-25 April 2020 for coronavirus testing.

The study demonstrated asymptomatic carriage of SARS-CoV-2 in 2.4% of health care workers highlighting the importance of transmission within hospitals. Over a third of these individuals subsequently became unwell with symptoms of COVID-19 suggesting that screening could be effective in reducing viral transmission in hospitals through the earlier identification and isolation of viral carriers before they become symptomatic.

Antibody testing helps identify individuals that have previously been exposed to the virus by measuring their immune response against it. Overall 24.4% of health care workers were found to be antibody positive at the time of the study, much higher than the equivalent general population rate, highlighting the occupation risk of working in a hospital.  Notably 17.1% of individuals had developed antibodies to the virus without experiencing any symptoms of COVID-19, providing further evidence for asymptomatic carriage.

Significant differences in antibody positivity were found between hospital departments. The study suggested that housekeepers and individuals working in acute or general medicine were most likely to have been previously exposed to the virus, in comparison to those working in intensive care medicine who were relatively protected despite caring for the sickest COVID-19 patients. 

As the study was observational, the study did not explore the reasons behind these findings, however, the differences could be down to the different levels of PPE (personal protective equipment) provided in different hospital areas, with those in ITU being allocated FFP3 masks which are believed to afford the highest protection.

Researchers also found that individuals belonging to Black, Asian and minority ethnic (BAME) groups were at significantly greater risk of having a positive antibody result.  This difference was not explained by differences in age, gender or external social circumstances between study participants.

This study highlights the occupational risk of infection exposure to health care workers and enables important lessons to be learnt going into this winter and for future viral outbreaks.

Dr Adrian Shields, from the University of Birmingham’s Clinical Immunology Service and lead author of the study, said: “Intensive care departments were designated high-risk environments and the use of enhanced personal protective equipment, including respirators was mandated in accordance with national guidelines. Other clinical areas within the hospital had different levels of PPE provision. More research is needed into the use of enhanced PPE but our results, which show significantly lower seroprevalence in intensive care workers, suggests that they were less exposed to the virus.

“Our study has also demonstrated that more work needs to be done to understand the higher proportion of antibody positive workers that we observed in BAME members of staff compared with white staff. Although individuals of BAME ethnicity within this study, on average, lived in significantly more deprived areas, the index of multiple deprivation of participants’ home postcode did not significantly influence  antibody status which is something that should be investigated further.”

Professor Alex Richter, who led the study, from the Institute of Immunology and Immunotherapy, said: “This study has highlighted the vulnerability of healthcare workers to major outbreaks of infections.  There are important lessons to be learnt not only for this pandemic but also for the annual influenza season and the winter pressure that this generates within the NHS. There are important lessons to be learnt around infection control and prevention that we will build on. “

The study, ‘SARS-CoV-2 seroprevalence and asymptomatic viral carriage in health care workers: a cross-sectional study’ was published today in Thorax. 

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