Risk factors for SARS-Cov-2 infection at a United Kingdom electricity-generating company: a test-negative design case-control study

Abstract

AbstractObjectivesIdentify workplace risk factors for SARS-Cov-2 infection, using data collected by a United Kingdom electricity-generating company.MethodsUsing a test-negative design case-control study we estimated the odds ratios (OR) of infection by job category, site, test reason, sex, vaccination status, vulnerability, site outage, and site COVID-19 weekly risk rating, adjusting for age, test date and test type.ResultsFrom an original 80,077 COVID-19 tests, there were 70,646 included in the final analysis. Most exclusions were due to being visitor tests (5,030) or tests after an individual first tested positive (2,968).Women were less likely to test positive than men (OR=0.71; 95% confidence interval=0.58-0.86). Test reason was strongly associated with positivity and although not a cause of infection itself, due to differing test regimes by area it was a strong confounder for other variables. Compared to routine tests, tests due to symptoms were highest risk (94.99; 78.29-115.24), followed by close contact (16.73; 13.80-20.29) and broader-defined work contact 2.66 (1.99-3.56). After adjustment, we found little difference in risk by job category, but some differences by site with three sites showing substantially lower risks, and one site showing higher risks in the final model.ConclusionsIn general, infection risk was not associated with job category. Vulnerable individuals were at slightly lower risk, tests during outages were higher risk, vaccination showed no evidence of an effect on testing positive, and site COVID-19 risk rating did not show an ordered trend in positivity rates.Key messagesWhat is already known on this topicIn the United Kingdom, there is now a considerable body of evidence showing occupational differences in Covid-19 infection and severity, but with understandable focus on high-risk industries like healthcare.Less is known about differences in risk of COVID-19 infection in other industries that do not involve directly working with the general public, in particular, there is relatively little evidence on the risks of transmission in the electricity-generating industry.What this study addsAt this company, infection risk was not associated with job category after adjusting for test reason; however women were less likely to test positive than men and the risk was higher when there was a power outage, requiring more staff to visit the site in person.How this study might affect research, practice or policyThe site risk rating showed a consistent (but modest) dose-response with infection risk, indicating that such risk rating may be useful for identifying “high risk” sites.This analysis demonstrates the importance of adjusting for both date of and reason for test, when prevalence and testing protocols differ over time.