Background
South Africa saw an estimated 10 million mild influenza cases in 2015. No influenza season has been observed since the COVID-19 pandemic started in 2020. In November 2021, increased influenza-positive individuals were observed in a closed community outside of the normal influenza season at a mine in a province in South Africa. Because of this, an outbreak investigation was launched. We aimed to calculate the influenza attack rate and risk factors, as well as identifying circulating influenza strains in the context of a local COVID-19 epidemic.
Methods
We performed a cohort study including all 10 030 mine employees. Symptomatic individuals, employees returning from leave and new recruits were screened for SARS-CoV-2, influenza and respiratory syncytial virus. We calculated the influenza attack rate and performed logistic regression to assess risk factors for influenza illness from 1 September – 13 December 2021. Where available, positive influenza specimens were further subtyped.
Results
The overall influenza attack rate was 1.6% (161/10 030), with 8/161 (5.0%) of tested individuals influenza positive in September, 47/161 (28.6%) in October, 96/161 (59.6%) in November and 12/161 (7.5%) in December. Risk factors for laboratory-confirmed influenza illness were being aged 18-35 years (aOR 2.4, p-value=0.003) compared to ≥60 years. Compared to office workers, those working as operators in shafts (aOR 3.28, p-value=0.025) had higher odds of being infected. The influenza outbreak was predominated by influenza A H1N1pdm09 (19/30, 63%) and influenza A H3N2 (8/30, 27%).
Conclusion
Influenza outbreaks in mines can occur outside of normal influenza seasons. Employees of working age and working in enclosed spaces (such as shafts) were at higher risk for laboratory-confirmed influenza A illness. This outbreak was driven by Influenza A H1pdm09, equally what was detected by respiratory surveillance programmes in South Africa. Immediate recommendations included isolation of infected individuals and education to increase adherence to non-pharmaceutical interventions.