The associations between invasive group A streptococcal disease and infection with influenza, varicella or hepatitis C viruses: A data linkage study, Victoria, Australia.

Background Invasive group A streptococcal disease (iGAS) causes significant disability and death. Reported associations between iGAS incidence and influenza, varicella and chronic hepatitis C (HCV) have not been well quantified, and these studies have been susceptible to bias. Methods We used individual-level linked data of iGAS cases from Victoria, Australia (2007-2017) to assess associations between these viral infections and iGAS. A self-controlled case series method was used to estimate the relative incidence of iGAS following an influenza or varicella infection, while the relative incidence of iGAS amongst HCV cases, and HCV cases who inject drugs, was estimated using population-level data and a negative binomial regression model. Results Of the 1,949 individuals with at least one iGAS diagnosis, 82 were diagnosed with influenza at least once, 30 with varicella and 118 with HCV during the study period. The relative incidence of iGAS increased substantially following infection with influenza (IRR: 34.5, 95% CI: 21.3-55.8) or varicella (IRR: 22.4, 95% CI: 10.3-48.8). iGAS incidence was higher among HCV cases (IRR 5.7, 95%CI: 4.4-7.3) compared to individuals without HCV. iGAS incidence was also higher among HCV cases who inject drugs (IRR: 17.9, 95% CI: 13.0-24.4) compared to individuals without HCV who did not inject drugs. Conclusion We found a significantly higher risk of iGAS following an influenza or varicella infection and for chronic HCV cases, particularly those who inject drugs. These finding are relevant to public health practice and support the timely identification of iGAS cases.