Invasive group A Streptococcus in Newfoundland and Labrador: Recent epidemiology and risk factors for severe infection

Background: This project aimed to describe the local epidemiology of invasive group A Streptococcus (iGAS) in Newfoundland and Labrador (NL) over the past 10 years, confirm suspicions of a provincial outbreak in 2024, and assess risk factors that may account for this outbreak.Methods: A retrospective chart review of a cohort of patients in NL with iGAS infection between January 1, 2015 and December 31, 2024 was conducted.Once an outbreak was confirmed through assessment of case counts over time, reference genotyping of cases and patient risk factor data were collected.Multivariate logistic regression was performed to compare patients within the outbreak period to the rest of the cohort and describe trends in infections across the province.Invasive group A Streptococcus rates were compared with publicly available provincial influenza A rates.Results: A total of 231 iGAS isolates from 199 patients met the inclusion criteria.Mean age was 49 years and 42.2% of patients were female.The background yearly incidence rate ranged from 0.94-4.36cases per 100,000 population per year.There was a significant outbreak from December 2023 to May 2024, with an incidence rate of 10.95 cases per 100,000 population per year.When comparing cases within (n=59) and outside (n=140) this outbreak period, there was a higher proportion of critical care admissions (39.0%[n=23] vs 16.4% [n=23], p=0.001) and higher proportional mortality (13.6% [n=8] vs 7.9% [n=11], p=0.147).There were no demographic or clinical risk factor differences between the pre-and post-outbreak groups.However, 57.6% (n=34) of epidemic cases were caused by emm1, compared to 28.6% (n=40) cases during the non-epidemic period (p<0.01).The emm1 type (odds ratio=2.620,p=0.010) was a significant predictor of critical care admission.Close correlations with provincial influenza rates are noted.Conclusion: A period of increased iGAS prevalence was defined, denoting an outbreak and association with the emm1 genotype.Respiratory infections such as influenza A played a likely role.