Invasive group iGAS surveillance in Island Health, British Columbia, 2022

Demographic distribution

The median age of cases was 53 years, with a range of 0–96 years and 64% of cases were male. The distribution and risk of infection were the highest in men (distribution: 64%, incidence: 15.0 cases per 100,000 population) and individuals 40 years of age and older (distribution: 76%, incidence: 14.7 cases per 100,000 population) (Table 1). The highest incidence was reported in men 40–59 years of age (21.3 cases per 100,000 population) (Figure 5).

Emm typing

In 2022, there was no single dominant emm type. The three most common reported emm types were emm92 (n=14), emm49 (n=13), and emm83 (n=12) (Table 2).

Severity

Twenty-seven percent of cases reported in 2022 were clinically classified as severe (Table 3). Severe cases are defined as cases of streptococcal toxic shock syndrome (STSS), soft-tissue necrosis (including necrotizing fasciitis, myositis, or gangrene), meningitis, GAS pneumonia, or death directly attributable to GAS infection^{Footnote 6}. Overall, 85% of cases were hospitalized, 21% were admitted to the intensive care unit (ICU), and 6% died (Table 4). The proportion of cases admitted to the hospital and ICU was below the average number admitted in the previous five years (hospitalizations: average=90%, range=85%–93%; ICU admissions: average=23%, range=15%–32%). The case fatality rate was the same as the average case fatality rate reported in the previous five years (average=6%, range=4%–8%). The deaths reported in 2022 occurred in men and women 52–89 years of age (median age=73 years, 67% female). All cases had multiple risk factors reported (median number of reported risk factors=4, range=2–5). Five different emm types were prevalent amongst these fatal cases: 74, 81, 83, 92, and 43.

There was no dominant emm type reported among severe cases. For both severe and non-severe cases, the most common emm types were the same (Figure 6).

Risk factors

The most common reported risk factors among cases were having a skin infection, 47% (n=47) and having a wound, 46% (n=46) (Table 5). Compared to the previous five years, skin infections, wounds, alcohol use disorder, unstable housing, chronic cardiac conditions, chronic respiratory conditions, and immunocompromised conditions were reported more frequently in 2022, while injection drug use was reported less frequently. Among severe cases (n=27), the most common reported risk factors were having a wound, 52% (n=14); using substances, 52% (n=14); or having a skin infection, 44% (n=12) (Table 6). For non-severe cases (n=74), the most common reported risk factors were having a skin infection, 47% (n=35) or having a wound, 43% (n=32).

Discussion

In 2022, 101 confirmed cases of iGAS were reported in the Island Health region, corresponding to an incidence rate of 11.4 cases per 100,000 population; the highest rate reported in the last six years and above the preliminary annual provincial rate (8.5 cases per 100,000 population). Since 2019, the incidence of iGAS has been increasing in the Island Health region. This includes throughout the pandemic period when implemented non-pharmaceutical containment measures were also associated with a decrease in invasive respiratory diseases worldwide^{Footnote 7}. Provincially, in British Columbia, rates of iGAS have been higher than expected since 2017, with the incidence in the last six years remaining stable^{Footnote 8}. Globally, an increase in the incidence of iGAS over time has also been observed in many countries, including Canada^{Footnote 9Footnote 10Footnote 11Footnote 12}. Previous analyses have hypothesized that the observed increase is linked to both the increase in genetic diversity of circulating emm types and compounding societal risk factors, such as homelessness and substance use^{Footnote 10Footnote 13Footnote 14Footnote 15Footnote 16Footnote 17}. Although the factors associated with the increased incidence seen in the Island Health region since 2019, and particularly in 2022, are not completely clear, it is likely that multiple factors have contributed to the observed trends. This includes increased circulation of respiratory viruses, an increase in the diversity in circulating emm types, and the impact of the coronavirus disease 2019 (COVID-19) pandemic on community services, specifically an increased demand paired with reduced capacity and availability.

In December 2022, several European countries and the United States reported recent increases in infections of iGAS in children^{Footnote 2Footnote 3}. Similar to the provincial picture in British Columbia, demographic analysis of Island Health cases showed no notable increase in infections among the paediatric population^{Footnote 8}. The highest risk of infection was observed in men 40 years of age and older. While men 40 years of age and older appear to be at a higher risk for iGAS in 2022, further analysis on iGAS in this demographic group would contribute to understanding whether this is a confounding factor, since other risk factors, such as substance use, are known to be higher in this population^{Footnote 18Footnote 19Footnote 20}.

In 2022, no single dominant emm type was identified in the Island Health region. The three most common reported emm types were emm92 (n=14), emm49 (n=13), and emm83 (n=12). Prior to 2021, these emm types were uncommon in the Island Health region and British Columbia, representing on average 0.4%–4% and 1% of subtyped cases reported from 2016 to 2020, respectively^{Footnote 8}. Nationally, emm1 has been the dominant emm type for the last decade^{Footnote 21}. Since 2014, the prevalence of emm1 has been decreasing nationally and was surpassed by emm76 in 2019 and emm49 in 2020^{Footnote 9Footnote 22Footnote 23Footnote 24}. To date in the available literature, emm types 49, 83, and 92 have not been associated with more life-threatening illness. Emm types 1 and 3 have been associated with more life-threatening illness, but only represented 1% of cases subtyped in the Island Health region in 2022^{Footnote 25Footnote 26Footnote 27}. Overall, indicators of severity in the Island Health region were either below the average or within range of the values reported in the previous five years.

Limitations

When breaking down case numbers by subgroups, cell sizes become small. Calculated rates where the numerator is less than 20 are unstable and should be interpreted with caution. The descriptive analyses where cases are broken down by month, by HSDA (applies to North Island), by age (applies to age categories younger than 20 years of age), and by age and sex are affected by small cell sizes. Fluctuations in these values may indicate random variation rather than significant change in the rate. As well, information on risk factors is predominantly collected through chart reviews. These reviews may not capture the full medical or social history of each case, therefore risk factors among iGAS cases may be under-reported. The regional data presented in this report have undergone data quality assessment by Island Health, but data reconciliation processes for the provincial data are underway for cases reported for 2019 through 2022. The provincial rates shown are based on preliminary numbers, and final numbers and rates for the province may change. Lastly, this report includes data from pandemic response years and an analysis on the impact of the response on the completeness and trends of respiratory surveillance data in the Island Health region has not yet been conducted. It is likely that due to the response, both burden of disease and data completeness decreased, therefore, observed trends during these years might have been higher than reported in this publication. This would affect the interpretation of observed trends in 2022 in comparison to the previous five years. Despite these limitations, this summary contributes descriptive epidemiology that is important for understanding iGAS in the Canadian context.

Conclusion

Overall, this surveillance study characterizes cases of iGAS in the Island Health region in 2022 and compares these cases to those reported over the last five years. The study highlights that incidence of iGAS in the Island Health region continued to increase throughout the COVID-19 pandemic, reaching its highest annual rate in 2022. In contrast to reports from Europe and the United States, there was no notable increase in infections in the paediatric population. The findings of this report contribute to the epidemiological characterization of iGAS in Canada. Given the continued local, provincial, and national increase in incidence of iGAS, it is imperative that the epidemiology of these cases continues to be monitored and described annually.

Authors’ statement

• AN — Formal analysis, data curation, visualization, writing–original draft, review and editing of final version
• AS — Data collection, writing–review and editing
• CAW — Data collection, writing–review and editing
• CU — Data collection, writing–review and editing
• CB — Data collection, writing–review and editing
• KT — Data collection, writing–review and editing
• JE — Data collection, writing–review and editing
• KM — Data collection, writing–review and editing
• LW — Data collection, writing–review and editing
• TR — Data collection, writing–review and editing
• TGalbraith — Data collection, writing–review and editing
• SM — Data collection, writing–review and editing
• SG — Data collection, writing–review and editing
• TGasper — Data collection, writing–review and editing
• CB — Data collection, resources, methodology, writing–review and editing
• FL — Data collection, resources, methodology, writing–review and editing
• DH — Resources, methodology, writing–review and editing
• SA — Resources, methodology, writing–review and editing
• PK — Data collection, resources, laboratory validation and methodology, writing–review and editing
• AR — Resources, methodology, supervision, writing–review and editing
• MG — Resources, methodology, supervision, writing–review and editing
• CS — Resources, writing–review and editing

Competing interests

None.

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