Ongoing epidemic of scarlet fever in Shanghai and the emergence of M1 lineage group A : a 14-year surveillance study across the COVID-19 pandemic period.

Background: The surge of group A (GAS) infections in regional countries warranted attention in the post-COVID-19 era. The resurgence of notifiable scarlet fever in China has been a public health issue since 2011. We conducted a consecutive 14-year surveillance study to analyse dynamic epidemiological profiles of scarlet fever from the perspective of epidemic trend and molecular evolution of GAS strains.

Methods: From 2011 to 2024, epidemiological data on notifiable cases of scarlet fever and 3370 GAS isolates from children with scarlet fever were analysed. Incidence rates were calculated as the mean rate per 100,000 population with 95% CIs. Whole genome sequencing was performed on 332 12 and 248 1 isolates.

Findings: From 2011 to 2024, a total of 25,539 cases of scarlet fever were reported in Shanghai. The mean incidence declined from 17.1/100,000 population (95%CI 9.7-24.3, pre-COVID-19 period) to 4.8/100,000 population (2.0-10.1, post-COVID-19 period). Children aged 4-9 years represented 85.6% of cases. Sixteen types were identified with predominance of 12 (66.4%) and 1 (29.8%). The annual proportions of 12 and 1 alternatively fluctuated with a cycle of six years. GAS isolates displayed >90% resistance rate to erythromycin, clindamycin, and tetracycline. The distribution of types, antimicrobial resistance rates and profiles, and differences in superantigen genes carried by the 12 and 1 isolates were similar in the pre- and post-COVID-19 periods. Phylogenetic analysis indicated 96.7% of 12 isolates belonged to Clade II, harbouring ICE-12 and ΦHKU.vir. Among the 1 isolates, the proportions of the ST1274 isolates increased from 10.5% (pre-COVID-19 period) to 73.7% (post-COVID-19 period). Four novel M1 isolates were found in Shanghai, with distinctive characteristics of presence of and .

Interpretation: Persistent and periodic predominance of 12 and 1 types was responsible for the annual epidemic of scarlet fever in Shanghai. A marked decline in the incidence of scarlet fever in the last 5 years in Shanghai may be associated with enhanced type-specific immunity among the susceptible population and enhanced infection control measures, awareness, and actions in child institutions in the post-COVID-19 era. The emergence of the M1 isolates with some distinctive characteristics and ST1274 lineage warrants further attention and continuous surveillance in China.

Funding: Shanghai municipal three-year action plan for strengthening the construction of the public health system 2023-2025 (GWVI-2.1.2); Research project of Shanghai Minhang District Health Commission (2024MW45).