What is the epidemiology of scarlet fever?

Epidemiology of Scarlet Fever

Scarlet fever is primarily a disease of children aged 3-9 years, with peak incidence in those aged 4-6 years, occurring predominantly during winter and spring months with characteristic bimodal seasonal peaks. 1, 2

Age Distribution

The disease overwhelmingly affects young children:

• Children aged 3-9 years account for approximately 83-86% of all cases 3, 4
• Peak incidence occurs in children aged 4-6 years, with the highest rates in 6-year-olds (5.0 per 100,000) 3
• Children aged 5-15 years represent the primary burden of disease 4
• The median age of cases is 4 years (IQR 3-7) 5

Gender Distribution

Males are consistently more affected than females, with male incidence approximately 21-51% higher than females across multiple studies 3, 6, 5. This male predominance is statistically significant (incidence rate ratio 1.21,95% CI 1.09-1.34) 6.

Seasonal Patterns

Scarlet fever demonstrates consistent bimodal seasonal peaks:

• Primary peak: March through July (higher peak)
• Secondary peak: November through January (lower peak) 3, 4
• In temperate climates, the disease typically occurs in winter and early spring 2
• Winter and spring months account for approximately 57% of cases 6

Geographic and Setting Distribution

Kindergartens and primary schools serve as the primary transmission hotspots:

• Kindergarten children represent 54.32% of cases
• Students account for 34.09% of cases 3
• Urban areas typically show higher incidence rates, with gradual spread to remote areas 3
• Spatial clustering is significant, with disease concentration in urban centers and adjacent districts 3

Recent Epidemiological Trends

Global Resurgence (2011-2019)

A dramatic global resurgence of scarlet fever began around 2011-2014 after decades of decline:

• England experienced a threefold increase between 2013-2014 (from 8.2 to 27.2 per 100,000), reaching the highest incidence in nearly 50 years by 2016 (33.2 per 100,000) 5
• China saw sustained increases from 2011 onward, with incidence rising 106.54% in the post-upsurge period (2015-2019) compared to pre-2014 levels 3
• Australia maintained stable incidence (mean 2.5 per 100,000) from 2007-2017, though remained vigilant for potential outbreaks 6

COVID-19 Pandemic Impact (2020-2022)

The pandemic period brought significant but temporary reductions in scarlet fever incidence:

• Incidence decreased by 68.61% in 2020,25.66% in 2021, and 10.59% in 2022 compared to predicted rates 3
• These reductions were attributed to enhanced infection control measures, social distancing, and increased awareness in child institutions 7

Post-Pandemic Period (2023-2025)

The post-pandemic epidemiology shows divergent patterns:

• Shanghai reported a mean incidence decline from 17.1/100,000 (pre-COVID) to 4.8/100,000 (post-COVID), with unexpected continued decrease in 2023 (1.5 per 100,000) 7
• However, predictions suggest resurgence in 2024-2025, with cases expected to increase 3
• The emergence of the M1UK lineage and ST1274 strains warrants heightened surveillance 7

Microbiology and Molecular Epidemiology

Group A Streptococcus (GAS) emm types show geographic and temporal variation:

• In China: emm12 (66.4%) and emm1 (29.8%) predominate, with alternating proportions in 6-year cycles 7
• In England: emm3 (43%), emm12 (15%), emm1 (11%), and emm4 (9%) are most common 5
• Antimicrobial resistance is concerning: >90% resistance to erythromycin, clindamycin, and tetracycline in some regions 7

Severity and Complications

Hospital admission rates range from 24% to 25% of cases 6, 5:

• One in 40 cases requires admission for management or complications 5
• Hospital admissions increased by 97% between 2013-2016 in England during the resurgence 5
• No deaths were reported in recent surveillance studies from Australia 6

Clinical Pitfalls

The epidemiological pattern of 4-yearly periodicity has been identified in longitudinal analysis, though the current escalation exceeds historical patterns 5. This cyclical nature may reflect population immunity dynamics and should inform public health planning.

Key surveillance priorities include monitoring for novel GAS strains (particularly M1UK lineage), maintaining vigilance in kindergartens and primary schools during peak seasons, and preparing for potential post-pandemic resurgence in regions that experienced significant declines during 2020-2022.