Research and analysis

Group A streptococcal infections: second update on seasonal activity in England, 2023 to 2024

Updated 25 July 2024

Applies to England

In the first 18 weeks of the 2023 to 2024 season, scarlet fever notifications followed a typical seasonal pattern, though numbers of notifications were at the higher end of what is normally expected. Notifications showed a small December peak, in line with normal seasonal patterns and, in the most recent week, are beginning a gradual increase again. Rates of notifications between regions are varied. Current notifications are significantly below those reported last season.

Similarly, the incidence of invasive group A streptococcus (iGAS) disease increased to a peak in week 52, in line with what would normally be expected for the time of year and significantly below what was observed this time last year. The age distribution of cases is following expected patterns, with the majority of cases in older age groups. This is in contrast to the 2022 to 2023 season where high rates of iGAS infection in children were observed in the early part of the season (1).

Given the potential for severe presentations, scarlet fever cases should be treated promptly with antibiotics to limit further spread and reduce risk of potential complications in cases and their close contacts. Clinicians should continue to be alert to complications of GAS infections and maintain a high degree of clinical suspicion when assessing patients, particularly those with preceding viral infection (including chickenpox) or their close contacts. New NHS England best practice principles to support clinicians when assessing patients presenting with a direct-to-consumer point-of-care device result for GAS were published on 19 January 2024.

Updated UK public health guidance on the management of close contacts of iGAS cases in community settings was published on 15 December 2022, with public health action extended to include patients with probable invasive GAS infection and additional close contact groups recommended for antibiotic prophylaxis.

National guidance on the management of scarlet fever outbreaks highlights essential tools to limit spread: prompt notification of scarlet fever cases and outbreaks to UK Health Security Agency (UKHSA) Health Protection Teams (HPTs); collection of throat swabs (prior to commencing antibiotics) when there is uncertainty about the diagnosis; and exclusion of cases from school and work until 24 hours of antibiotic treatment has been received.

Data presented within this seasonal activity update is based on information available as of 17 January 2024 and presents data to 14 January 2024 (the end of week 2). Numbers presented in this report may change as updated data becomes available. Weekly notifiable disease reports are published each week throughout the year to provide a regular update of scarlet fever notifications.

Key definitions are available at the end of the report. Seasons extend from week 37 (mid-September), of one year, to week 36 (mid-September) of the next.

Scarlet fever

During the first 18 weeks of the current 2023 to 2024 season (week 37 to week 2), scarlet fever notifications remained slightly higher than what is normally expected for the time of year, though considerably lower than the large number observed during the previous, 2022 to 2023 season. Notifications peaked at 1,020 in week 51 of 2023, before decreasing. Early signs of a possible increase were evident in week 2, suggesting the start of increased activity typically peaking in the Spring

A total of 8,350 scarlet fever notifications were made up to week 2 of 2024, with 1,011 notifications made in 2024. Compared to the previous five seasons (2017 to 2018 season to the 2021 to 2022 season), this is outside the range (532 to 5,584) and higher than the average (3,240).

Scarlet fever notifications for the 2022 to 2023 season remain provisional while the NOIDs annual report is being compiled. As a result, numbers may change in future reports.

Figure 1. Weekly scarlet fever notifications in England, by season, 2017 to 2018 onwards (weeks 37 to 48)

Note: Data shown for the current season goes up to week 2 (14 January 2024); data for the most recent weeks may change, as further notifications are received and processed, and are therefore represented by a dotted line between weeks 1 and 2 of 2024.

Rates of scarlet fever notifications varied between regions, with the North West region reporting the highest rate (22.4 per 100,000) and the East of England the lowest (9.5 per 100,000). The rate in the North West is followed by 21.2 per 100,000 in the East Midlands and 14.9 per 100,000 in both the West Midlands and Yorkshire and the Humber (Table 1).

Invasive group A streptococcal infection

Table 1. Number and rate per 100,000 population of scarlet fever and iGAS notifications in England: 2023 to 2024 season

Region	Number of cases of scarlet fever	Rate of scarlet fever	Number of cases of iGAS	Rate of iGAS
East of England	630	9.5	84	1.3
East Midlands	1,037	21.2	71	1.5
London	1,003	11.4	106	1.2
North East	450	17.0	67	2.5
North West	1,664	22.4	126	1.7
South East	1,174	13.0	138	1.5
South West	689	12.1	89	1.6
West Midlands	889	14.9	115	1.9
Yorkshire and the Humber	814	14.9	123	2.2
England	8,350	14.8	919	1.6

Laboratory notifications of iGAS infection so far this season (weeks 37 to 2) are higher than usually seen by this point in the season (Figure 2); there have been 919 cases reported so far this season, 116 of which were in 2024. This total is significantly below the previous, 2022 to 2023 season (1,698 cases). Notifications so far in the current season are higher than the average (584) and the range (300 to 855) seen for the same period in the prior 5 seasons.

This follows a season of exceptionally high iGAS notifications with a total of 4,413 iGAS episodes received in 2022 to 2023, peaking at 213 notifications in week 52 of 2022.

During the current season to date, the highest rates were reported in the North East region (2.5 per 100,000 population), followed by the Yorkshire and Humber region (2.2 per 100,000), and the West Midlands region (1.9 per 100,000) (Table 1).

Figure 2. Weekly laboratory notifications of iGAS, England, by season, 2017 to 2018 onwards

Note: In this graph, notifications for the most recent weeks of the 2023 to 2024 season are expected to increase due to lags in reporting timelines from laboratories. The decline in notifications shown for recent weeks should therefore be interpreted with caution and is represented here by a dashed line between weeks 1 and 2.

Of reports received this season to date, iGAS rates were highest for the 75 years and older age group (5.2 per 100,000), followed by under 1 year olds (4.0 per 100,000) and 65 to 74 year olds (2.2 per 100,000). The lowest rate of infection was observed in 10 to 14 year olds (0.4 per 100,000). The variation in rates for different age groups is similar to what has been observed in previous seasons and does not resemble the unusual pattern noted in children during the 2022 to 2023 season (1).

The median age of patients with iGAS infection so far this season has been 57 years (range 1 year and under to 99 years) in line with the range seen at this point in the preceding 6 seasons (51 to 59 years).

Antimicrobial susceptibility results obtained from routine laboratory surveillance of iGAS isolates this season (weeks 37 to 2) were broadly consistent with prior years, in particular:

• isolates were reported as universally (100%) susceptible to penicillin
• 7% were resistant to clindamycin (compared with 4% last season; range 4% to 21% in the last 5 years)
• 10% were resistant to erythromycin (compared with 4% last season; range 4% to 21% in the last 5 years), and
• 28% were resistant to tetracycline (13% last season; range 13% to 45% in the last 5 years)

The most commonly reported serotype so far this season is emm 1 (16% of referrals), followed by emm 3 (8%), and emm 89 with 6% of referrals. At this point last season, emm 1 was the most frequently identified (48%), followed by  emm 12 (15%) and emm 89 (4%). This increase in the proportion of iGAS due to emm 3 (specifically subtype emm 3.93) in the winter of the 2023 to 2024 season is of note; the last time an increase in the prevalence of this type was observed was in the 2017 to 2018 season (2).

Discussion

After a period of elevated notification during last winter, scarlet fever notifications throughout 2023 fell to levels at the top end of what would normally be reported at this point in the season.

Similarly, the rate of iGAS infection in the early part of the current season is following a normal seasonal pattern, albeit at the higher end of the range that would normally be expected. Incidence remains significantly below the exceptional levels being recorded this time last year, with laboratory notifications and age group distribution being within the range normally expected.

This season, a clonal expansion has been observed in the emm 3.93 subtype; an expansion on this subtype was previously seen during the 2017 to 2018 season (2). Detailed genomic and biological investigations are under way to investigate this emergence.

Reduced exposure to GAS infections during the pandemic was likely to have increased the population’s susceptibility to these infections last season. Prompt treatment of scarlet fever with antibiotics is recommended to reduce risk of possible complications and limit onward transmission.

GPs and other frontline clinical staff are also reminded of the increased risk of invasive disease among household contacts of scarlet fever cases (3, 4). Clinicians should continue to maintain a high index of suspicion in relevant patients for invasive disease as early recognition facilitates prompt initiation of specific and supportive therapy for patients with iGAS infection. To support clinician-led decision making when assessing patients with sore throat who present with a direct-to-consumer point-of-care in vitro diagnostic device for GAS, best practice principles are now available.

Relevant guidelines and information can be found on GOV.UK:

• Guidelines for the public health management of scarlet fever outbreaks in schools, nurseries and other childcare settings
• Scarlet fever: symptoms, diagnosis and treatment (factsheet)
• Guidelines for the management of close community contacts of invasive GAS cases and the prevention and control of GAS transmission in acute healthcare and maternity settings
• Prevention and control of group A streptococcal infection in acute healthcare and maternity settings
• Weekly notifiable disease reports (“NOIDS”)
• Group A strep – what you need to know (blog)

All invasive disease isolates – and also non-invasive isolates – from suspected clusters or outbreaks should be submitted for typing to:

Staphylococcus and Streptococcus Reference Section
Antimicrobial Resistance and Healthcare Associated Infections (AMRHAI)
UK Health Security Agency
61 Colindale Avenue
London
NW9 5HT

Data sources and methods

Scarlet fever notification data was extracted from the notifications of infectious diseases (NOIDs) reports. Data for England was extracted on 17 January 2024. Weekly totals include a few scarlet fever notifications identified in port health authorities; this means that the regional totals will not equal the season total for England.

Invasive GAS laboratory notification data was extracted from the UKHSA Second Generation Surveillance System (SGSS) and combined with specimen referrals to the Staphylococcus and Streptococcus Reference Section to produce a total number of episodes for England. Data was extracted on 19 January 2024.

The sharp increase in scarlet fever and other group A strep infections alongside increased awareness and vigilance among clinicians led to a significant rise in scarlet fever notifications during winter 2022. This resulted in a backlog of notifications of scarlet fever cases being entered into the national database after being processed. As a result, notifications for the 2022 to 2023 season are still being finalised and numbers presented here may change in future reports.

The GAS surveillance season runs from week 37 in one year to week 36 in the following year (mid-September to mid-September). The 2023 to 2024 season data in this report covers the period 11 September 2023 to 17 January 2024.

Population rates are calculated per 100,000 using the relevant year’s ONS mid-year population estimate.

The M protein gene (emm) encodes the cell surface M virulence protein.

Prior to the COVID-19 pandemic, there were a number of seasons with elevated incidence of scarlet fever and iGAS, in particular the 2017 to 2018 season.

During the pandemic there was an unprecedented reduction in the number of scarlet fever and iGAS notifications, affecting the 2019 to 2020 season and the 2021 to 2022 season.

References

1. Guy R, Henderson KL, Coelho J, Hughes H, Mason EL, Gerver SM, and others (2023). Increase in invasive group A streptococcal infection notifications, England, 2022. Eurosurveillance: volume 28, number 1

2. Public Health England (2018), Group A streptococcal infections: third report on seasonal activity, 2017 to 2018

3. Lamagni T, Guy R, Chand M, Henderson KL, Chalker V, Lewis J, and others (2018). ‘Resurgence of scarlet fever in England, 2014–16: a population based surveillance study’. The Lancet Infectious Diseases: volume 18, number 2, pages 180 to 187

4. Watts V, Balasegaram S, Brown CS, Mathew S, Mearkle R, Ready D, and others (2019). ‘Increased risk for Invasive Group A Streptococcus disease for household contacts of scarlet fever cases, England, 2011–2016’. Emerging Infectious Diseases: volume 25, number 3, pages 529 to 537

Acknowledgements

These reports would not be possible without the weekly contributions from microbiology colleagues in laboratories across England, without whom there would be no surveillance data.

The support from colleagues within UKHSA, and the AMRHAI Reference Unit in particular, is valued in the preparation of the report.

Feedback and specific queries about this report are welcome via [email protected]