1 Background

Group A Streptococcus (GAS) are common bacteria. GAS can be present on or in the human body, without causing infection which is known as colonisation. However these bacteria can cause skin, soft tissue and respiratory infections, including scarlet fever. GAS infections range from mild to very severe. Invasive GAS (IGAS) occurs when the bacteria proliferate in a normally sterile body site, such as the blood, soft tissues or joints. Streptococcal toxic shock syndrome (STSS) can be a severe complication of IGAS, caused by toxins that are produced by GAS, which act on host cells or tissues.

GAS is spread by close contact between individuals, through respiratory droplets and direct skin contact. GAS can also be transmitted indirectly through contact with inanimate objects, such as towels or bedding. GAS is usually diagnosed by microbiological culture of a specimen from the affected site or by a characteristic illness, such as scarlet fever.

During the COVID-19 pandemic there were a reduced number of GAS cases reported during 2020 and 2021, likely in part due to reduced social contact and increased use of preventive measures. The reduction might also partly reflect changes in healthcare seeking or access during that period.

During December 2022, there was a substantial increase in GAS and Scarlet fever cases across the UK, with the highest number of notifications in Northern Ireland recorded in January 2023. Notifications for GAS and Scarlet fever remained higher than usual between December 2022 to March 2023 before returning to usual seasonal levels towards the end of Spring 2023.

Public and clinician awareness of GAS infections has changed over time, especially during December 2022, which may contribute to changes in trends. IGAS, being more severe, is likely more consistent over time.

Detail of case definitions and methods are at the end of this document. Data were correct at 9am on 04 April 2025.

2 Cumulative number of scarlet fever and IGAS cases, 2022-2025.

Annual figures for Scarlet fever cases from 2022 onwards are shown in (Table 2.1). Reported cases of IGAS during the same time period are shown in (Table 2.2).

*Please note that 2025 is not a complete year. Data shown in the below figures is up to 2nd April 2025.

3 Scarlet Fever

3.1 Trends

Scarlet fever notifications in Northern Ireland slightly increased in December 2024 but have since steadily declined. Scarlet fever notifications during January to March 2025 remain lower than what was observed during the same period last year.

(Figure 3.1) shows incident cases by date of recording by PHA.

Supplementary Table 7.1 shows counts of probable and confirmed scarlet fever notifications recorded by week.

Figure 3.1: Incidence of scarlet fever notifications, by month of notification, all ages, 2016 - 2025.

Please note: in the figure above the most recent month is incomplete

4 Invasive Group A Streptococcal Infection

4.1 Trends

IGAS reports typically follow a seasonal pattern, with cases typically highest in spring. NI experienced unusually high levels in December 2022 and January 2023 (Figure 4.1). IGAS reports increased in Dec 2024, prior to decreasing from Jan 2025 onwards. Notifications of IGAS remain lower than the number of cases reported during the same period in the previous year.

Figure 4.1: Incidence of IGAS reports, by month of report, all ages, 2016 - 2025.

Please note: in the figure above the most recent month is incomplete

5 Discussion

There was an increase in scarlet fever incidence during January 2023, this increase was also observed in other European nations. There has been an increase in the number of IGAS infections compared to the pandemic period. While IGAS incidence has steadily fallen since December 2022 and January 2023, the monthly incidence remains at levels in keeping with previous moderate seasons.

6 Methods

Scarlet fever notifications and IGAS cases are reported by clinicians to the Public Health Agency and were extracted from the case management database. Laboratory reports of IGAS are also reported and validated with those reported by clinicians. Scarlet fever is notifiable in Northern Ireland under the Public Health Act (1967) Northern Ireland and IGAS is not notifiable.

Cases are presented by the date that they were recorded on the PHA database. Notified cases will be removed from future reports if PHA is notified that clinical investigations later result in an alternative diagnosis. Numbers may therefore reduce.

Data were processed in R.

7 Supplementary Table