3.1 Consultation rates for flu/FLI

Since the beginning of the COVID-19 pandemic, the offer and uptake of GP consultations has changed. As a result, consultation rates in the most recent period are unlikely to be directly comparable to pre-pandemic and pandemic years.

The GP flu/FLI consultation rate exceeded the pre-epidemic threshold (low activity) of 10.7 per 100,000 population in week 01, 2024 and rates remained at low activity for a further eight weeks before returning to baseline activity in week 09, 2024 (Figure 3.1). The highest consultation rate was reported in week 04, 2024 (19.2 per 100,000 population). This peak was later and lower compared to the 2022/23 season.

Consultation rates were highest in the 0-4 age group in week 06, 2024 (22.4 per 100,000 population), followed by the 15-64 age group in week 04, 2024 (20.4 per 100,000 population).

A supplementary table of Flu/FLI consultation rates by age groups is shown at the end of this report.

Figure 3.1: Northern Ireland GP consultation rates for ‘flu/FLI’ 2019/20 – 2022/23

The baseline MEM threshold for Northern Ireland is 10.7 per 100,000 population for 2023-24. Low activity is 10.7 to <21.1, moderate activity 21.1 to <47.7, high activity 47.7 to <68.3 and very high activity is >68.3 per 100,000 population.

3.2 Episodes of influenza

The number of new influenza episodes increased steadily from week 48, 2023, with the highest number of unique episodes being reported in week 04, 2024 (500 episodes). The number of unique episodes of influenza B surpassed influenza A from week 17, 2024 (Figure 3.2). From week 40, 2023 to week 20, 2024, 463 episodes were typed as Flu A(H1), 1,580 were Flu A(H3), 2,350 were Flu A(not subtyped) and 211 were Flu B.

Episode rates were highest in the 0-4 age group in week 06, 2024 (77.0 per 100,000 population), followed by an earlier peak in week 04, 2024 (73.4 per 100,000 population). The 65+ age group reported the second highest peak in week 03, 2024 (52.2 per 100,000 population). The other age groups reported similar, but lower increases in episode rates during the same period in the influenza season. These peaks differ from what was reported in the 2022/23 season across age groups (Figure 3.3)

Supplementary tables of unique episodes and weekly episode rates by age groups are shown at the end of this report.

Figure 3.2: Weekly number of unique episodes of influenza, by epidemiological week, 2022/23

Figure 3.3: Weekly episode rates of influenza per 100,000 population, by age group, by epidemiological week, 2022/23

3.3 Episodes of RSV

The number of new RSV episodes began increasing later than what was seen in the 2022/23 season, with episodes steadily increasing from week 39, 2023. The highest number of unique episodes was reported in week 46, 2023 (175 episodes) (Figure 3.4). From week 40, 2022 to week 20, 2023, 1,570 episodes of RSV were identified.

Episode rates were highest in the 0-4 age group in week 46, 2023 (108.8 per 100,000 population). The other age groups saw small increases and the second highest episode rate was reported by the 65+ age group in week 47, 2023 (11.2 per 100,000 population) (Figure 3.5).

Supplementary tables of unique episodes and weekly episode rates by age groups are shown at the end of this report.

Figure 3.4: Weekly number of unique episodes of respiratory syncytial virus, by epidemiological week, 2022/23

Figure 3.5: Weekly episode rates of respiratory syncytial virus per 100,000 population, by age group, by epidemiological week, 2022/23

3.4 Virology

3.4.3 Sentinel and non-sentinel

Total influenza positivity peaked later and was higher than what was seen in the 2022/23 season, with positivity peaking at 21.9% in week 03, 2024 (501 samples were positive for influenza from 2,289 samples submitted) (Figure 3.6). Total RSV positivity peaked later and was lower than what was seen in the 2022/23 season, with positivity peaking at 18.3% in week 46, 2023 (186 samples were positive for RSV from 1,014 samples submitted) (Figure 3.7).

Figure 3.6: Sentinel and non-sentinel weekly positivity for influenza, by epidemiological week, 2019/20 – 2022/23

Figure 3.7: Sentinel and non-sentinel weekly positivity for RSV, by epidemiological week, 2019/20 – 2022/23

5.1 Inpatients and occupancy

There were a total of 2,018 community-acquired emergency influenza hospital admissions (Figure 5.1). The highest number of admissions were reported in week 04, 2024; 207 admissions, of which 19 were typed as Flu A(H1), 58 were Flu A(H3), 128 were Flu A(not subtyped) and two were Flu B. This peak was later and lower compared to the 2022/23 season. Overall, the 65+ age group represented 40.0% of all admissions.

The 7-day rolling average of daily cases of community-acquired emergency influenza inpatients followed a similar pattern, with influenza A being reported for majority of inpatients during the peak weeks of the influenza season. Peak occupancy was reported during week 04, 2024 (week ending 28 January 2024). Similar to admissions, this peak was later and lower compared to the 2022/23 season. Influenza B remained at low levels throughout the season (Figure 5.2).

Figure 5.1: Weekly number of community-acquired emergency influenza hospital admissions, by year and epidemiological week

Figure 5.2: 7-day rolling average of community-acquired emergency influenza inpatients

There were a total of 2,160 community-acquired emergency RSV hospital admissions (Figure 5.3). The highest number of admissions were reported in week 47, 2023; 121 admissions. This peak was later and higher compared to the 2022/23 season. Overall, the 0-4 age group represented 79.9% of all community-acquired emergency RSV hospital admissions.

The 7-day rolling average of daily cases of community-acquired emergency RSV inpatients followed a similar pattern, with peak occupancy being reported during week 47, 2023 (week ending 26 November 2023) (Figure 5.4).

Figure 5.3: Weekly number of community-acquired emergency respiratory syncytial virus hospital admissions, by epidemiological week

Figure 5.4: 7-day rolling average of community-acquired emergency respiratory syncytial virus inpatients

5.2 Medical certificate of cause of death for respiratory-associated deaths

The Northern Ireland Statistics and Research Agency (NISRA) provides the weekly number of respiratory-associated deaths and the proportion of all-cause registered deaths (by week of death registration, not by week of death).

Respiratory-associated deaths include those that are attributable to influenza, other respiratory infections or their complications. This includes “bronchiolitis, bronchitis, influenza or pneumonia” keywords recorded on the death certificate.

From week 40, 2023 to week 20, 2024, 3,115 respiratory associated deaths out of 11,928 all-cause deaths were reported (26.1%). This is similar to the 2023/23 influenza season (2,993 respiratory deaths out of 11,520 all-cause deaths, 26.0%). The highest proportion of respiratory-associated deaths was reported in week 07, 2024 (115 respiratory deaths out of 339 all-cause deaths, 33.9%) (Figure 5.5) and (Figure 5.6).

Figure 5.5: Weekly number of deaths with repiratory keywords, to current week, 2023

Figure 5.6: Percentage of deaths with repiratory keywords (%), to current week, 2023

Figures may be impacted by General Registration Office closures over public holidays.

5.3 All-cause excess deaths (EuroMOMO)

Based on NISRA death registrations and the EuroMOMO model, excess all-cause mortality for all ages was reported for one week during the season (week 44) (Figure 5.7).

Figure 5.7: Weekly observed and expected number of all-cause deaths, in all ages, to current week, 2023

6.1 Public Programme

In the population aged 65 years and older uptake was 78.0% (compared to 83.0% in 2022/23). Uptake was 31.1% in the population of 50-64 year olds in 2023/24, compared to 51.5% in 2022/23. Those aged 50-64 years were eligible for vaccination in 2022/23 for the duration of the programme, however the vaccination offer in 2023/24 was extended to this group from January 2024 onwards. Vaccine uptake in those aged 18 to 49 years at clinical risk was approximately 11.9% in 2023/24. Uptake in pregnant women (counted as mothers who delivered during the flu vaccine programme) was 24.9% in 2023/24 (based on data from four Trust areas) compared to 29.8% in 2022/23 (based on data from five Trust areas).

6.2 Healthcare workers

Uptake in Trust employed frontline health and social care workers was 21.5% in 2023/24 compared to 33.9% in 2022/23.

6.3 Influenza vaccine (LAIV) programme for children

In 2023/24 the childhood influenza vaccination programme continued to include all pre-school children aged two to four years old, all primary school aged children (years 1-7) and post primary school children in years 8 to 12. The former group were offered vaccination through primary care, with the latter two groups offered vaccination through school health teams. The vaccination uptake rate in 2023/24 for pre-school children aged two to four years old was 32.9% (compared to 33.0% in 2022/23). The vaccination uptake rate for children in primary school (aged approximately four to 11 years old) was 68.6% (compared to 70.7% in 2022/23). In 2021/2022, Northern Ireland expanded the vaccination programme from all year 8 children (introduced in 2020/21) to include post-primary school children (years 8 to 12) with an uptake rate of 56.5% in 2023/24. In 2022/23, uptake within this group was 61.0% These year groups were vaccinated through school clinics and a small number by GP.

8.1 Surveillance systems used to monitor influenza activity in Northern Ireland include:

• GP ‘flu/flu-like-illness’ (‘flu/FLI’) surveillance representing ~95% of the population - General Practice Intelligence Platform (GPIP).

• Sentinel GP practices representing ~18% of the population.

• Virological reports of influenza and RSV from the Regional Virus Laboratory (RVL) and all local laboratories - The Northern Ireland Health Analytics platform (NIHAP).

• Laboratory confirmed flu outbreak notifications reported to PHA Health Protection duty room.

• Hospital admissions and occupancy from the Patient Administration System (PAS) combined with infection episodes data from virological reports of influenza and RSV in NIHAP.

• Mortality data from Northern Ireland Statistics and Research Agency (NISRA) of selected respiratory infections (some of which may be attributable to influenza).

• Excess mortality estimations are calculated using the EuroMOMO (Mortality Monitoring in Europe) model based on raw death data supplied by NISRA.

• Vaccinations - Vaccine Management System (VMS) and Child Health System (school-based flu programme).

8.2 Presentation of data

Unless otherwise stated, data are presented using epidemiological weeks (a standardised method of counting weeks [Monday-Sunday] to allow for the comparison of data year after year). This is dependent on the data available. The data included in this report are the most up to date data available at the time of the report; however, this is subject to change as the data are subject to ongoing quality assurance.

8.3 Episodes of infection

This report includes information on episodes of both influenza and RSV infections.

For influenza infection episodes, they are defined by a rolling 42-day (6-week) period from the date of the first positive test result (utilising any test method, including PCR and Point of Care Tests, or source of sample, including hospital, GP, other source). This episode begins with the earliest positive specimen date. Any subsequent positive specimen dates within 42 days for the same individual are considered part of the same episode. Positive specimens for the same individual occurring more than 42 days after the last one are counted as a separate episode.

As for RSV infection episodes, the same methodology is employed, but with a rolling 14-day (2-week) period between positive test results.

Rates per 100,000 population are calculated using the NISRA 2021 Mid-Year Population Estimates.

8.4 Virology (including positivity)

SARS-CoV-2 and influenza testing capacity across the laboratory network was adversely affected in 2022/23 due to a critical failure in the detection of circulating Flu A(H1) using the main Roche CE-IVD multiplex PCR kit on the high throughput COBAS system. Significant genetic changes in circulating Flu A(H1) resulted in the performance failure of the influenza component of the Roche CAB (Covid/Flu A & B multiplex PCR kit) on the COBAS platform. This kit could no longer be used for testing in all Trusts excluding SEHSCT who continued to deliver testing using the Seegene platform. The Pathology Network position to consolidate this testing to RVL, Belfast to allow use of the alternative testing platform for influenza testing was necessary for winter 2022/23 in the absence of a replacement assay by the manufacturer.

In contrast to influenza episodes, sentinel and non-sentinel influenza virological data are managed separately. Instead of utilising an episode-based approach, the data is analysed on an epidemiological week basis. Within each epidemiological week, an individual is limited to one influenza test, whether positive or negative, with separate reports for sentinel and non-sentinel virological data. If an individual tests positive for influenza during a specific epidemiological week and subsequently tests positive again within the same week, the second positive test is not counted. Regardless of whether it occurs before or after a negative test within the same epidemiological week, a positive test always takes precedence and is recorded. Similarly, only the first test of multiple negative results is counted for each individual within any given epidemiological week. This helps prevent the double-counting of tests, particularly for individuals who may be hospitalised and routinely tested.

The same methodology is applied when analysing RSV data.

8.5 Influenza and respiratory syncytial virus (RSV) outbreaks

PHA conducts surveillance of outbreaks across multiple settings, including care homes (nursing homes and residential homes) in NI that are registered with the Regulation and Quality Improvement Agency. All care homes have a requirement to notify the PHA Health Protection duty room of suspected outbreaks of any infectious disease. A confirmed outbreak of influenza or RSV can be defined as where there are two or more confirmed cases with onset within a 14 day period, where transmission within the Care Home facility is considered the likely cause.

8.6 Admissions and occupancy

Community-acquired influenza and RSV emergency admissions to acute hospitals are estimated by combining data from PAS and virological reports in NIHAP. Admissions are counted where there was a positive test up to seven days before admission or up to one day after admission, and the method of admission was ‘Emergency’. The number of inpatients is counted at midnight. Admissions and occupancy refer to the first admission per infection episode. It is not currently possible to distinguish emergency from other community acquired admissions in the South Eastern Health and Social Care Trust (SEHSCT) and Belfast Health and Social Care Trust (BHSCT) hospital data used for this bulletin following the introduction of a new electronic healthcare record on 6th November 2023 and 6th June 2024, respectively. In the preceding influenza season, 95.7% (SEHSCT) and 89.1% (BHSCT) of all community acquired influenza admissions were emergency admission. For this bulletin, all community acquired influenza and RSV admissions for SEHSCT (from 6th November 2023 onwards) and BHSCT (from 6th June 2024 onwards) are included (emergency and other). Work is ongoing to adapt systems and validation is ongoing.

8.7 Medical certificate of cause of death for respiratory-associated deaths

PHA report weekly counts of selected respiratory infection death registrations in NI, as supplied by NISRA. Deaths occurring in NI are registered on the NI General Register Office’s Registration System.Provisional data on deaths registered in each week (ending on a Friday) are compiled at the end of the following week. The data presented here is based on registrations of deaths, not occurrences. The majority of deaths are registered within five days in NI. The selected respiratory infections include deaths due to influenza, bronchitis, bronchiolitis, and pneumonia. These figures may be impacted by General Registration Office closures over public holidays.

8.8 Excess mortality surveillance

PHA reports the weekly number of excess deaths from any cause in NI using the Mortality Monitoring in Europe (EuroMOMO) model. EuroMOMO provides a coordinated, timely and standardised approach to monitoring and analysing mortality data across the UK and Europe. Based on mortality data supplied by NISRA, the EuroMOMO model produces the number of expected and observed deaths every week, corrected for reporting delay and standardised for the population by age group and region. Excess mortality is defined as a statistically significant increase in the number of deaths reported over the expected number for a given point in time. Results are provisional due to the time delay in deaths registration.

8.9 Vaccination

In Northern Ireland, the uptake of seasonal influenza vaccine is monitored by the Public Health Agency (PHA) of Northern Ireland. From 2021/22 onwards influenza vaccine uptake has been determined using data extracted from regional Immunisation Information System developed by the Department of Health (DoH) Digital team; known as the Vaccine Management System (VMS). Influenza vaccinations administered by Trust School Nursing Teams are recorded in the Child Health System (CHS), similar to previous seasons. Caution should be used when considering influenza vaccination uptake rates from 2021/22 in comparison to previous seasons, due to the introduction of the VMS involving new methods of recording and extracting influenza vaccine data. Uptake in pregnant women is based on data available from four Health and Social Care Trusts due to the introduction of a new data system, Encompass. A surveillance definition of frontline health and social care workers has been applied to estimate uptake. Uptake figures may be subject to revision with improvements in data collection and reporting.

9.1 Flu/FLI consultation rates per 100,000 population, by age group, 2022-2023

9.2 Unique episodes of influenza and RSV by epidemiological week, 2022-2023

9.3 Weekly influenza episode rates per 100,000 population, by age group, 2022-2023

9.4 Weekly RSV episode rates per 100,000 population, by age group, 2022-2023

9.5 Total sentinel tests and positivity for influenza and RSV by epidemiological week, 2022-2023

9.6 Total non-sentinel tests and positivity for influenza and RSV by epidemiological week, 2022-2023

3.1 Consultation rates for flu/FLI

Since the beginning of the COVID-19 pandemic, the offer and uptake of GP consultations has changed. As a result, consultation rates in the most recent period are unlikely to be directly comparable to pre-pandemic and pandemic years.

The GP flu/FLI consultation rate exceeded the pre-epidemic threshold (low activity) of 11.3 per 100,000 population in week 49, 2022 and reached moderate activity in week 51, 2022 (≥21.8 per 100,000 population). Rates remained at moderate activity for a further two weeks before returning to low activity in week 2, 2023, and then returning to baseline activity from week 3, 2023 and remained low until the end of the season (Figure 3.1). The highest consultation rate was reported in week 51, 2022 (35.8 per 100,000 population).

Consultation rates were highest in the 0-4 age group in week 51, 2022 (40.9 per 100,000 population), followed by the 65+ age group in week 1, 2023 (39.9 per 100,000 population).

Supplementary tables of Flu/FLI consultation rates by Local Government District (LGD) and age groups are shown at the end of this report.

Figure 3.1: Northern Ireland GP consultation rates for ‘flu/FLI’ 2019/20 – 2022/23

The baseline MEM threshold for Northern Ireland is 11.3 per 100,000 population for 2022-23. Low activity is 11.3 to <21.8, moderate activity 21.8 to <57.0, high activity 57.0 to <87.1 and very high activity is ≥87.1 per 100,000 population.

3.2 Episodes of influenza

The number of new influenza episodes increased steadily from week 40, 2022, with the highest number of unique episodes being reported in week 52, 2022 (636 episodes). The number of unique episodes of influenza B surpassed influenza A from week 6, 2023 (Figure 3.2). From week 40, 2022 to week 20, 2023, 1,119 episodes were typed as Flu A(H1), 498 were Flu A(H3), 1,591 were Flu A(not subtyped) and 487 were Flu B.

Episode rates were highest in the 65+ age group in week 52, 2022 (83.5 per 100,000 population), followed by the 0-4 age group in the same week (71.7 per 100,000 population). The other age groups reported similar, but lower increases in episode rates during the same period in the influenza season (Figure 3.3).

Supplementary tables of unique episodes and weekly episode rates by age groups are shown at the end of this report.

Figure 3.2: Weekly number of unique episodes of influenza, by epidemiological week, 2022/23

Figure 3.3: Weekly episode rates of influenza per 100,000 population, by age group, by epidemiological week, 2022/23

3.3 Episodes of RSV

The number of new RSV episodes began increasing before the beginning of the influenza season, with episodes beginning to increase from week 27, 2022 (week ending 10 July 2022). The highest number of unique episodes was reported in week 44, 2022 (159 episodes) (Figure 3.4). From week 40, 2022 to week 20, 2023, 1,443 episodes of RSV were identified.

Episode rates were highest in the 0-4 age group in week 44, 2022 (115.0 per 100,000 population). The other age groups saw small increases and the second highest episode rate was reported by the 65+ age group in week 52, 2022 (7.0 per 100,000 population) (Figure 3.5).

Supplementary tables of unique episodes and weekly episode rates by age groups are shown at the end of this report.

Figure 3.4: Weekly number of unique episodes of respiratory syncytial virus, by epidemiological week, 2022/23

Figure 3.5: Weekly episode rates of respiratory syncytial virus per 100,000 population, by age group, by epidemiological week, 2022/23

3.4 Virology

3.4.3 Sentinel and non-sentinel

Total influenza positivity was highest in week 52, 2022; 647 samples were positive for influenza from 3,262 samples submitted (19.8% positivity). Total RSV positivity was highest in week 41, 2022; 147 samples were positive for RSV from 586 samples submitted (25.1% positivity) (Figure 3.6 and (Figure 3.7).

Figure 3.6: Sentinel and non-sentinel weekly positivity for influenza, by epidemiological week, 2019/20 – 2022/23

Figure 3.7: Sentinel and non-sentinel weekly positivity for RSV, by epidemiological week, 2019/20 – 2022/23

5.1 Inpatients and occupancy

There were a total of 1,761 community-acquired emergency influenza hospital admissions (Figure 5.1). The highest number of admissions were reported in week 52, 2022; 342 admissions, of which 88 were typed as Flu A(H1), 45 were Flu A(H3), 204 were Flu A(not subtyped) and 5 were Flu B. Overall, the 15-64 and 65+ age groups represented 36.6% and 36.9% of all admissions, respectively.

The 7-day rolling average of daily cases of community-acquired emergency influenza inpatients followed a similar pattern, with influenza A being reported for majority of inpatients during the peak weeks of the influenza season. Peak occupancy was reported during week 1, 2023 (week ending 08 January 2023). Influenza B surpassed influenza A later in the season, in line with other influenza indicators (Figure 5.2).

Figure 5.1: Weekly number of community-acquired emergency influenza hospital admissions, by year and epidemiological week

Figure 5.2: 7-day rolling average of community-acquired emergency influenza inpatients

There were a total of 914 community-acquired emergency RSV hospital admissions (Figure 5.3). In line with other RSV indicators, peak admissions occurred early in the influenza season. The highest number of admissions were reported in week 44, 2022, 115 hospital admissions were reported. Overall, the 0-4 age group represented 80.3% of all community-acquired emergency RSV hospital admissions.

The 7-day rolling average of daily cases of community-acquired emergency RSV inpatients followed a similar pattern, with peak occupancy being reported during week 45, 2022 (week ending 13 November 2022) (Figure 5.4).

Figure 5.3: Weekly number of community-acquired emergency respiratory syncytial virus hospital admissions, by epidemiological week

Figure 5.4: 7-day rolling average of community-acquired emergency respiratory syncytial virus inpatients

5.2 Medical certificate of cause of death for respiratory-associated deaths

The Northern Ireland Statistics and Research Agency (NISRA) provides the weekly number of respiratory-associated deaths and the proportion of all-cause registered deaths (by week of death registration, not by week of death).

Respiratory-associated deaths include those that are attributable to influenza, other respiratory infections or their complications. This includes “bronchiolitis, bronchitis, influenza or pneumonia” keywords recorded on the death certificate.

From week 40, 2022 to week 20, 2023, 2,993 respiratory associated deaths out of 11,520 all-cause deaths were reported (26.0%). This is higher to the 2021/22 influenza season (2,796 respiratory deaths out of 11,370 all-cause deaths, 24.3%). The highest proportion of respiratory-associated deaths was reported in week 2, 2023 (171 respiratory deaths out of 515 all-cause deaths, 33.2%) (Figure 5.5) and (Figure 5.6).

Figure 5.5: Weekly number of deaths with repiratory keywords, to current week, 2023

Figure 5.6: Percentage of deaths with repiratory keywords (%), to current week, 2023

Figures may be impacted by General Registration Office closures over public holidays.

5.3 All-cause excess deaths (EuroMOMO)

Based on NISRA death registrations and the EuroMOMO model, excess all-cause mortality for all ages was reported for eight weeks during the season (weeks 42-44, 48, 50-52 and 01) (Figure 5.7).

Figure 5.7: Weekly observed and expected number of all-cause deaths, in all ages, to current week, 2023

6.1 Public Programme

In the population aged 65 years and over uptake was 83.0% (compared with 57.7% in 2021 to 2022). Uptake was 51.4% in the population of 50 to 64 year olds in 2022 to 2023, compared with 43.5% in 2021 to 2022. The first year of rollout in this age group was 2020 to 2021. Note that the Vaccine Management System (VMS) does not currently collect data on the specific clinical risk conditions for influenza vaccination. Uptake in pregnant women (counted as mothers who delivered during the flu vaccine programme) was 29.8% in 2022 to 2023 compared with 45.9% in 2021 to 2022.

6.2 Healthcare workers

Uptake in frontline health and social care workers including social care was 37.5% in 2022 to 2023 compared with 49.8% in 2020 to 2021. Uptake in 2021 to 2022 is not comparable as it was not possible to disaggregate frontline and non-frontline staff. It should be noted that data sources differ between 2020 to 2021 and 2022 to 2023.

6.3 Influenza vaccine (LAIV) programme for children

Caution should be used when considering the 2021 to 2022 and 2022 to 2023 influenza vaccination uptake rates in pre-school children in comparison with previous seasons, due to the introduction of the new VMS involving new methods of recording and extracting influenza vaccine data. Influenza vaccinations administered by trust school nursing teams are recorded in the Child Health System, similar to previous seasons.

In 2022 to 2023, the childhood influenza vaccination programme continued to include all pre-school children aged 2 to 4 years old, all primary school aged children (year groups 1 to 7) and post primary school children in year groups 8 to 12. The former group was offered vaccination through primary care, while the latter two groups were offered vaccination through school health teams. The vaccination uptake rate in 2022 to 2023 for pre-school children aged 2 to 4 years old was 33.0% (compared with 25.4% in 2021 to 2022). The vaccination uptake rate for children in primary school (aged approximately 4 to 11 years old) was 70.6% (compared with 72.7% in 2021 to 2022). In 2021 to 2022, Northern Ireland expanded the vaccination programme from all year 8 children (introduced in 2020 to 2021) to include post-primary school children (years 8 to 12) with an uptake rate of 63.8%. In 2022 to 2023, uptake within this group was 60.2%. These year groups were vaccinated through school clinics.

8.1 Surveillance systems used to monitor influenza activity in Northern Ireland include:

• GP ‘flu/flu-like-illness’ (‘flu/FLI’) surveillance representing ~95% of the population - General Practice Intelligence Platform (GPIP).

• Sentinel GP practices representing ~8% of the population.

• Virological reports of influenza and RSV from the Regional Virus Laboratory (RVL) and all local laboratories - The Northern Ireland Health Analytics platform (NIHAP).

• Laboratory confirmed flu outbreak notifications reported to PHA Health Protection duty room.

• Hospital admissions and occupancy from the Patient Administration System (PAS) combined with infection episodes data from virological reports of influenza and RSV in NIHAP.

• Mortality data from Northern Ireland Statistics and Research Agency (NISRA) of selected respiratory infections (some of which may be attributable to influenza).

• Excess mortality estimations are calculated using the EuroMOMO (Mortality Monitoring in Europe) model based on raw death data supplied by NISRA.

• Vaccinations - Vaccine Management System (VMS) and Child Health System (school-based flu programme).

8.2 Presentation of data

Unless otherwise stated, data are presented using epidemiological weeks (a standardised method of counting weeks [Monday-Sunday] to allow for the comparison of data year after year). This is dependent on the data available. The data included in this report are the most up to date data available at the time of the report; however, this is subject to change as the data are subject to ongoing quality assurance.

8.3 Episodes of infection

This report includes information on episodes of both influenza and RSV infections.

For influenza infection episodes, they are defined by a rolling 42-day (6-week) period from the date of the first positive test result (utilising any test method, including PCR and Point of Care Tests, or source of sample, including hospital, GP, other source). This episode begins with the earliest positive specimen date. Any subsequent positive specimen dates within 42 days for the same individual are considered part of the same episode. Positive specimens for the same individual occurring more than 42 days after the last one are counted as a separate episode.

As for RSV infection episodes, the same methodology is employed, but with a rolling 14-day (2-week) period between positive test results.

Rates per 100,000 population are calculated using the NISRA 2021 Mid-Year Population Estimates.

8.4 Virology (including positivity)

SARS-CoV-2 and influenza testing capacity across the laboratory network was adversely affected in 2022/23 due to a critical failure in the detection of circulating Flu A(H1) using the main Roche CE-IVD multiplex PCR kit on the high throughput COBAS system. Significant genetic changes in circulating Flu A(H1) resulted in the performance failure of the influenza component of the Roche CAB (Covid/Flu A & B multiplex PCR kit) on the COBAS platform. This kit could no longer be used for testing in all Trusts excluding SEHSCT who continued to deliver testing using the Seegene platform. The Pathology Network position to consolidate this testing to RVL, Belfast to allow use of the alternative testing platform for influenza testing was necessary for winter 2022/23 in the absence of a replacement assay by the manufacturer.

In contrast to influenza episodes, sentinel and non-sentinel influenza virological data are managed separately. Instead of utilising an episode-based approach, the data is analysed on an epidemiological week basis. Within each epidemiological week, an individual is limited to one influenza test, whether positive or negative, with separate reports for sentinel and non-sentinel virological data. If an individual tests positive for influenza during a specific epidemiological week and subsequently tests positive again within the same week, the second positive test is not counted. Regardless of whether it occurs before or after a negative test within the same epidemiological week, a positive test always takes precedence and is recorded. Similarly, only the first test of multiple negative results is counted for each individual within any given epidemiological week. This helps prevent the double-counting of tests, particularly for individuals who may be hospitalised and routinely tested.

The same methodology is applied when analysing RSV data.

8.5 Influenza and respiratory syncytial virus (RSV) outbreaks

PHA conducts surveillance of outbreaks across multiple settings, including care homes (nursing homes and residential homes) in NI that are registered with the Regulation and Quality Improvement Agency. All care homes have a requirement to notify the PHA Health Protection duty room of suspected outbreaks of any infectious disease. A confirmed outbreak of influenza or RSV can be defined as where there are two or more confirmed cases with onset within a 14 day period, where transmission within the Care Home facility is considered the likely cause.

8.6 Admissions and occupancy

Community-acquired influenza and RSV emergency admissions to acute hospitals are estimated by combining data from PAS and virological reports in NIHAP. Admissions are counted where there was a positive test up to seven days before admission or up to one day after admission, and the method of admission was ‘Emergency’. The number of inpatients is counted at midnight. Admissions and occupancy refer to the first admission per infection episode.

8.7 Medical certificate of cause of death for respiratory-associated deaths

PHA report weekly counts of selected respiratory infection death registrations in NI, as supplied by NISRA. Deaths occurring in NI are registered on the NI General Register Office’s Registration System.Provisional data on deaths registered in each week (ending on a Friday) are compiled at the end of the following week. The data presented here is based on registrations of deaths, not occurrences. The majority of deaths are registered within five days in NI. The selected respiratory infections include deaths due to influenza, bronchitis, bronchiolitis, and pneumonia. These figures may be impacted by General Registration Office closures over public holidays.

8.8 Excess mortality surveillance

PHA reports the weekly number of excess deaths from any cause in NI using the Mortality Monitoring in Europe (EuroMOMO) model. EuroMOMO provides a coordinated, timely and standardised approach to monitoring and analysing mortality data across the UK and Europe. Based on mortality data supplied by NISRA, the EuroMOMO model produces the number of expected and observed deaths every week, corrected for reporting delay and standardised for the population by age group and region. Excess mortality is defined as a statistically significant increase in the number of deaths reported over the expected number for a given point in time. Results are provisional due to the time delay in deaths registration.

8.9 Vaccination

From 2021 to 2022 onwards, influenza vaccine uptake has been determined using data extracted from regional Immunisation Information System developed by the Department of Health Digital team; known as the Vaccine Management System (VMS). Caution should be used when considering the 2021 to 2022 and 2022 to 2023 influenza vaccination uptake rates in comparison with previous seasons, due to the introduction of the new VMS involving new methods of recording and extracting influenza vaccine data.

9.1 Flu/FLI consultation rates per 100,000 population, by age group, 2022-2023

9.2 Unique episodes of influenza and RSV by epidemiological week, 2022-2023