2.1 Consultation rates for influenza/influenza-like-illness (‘flu/FLI’)

The GP flu/FLI consultation rate during week 18 was 3.6 per 100,000 population. This is slightly lower compared to week 17 (3.9 per 100,000 population). Activity remains at baseline levels (≤10.7 per 100,000 population) (Figure 2.1).

The 0-4 age group observed an increase in consultation rates when compared to week 17, while the 15-64 age group saw a decrease. The 5-14 and 65+ age ranges remained stable. The highest rate in week 18 was seen in the 15-64 age group (4.5 per 100,000 population).

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.

A supplementary table of Flu/FLI consultation rates by age group are shown at the end of this bulletin.

Figure 2.1: Northern Ireland GP consultation rates for ‘flu/FLI’ 2020/21 – 2023/24

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.

2.2 Episodes of influenza

All virology data provided here are preliminary. Virology data for current and prior weeks, as included in this or future bulletins, are subject to updates based on laboratory returns received after the last report was produced. The current bulletin offers the most current information available.

The number of new influenza episodes remained stable in week 18, with 19 unique episodes identified. There were 20 episodes reported in week 17 (Figure 2.2). Of the 19 episodes identified, two were Flu A(H3), five were Flu A (not subtyped) and 12 were Flu B.

Episode rates for the 0-4 and 65+ age groups remained at a similar level when compared to week 17. The 5-14 age group observed a decrease whilst the 15-64 age group saw an increase. The highest episode rate in week 18 was in the 0-4 age group (1.8, per 100,000 population) (Figure 2.3).

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

Figure 2.2: Weekly number of unique episodes of influenza, by epidemiological week

Figure 2.3: Weekly episode rates of influenza per 100,000 population, by age group, by epidemiological week

2.3 Episodes of respiratory syncytial virus

There were no new unique episodes of RSV identified in week 18. There was one episode reported in week 17 (Figure 2.4).

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

Figure 2.4: Weekly number of unique episodes of respiratory syncytial virus, by epidemiological week

Figure 2.5: Weekly episode rates of respiratory syncytial virus per 100,000 population, by age group, by epidemiological week

2.4 Virology

4.1 Inpatients and occupancy

There were seven community-acquired emergency influenza hospital admissions during week 18 (Figure 4.1).

The 7-day rolling average of daily cases of community-acquired emergency influenza A and influenza B inpatients decreased during week 18, remaining at low levels. (Figure 4.2).

Figure 4.1: Weekly number of community-acquired emergency influenza hospital admissions, by year and epidemiological week

Figure 4.2: 7-day rolling average of community-acquired emergency influenza inpatients

There were no community-acquired emergency RSV hospital admissions during week 18 (Figure 4.3), (Figure 4.4).

Figure 4.3: Weekly number of community-acquired emergency respiratory syncytial virus hospital admissions, by epidemiological week

Figure 4.4: 7-day rolling average of community-acquired emergency respiratory syncytial virus inpatients

4.2 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.

In week 18, 95 respiratory associated deaths out of 359 all-cause deaths were reported (26.5%). This is higher to the same period in 2022/23 (66 respiratory deaths out of 298 all-cause deaths, 22.2% (Figure 4.5) and (Figure 4.6).

Figure 4.5: Weekly number of deaths with respiratory keywords, to current registration week

Figure 4.6: Percentage of deaths with respiratory keywords (%), to current registration week

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

4.3 All-cause excess deaths (EuroMOMO)

In 2023/24, based on NISRA death registrations and the EuroMOMO model, excess deaths were reported in weeks 04, 05 and 06, 2024, particularly in those aged 65+. Despite delay correction, reported mortality data are still provisional due to the time delay in registration and observations which can vary from week to week; not all registrations for the current week will have been included this bulletin (Figure 4.7).

Figure 4.7: Weekly observed and expected number of all-cause deaths, in all ages, to current registration week

5.1 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.

5.2 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.

5.3 Episodes of infection

This bulletin 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.

5.4 Virology (including positivity)

All virology data provided here are preliminary. Virology data for prior weeks, as included in this or future bulletins, are subject to updates based on laboratory returns received after the last report was produced. The current bulletin offers the most current information available. Cumulative reports of influenza types may fluctuate from week to week, as Flu A (not subtyped) specimens may be subsequently typed in later reports.

Positive influenza results (dual positive influenza A and influenza B) can arise when vaccine virus is detected in a specimen taken from a person (e.g. a child under 16 years) who recently received intranasal administration of live attenuated influenza virus vaccine (LAIV). Therefore, the number of positive influenza results should be interpreted cautiously.

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.

5.5 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.

5.6 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) hospital data used for this bulletin following the introduction of a new electronic healthcare record on 6th November 2023. In the preceding influenza season, 95.7% of all community acquired influenza admissions were emergency admission. For this bulletin, all community acquired influenza and RSV admissions for SEHSCT are included from 6th November 2023 onwards (emergency and other). Work is ongoing to adapt systems and validation is ongoing.

5.7 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 (NIROS).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.

5.8 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.

6.1 Flu/FLI consultation rates per 100,000 population, by age group, over a six week period

6.2 Unique episodes of influenza and RSV by epidemiological week, over a six week period

6.3 Weekly influenza episode rates per 100,000 population, by age group, over a six week period

6.4 Weekly RSV episode rates per 100,000 population, by age group, over a six week period

6.5 Total sentinel tests and positivity for influenza and RSV by epidemiological week, over a six week period

6.6 Total non-sentinel tests and positivity for influenza and RSV by epidemiological week, over a six week period

6.7 Total influenza and RSV outbreaks, by subtype and setting, from week 40, 2023

More than one virus can be co-circulating at the same time in the same setting.