Influenza activity in Ireland is continuing to increase, with case and hospitalisation numbers both rising in recent weeks
Influenza, commonly referred to as the flu, is a highly contagious and communicable respiratory illness caused by influenza viruses.1 There are four types of influenza viruses, A, B, C, and D. Influenza types A and B cause human infection annually and are notable for their ability to cause seasonal epidemics and, occasionally, pandemics, leading to significant morbidity and mortality worldwide.2 Influenza A(H3), A(H1)pdm09, and influenza B viruses have all been detected in Ireland this season.3
Influenza viruses spread through respiratory droplets released when a person coughs, talks, or sneezes. They can also be transmitted if people touch surfaces contaminated with the virus and then touch their nose, mouth, or eyes. A person with influenza can spread the virus even before they show symptoms and continue to do so for five-to-seven days after becoming infected. Following an infection, most healthy individuals recover in seven-to-10 days. However, complications such as pneumonia and death are more prevalent among certain high-risk groups.1
Those at higher risk of experiencing severe illness or complications from an influenza infection include pregnant women, children under five, the elderly, people with chronic medical conditions such as heart, lung, kidney, metabolic, neurodevelopmental, liver, or blood disorders, and individuals with compromised immune systems such as those with HIV, those undergoing chemotherapy or steroid treatment, or those dealing with malignancies. Healthcare workers have an increased risk of contracting the influenza virus due to their frequent exposure to patients, which also increases the likelihood of spreading the virus to vulnerable populations.1,2
Influenza surveillance and trends in Ireland (2019-2023)
In Ireland, the Health Protection Surveillance Centre (HPSC) is responsible for monitoring influenza activity through a comprehensive national surveillance system. The influenza season in Ireland runs from October to May, with peak activity often observed between December and February, impacting the healthcare system with increased hospital admissions and extra pressure on general practice services.3
The period from 2019-2023 saw significant changes in influenza activity in Ireland, influenced largely by the Covid-19 pandemic and the resultant public health measures.3 During the 2019-2020 influenza season, Ireland experienced over 4,000 laboratory-confirmed cases of influenza. This season was marked by a substantial number of hospital admissions due to influenza complications, particularly affecting vulnerable populations such as the elderly and those with underlying health conditions.3
The 2020-2021 influenza season was unprecedented due to the Covid-19 pandemic. Public health interventions aimed at controlling Covid-19 such as lockdowns, social distancing, mask mandates, and enhanced hygiene practices significantly reduced influenza transmission. Consequently, the HPSC reported historically low influenza levels, with minimal laboratory-confirmed cases and reduced influenza-like illness (ILI) rates. Additionally, there was an increased uptake of the influenza vaccine driven by heightened public awareness of respiratory illnesses, resulting in higher vaccination rates among high-risk groups compared to previous seasons. Ireland had the highest vaccination rate in the EU, with 75.4 per cent of people over 65 being vaccinated against influenza in 2021.3
As Covid-19 restrictions eased during the 2021-2022 influenza season, there was concern about the potential resurgence of influenza. The HPSC observed a gradual increase in influenza activity, though it remained below pre-pandemic levels.3
During the 2022-2023 season, influenza activity in Ireland normalised as public health measures for Covid-19 were largely relaxed. Influenza activity returned closer to pre-pandemic levels, with a significant number of laboratory-confirmed cases and increased ILI rates. While vaccine uptake remained strong, there was a slight decline compared to the previous season.3
Presentation
Influenza presents with a wide range of symptoms, from mild-to-severe. The clinical presentation often includes the sudden onset of high fever, chills, sweats, headache, myalgia, fatigue, and weakness. Respiratory symptoms such as nasal congestion, sore throat, and a severe cough are also common. Gastrointestinal symptoms such as nausea, vomiting, and diarrhoea may occur, especially in children.1,6
Complications of influenza can be severe and include viral pneumonia, secondary bacterial pneumonia, sinus infections, and exacerbation of chronic medical conditions such as asthma, chronic obstructive pulmonary disease, and cardiovascular disease. Vigilance is necessary in populations at higher risk for complications, including young children, the elderly, pregnant women, and individuals with underlying health conditions.1,6,7
Diagnosis
The diagnosis of influenza is primarily clinical, based on the presence of typical signs and symptoms during the influenza season. However, laboratory testing can confirm the diagnosis and is particularly useful during outbreaks or for patients with severe disease.1,2,6,7
Several diagnostic methods are available. Rapid influenza diagnostic tests detect viral antigens in respiratory specimens and provide results within 15 to 30 minutes. While convenient, they have variable sensitivity and may produce false-negative results. Reverse transcription polymerase chain reaction (RT-PCR) is the most sensitive and specific method for detecting influenza viruses. It can differentiate between influenza A and B and identify specific subtypes. Results are typically available within a few hours to a few days. A chest x-ray should be obtained in patients with pulmonary symptoms to
exclude bacterial pneumonia.1,2,6,7
Treatment
The treatment of influenza includes antiviral medications and supportive care. Initiating antiviral treatment within 36 to 48 hours of symptom onset for at-risk groups can lower the risk of influenza complications, such as otitis media in young children, pneumonia, and respiratory failure. It can also shorten the duration of illness in acutely ill patients and reduce morbidity, hospitalisation, and mortality in those with severe infections. Symptomatic treatment, however, is the recommended approach for previously healthy individuals, excluding pregnant women, unless the clinician believes the patient is very ill or at high risk for influenza complications.8
Two antiviral medications are recommended for use in Ireland during the influenza season – oral oseltamivir and inhaled zanamivir. These antiviral NA inhibitors exert activity against seasonal influenza A and B. NA inhibitors block the release of new virions (entire virus particles) from infected cells. Oseltamivir is the most used antiviral and is approved for treatment and prophylaxis of influenza in individuals aged one year and older. Zanamivir is recommended for patients who cannot take oral medications or for those with oseltamivir-resistant strains. Antivirals are recommended for pregnant women due to the adverse clinical outcomes that have been observed for influenza in this group. Oseltamivir is the first-line option for most pregnant women with influenza, including during seasons that are dominated by influenza A (H1N1).
Chemoprophylaxis should be reserved for at-risk individuals who have recently been in close contact with someone with influenza or an influenza-like illness within the same household or residential setting. Prior influenza vaccination does not rule out the use of post-exposure prophylaxis, especially in cases of localised outbreaks in residential care facilities.
Supportive care includes maintaining adequate hydration, rest, and the use of over-the-counter medications to relieve symptoms. For severe cases, hospitalisation may be required and supportive measures such as oxygen therapy, mechanical ventilation, and treatment of secondary bacterial infections may be necessary.
Prevention
In Ireland, the HSE is providing free flu vaccines through participating GPs and pharmacies to high-risk groups, including individuals aged 60 and over, all children aged two-to-17 years (nasal vaccine), pregnant women (at any stage of pregnancy), healthcare workers (staff and students in healthcare and social care), people with underlying medical conditions including chronic heart, lung, kidney, liver or neurological disease, cancer, weak immune system, diabetes, serious mental health conditions, BMI ≥40, children on long-term aspirin therapy; carers and household contacts of people at higher risk, residents of nursing homes and other long-stay facilities, and people with regular close contact with poultry, waterfowl, or pigs.4
Other preventive measures include good respiratory hygiene, such as covering the mouth and nose with a tissue or elbow when coughing or sneezing, frequent hand washing with soap and water, and the use of alcohol-based hand sanitisers. During peak flu season, avoiding close contact with infected individuals and wearing masks in crowded or high-risk settings can further reduce the spread of the virus.2,5,6
Antiviral resistance
The emergence of antiviral-resistant influenza strains is a growing concern. Continuous monitoring and research are necessary to identify resistant strains and develop new antiviral agents. Potential strategies include combination antiviral therapy and the development of broad-spectrum antivirals to enhance treatment efficacy and reduce the risk of resistance.2,6,7
Conclusion
Influenza remains a significant public health concern, causing substantial morbidity and mortality each year. Efforts to enhance surveillance, increase vaccination rates, and educate the public on preventive measures are important to mitigate the impact of influenza. The expansion of free flu vaccination in recent years is a positive development, aimed at protecting more high-risk individuals from the disease.
References
Boktor SW, Hafner JW. Influenza. [Updated 2023 Jan 23]. In: StatPearls [Internet]. Treasure Island: StatPearls Publishing; 2024. Available at: www.ncbi.nlm.nih.gov/books/NBK459363/
World Health Organisation. Influenza (Seasonally). Geneva: WHO; 2024. Available at: www.who.int/news-room/fact-sheets/detail/influenza-(seasonal)
Health Protection Surveillance Centre. Influenza surveillance. Ireland: HPSC; 2025. Available at: www.hpsc.ie/az/respiratory/influenza/seasonalinfluenza/surveillance/
Uyeki T, Hui D, Zambon M, et al. Influenza. Lancet. 2022; 400(10355):693-705
Health Service Executive. Influenza. Ireland: HSE; 2025. Available at: www2.hse.ie/conditions/flu/symptoms-diagnosis/
Medscape [internet]. Influenza: Clinical presentation. 2024. Available at: https://emedicine.medscape.com/article/219557-clinical
HSE antibiotic prescribing (2024). Influenza (Seasonal). Health Service Executive, Ireland. Available at: www.hse.ie/eng/services/list/2/gp/antibiotic-prescribing/conditions-and-treatments/influenza/
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