1.1.1 Geographical Disparity in Seasonal Influenza Vaccination

Considering the WHO recommendation that 75% of older adults should be vaccinated against influenza each year, there are significant gaps in influenza immunisation in Europe, and vaccination rates vary over time and between countries in this continent [6, 29]. Data from 2018 to 2019 showed that only one of 28 countries reporting coverage in the WHO European Region achieved that target (Belarus), and only four other countries achieved greater than 60% coverage (Ireland, Portugal, Russian Federation, and the UK) [29]. Vaccination coverage was less than 35% of the older population in half the countries during this period, with the lowest coverage less than 1% [29]. Few countries had mechanisms to monitor vaccination coverage in target groups such as older adults [29].

Data from 2021 confirm that influenza vaccination rates in older populations vary between EU Member States, ranging from the highest rate in Ireland (75.4%), followed by Denmark (75.0%), the Netherlands (72.6%) and Spain (67.7%), to low rates in Slovakia (12.9%), Poland (10%), Bulgaria (8.9%) and Latvia (7.7%) [6]. Overall, just over half (50.8%) of individuals aged at least 65 years in the EU were vaccinated against influenza in 2021 [6].

1.1.2 Disparity in Seasonal Influenza Vaccination Between Different Ethnic Groups

Research into disparities in influenza vaccination between different ethnic populations is primarily conducted in the USA, with Black, Hispanic, and Asian individuals noted to be substantially less likely to receive influenza vaccine than non-Hispanic White individuals [36-40]. Disparities in maternal vaccine acceptance among ethnic groups in the USA are well documented, with Black women the least confident in the safety of the maternal influenza vaccine [41]. In addition, Black and Hispanic women were less likely than White women to perceive the risk of acquiring vaccine-preventable diseases and to trust vaccine information from healthcare providers and public health authorities [41].

In a study of at-risk adults in England, relative to White patients, Black patients were least likely to be vaccinated (odds ratio [OR]18–64 years: 0.82, and OR65+ years: 0.59), whereas Asian patients among 18–64 year olds were most likely to be vaccinated (OR18–64 years: 1.10) [42]. Another study in England showed that compared to the ‘White British’ group, inequalities in influenza vaccine uptake were most prominent in the groups ‘White and Black Caribbean’ (hazard ratio [HR] 0.63) and ‘White and Black African’ (HR 0.67) [43]. In contrast, uptake was slightly higher than the White British group in the groups ‘other ethnic group’ (HR 1.11) and ‘Bangladeshi’ (HR 1.08) [43]. Lower uptake rates for influenza vaccine were also linked to nonwhite ethnicity among pregnant women in an additional study in England, with the lowest among Black women [44].

In addition, lower seasonal influenza vaccine uptake compared with the host population has been reported in immigrants [45], and certain mobile population groups, such as some Gypsy, Roma, Travellers and migrants [46].

1.1.3 Socioeconomic Disparity in Seasonal Influenza Vaccination

Socioeconomic position is one of the principal determinants of health, and affects an individual's ability to fully comply with preventive measures, including vaccination, and healthcare prescriptions [47]. Socioeconomic deprivation is correlated with low influenza vaccination coverage among groups at risk, such as older adults [42, 47, 48]. Effective targeted interventions to increase influenza vaccination rates should take account of socioeconomic determinants of inequalities in vaccine uptake [45, 49].

Living alone is an overlooked factor in vaccination programmes, particularly in older adults [45]. Higher seasonal influenza vaccine uptake in individuals aged ≥ 60 years from Europe was reported for those not living alone (OR 1.39), with higher income (OR 1.26) and with higher education (OR 1.05) [45]. Lower seasonal influenza vaccine uptake was observed in populations in more deprived areas (risk ratio 0.93) [45].

A study of differences in vaccination coverage, by socioeconomic position, among people aged ≥ 65 years in Italy showed that belonging to families with three or four members was associated with increased coverage rates [49]. In the most deprived group, vaccine uptake was positively associated with the dependency ratio (a measure of how many people depend on the working-age population for economic support [49, 50].

Vaccine uptake levels in the homeless population have been reported to be less than half those seen among eligible general practitioner (GP) patient groups in a study in England [51]. This finding clearly shows the health inequalities and ‘inverse care law’ that impact this vulnerable population, and highlights the need to strengthen efforts to ensure homeless individuals have access to influenza vaccination [51].

Socioeconomic inequalities in annual seasonal influenza vaccine uptake were exacerbated during the COVID-19 pandemic, as noted in a study in England, likely due to changes in demand for vaccination, new delivery models, and disruptions to healthcare and schooling [52]. It is crucial to improve understanding of drivers for vaccine uptake inequalities and to address and reverse these growing inequalities [52].

1.1.4 Disparities in Seasonal Influenza Vaccine Uptake Related to Sex and Gender

Biological (sex) differences as well as gender norms, roles, and relations can lead to differences in males and females in terms of acceptance of vaccination [53, 54]. This variability is seldom taken into account in the planning and execution of vaccination strategies [54].

A multi-site, retrospective chart review study in the USA comprising 1193 patients showed that the rate of influenza vaccination is higher for females than for males who presented for an annual preventive physical examination and who were younger than 75 years old [55]. The study authors stated that the reasons for the difference between males and females in the rate of influenza vaccination are not clear; however, there are likely influences on vaccination rate between sexes based on culture, age, and sex-based differences in biological response to vaccination [55]. Similarly, in a study of at-risk adults in England, females were more likely than males to be vaccinated among 18–64 year olds (OR18–64 years: 1.19) [42]. However, conversely, a nationwide cross-sectional study in Poland showed that males were more often vaccinated than females (OR 1.56) [56].

1.1.5 Disparities in Seasonal Influenza Vaccine Uptake Related to Age

Uptake of influenza vaccine in adults aged 18–64 years is relatively low, even among the older adults in this population (i.e., aged 50–64 years), perhaps indicating a lack of perceived need for vaccination [3]. In a study of at-risk adults in England, overall seasonal influenza vaccine uptake was 35.3% and 74.0% for individuals aged 18–64 and over 65 years, respectively [42]. A nationwide cross-sectional study in Poland also showed that people aged over 65 years received influenza vaccination more often than younger counterparts (OR 1.64) [56].

Considering the increased risk of comorbidity and general declining health in older age, extending the prioritisation of influenza vaccination to the 18–64 years population, or at least to individuals aged 50–64 years, may reduce the risk of hospitalisation and death in this large, working-age population [3].

Vaccinating children and adolescents against seasonal influenza may reduce disease burden in both vaccinated and unvaccinated individuals because of the pivotal role that younger age groups play in the transmission of infection [57]. Furthermore, young children are prone to severe influenza disease [58]. In the Netherlands, GP consultation rates associated with influenza infection were reported to be highest for children aged under 5 years [59].

Several European countries, including the UK and Finland, have initiated immunisation programmes for healthy toddlers and young children [58]. Efforts to improve influenza burden across Europe could include extending vaccination programmes to younger age groups in those countries that have not already done so. The introduction of new childhood influenza vaccination programmes provides an opportunity to further reduce the impact of influenza in the targeted age groups, and in the wider community [8].

1.1.6 Disparities in Seasonal Influenza Vaccine Uptake Related to Comorbidities

Influenza is associated with high levels of hospitalisation and outpatient visits, and places a burden on patients and healthcare systems that is exacerbated by comorbidities [3]. There are few studies on seasonal influenza vaccine uptake among individuals with disabilities or comorbidities, with many of these conducted outside Europe and showing mixed results [60-65]. For example, in the USA, vaccination rates have been reported to be low for children with neurodevelopmental disorders (52%) or epilepsy (59%), and only one in two physicians appeared to recognise these conditions as high-risk in the context of influenza [66, 67].

Focussing on Europe, a nationwide population study in France showed that individuals with disabilities were more frequently vaccinated than other at-risk groups, mainly because of their higher levels of morbidity and healthcare use [60]. In a cross-sectional study of adults with chronic conditions in Italy, 64.7% of participants were aware that influenza can be prevented with vaccines and that patients with chronic diseases are at higher risk of developing severe complications; however, only 42.1% had received influenza vaccine in the previous season, and 46.9% planned to receive it in the next season [68]. The level of awareness was significantly lower among the elderly (≥ 65 years) and those with a higher self-reported health [68]. Factors including older age, greater knowledge of vaccine utility and safety, chronic respiratory disease, or taking more drugs were associated with higher likelihood of vaccination in participants [68].

Patients with immune-mediated inflammatory diseases, such as rheumatoid arthritis, are at increased risk of infection, in part because of the disease itself, but the primary reason is treatment with immunomodulatory or immunosuppressive drugs [69]. However, self-reported uptake of vaccination against seasonal influenza was only 59% in a study in Denmark of patients with rheumatoid arthritis receiving immunosuppressive drugs [70]. Furthermore, the majority of patients with multiple sclerosis in a cross-sectional, single-centre study in Austria did not fulfil vaccination recommendations for 17 different diseases, with only 10% vaccinated against influenza [71]. Influenza vaccination rate was also low (6.7%) in patients with newly diagnosed HIV infection in a cross-sectional study of vaccination coverage in the Czech Republic [72]. Higher level of education and less advanced HIV infection were associated with higher vaccination rates in this population [72].

Suboptimal influenza vaccine uptake (41.8% in the latest season) was also reported in a Danish study in kidney transplant recipients and patients on the kidney transplant waiting list [73]. Prior influenza vaccine uptake was positively associated with influenza vaccine uptake in the latest season (p < 0.001), whereas recommendations given by non-physicians were negatively associated [73]. In a nationwide cohort study including all patients aged > 18 years with heart failure in Denmark, influenza vaccination coverage ranged from 16% to 54% during the study period [74]. Influenza vaccination was associated with a reduced risk of both all-cause and cardiovascular death after extensive adjustment for confounders, with frequent vaccination and vaccination earlier in the year associated with larger reductions in risk of death compared with intermittent and late vaccination [74].

Odds of vaccine uptake in a study of at-risk adults in England were highest among younger patients with diabetes (OR18–64 years: 4.25) and older patients with chronic respiratory disease (OR65+ years: 1.60), whereas they were lowest among morbidly obese patients of all ages (OR18–64 years: 0.68; OR65+ years: 0.97) [42].

1.1.7 Disparities in Seasonal Influenza Vaccination Among Healthcare Professionals

Uptake of seasonal influenza vaccination among healthcare professionals varies considerably across Europe, as shown by the following examples.

Self-declared vaccination coverage was 34.8% for influenza among healthcare professionals in a national cross-sectional study in 2019 in France [75]. Higher influenza vaccination coverage was observed in healthcare professionals working in wards where there was a staff vaccination contact person, staff vaccination was organised in the ward, information on influenza vaccines was provided, and/or the ward manager supported the healthcare professional vaccination campaign [75].

A survey by O'Lorcain et al. of hospitals and long-term/residential healthcare facilities in Ireland during each season from 2011 to 2012 to 2019–2020 showed that influenza vaccine uptake by healthcare professionals increased over the nine seasons [76]. Uptake among healthcare professionals employed in publicly-funded hospitals increased from 18.1% (2011–2012) to 58.9% (2019–2020), and uptake in publicly-funded long-term/residential healthcare facilities increased from 17.8% (2011–2012) to 45.5% (2019–2020) [76]. Overall, uptake among hospital nursing staff was lowest among all staff categories for most seasons, but increased from 12.4% in 2011–2012% to 58.1% in 2019–2020 [76]. O'Lorcain et al. suggested that reasons for this improvement in seasonal influenza vaccine uptake were most likely associated with the development of strategic plans, and implementation of specific actions taken by the Health Service Executive and led and facilitated by staff, including occupational health, trained peer vaccinators, influenza vaccination mobilisers and management [76].

In a Danish study published in 2021, 51% of hospital healthcare professionals were reported to have received influenza vaccination, with formerly vaccinated participants more likely to be vaccinated again [77]. Perception of own gain, patient gain and a workplace recommendation were specified as key incentives for vaccine uptake [77].

High rates of vaccination coverage among healthcare professionals in hospitals in Finland were reported for 2017–2018 (83.7%), 2018–2019 (90.8%) and 2019–2020 (87.6%) [78]. The study authors proposed that the legal situation (semi-mandatory system) in Finland, which defined that it was the employer's responsibility to appoint only vaccinated healthcare professionals to care for vulnerable patients, seemed to provide a good background for the high coverage rates during these seasons [78].

Table 1 shows a snapshot of current disparities in seasonal influenza vaccination in Europe.

1.2.1 Knowledge, Level of Education and Seasonal Influenza Vaccination

Self-reported data on influenza vaccination from 2019 show that higher proportions of individuals who completed tertiary education were vaccinated than those with a lower level of education in Hungary, Poland, Finland, Estonia and Norway, and the opposite was observed in Portugal, Belgium, Spain, Italy and Iceland [6]. Higher level of education was also consistently associated with higher coverage in a study in Norway reported in 2023 [79].

Higher level of education, being more knowledgeable about influenza vaccination, and taking more medications were independent predictors for being ever-immunised against seasonal influenza in individuals aged over 65 years in Serbia [80]. In addition, older age, educational level, and information from, and recommendation by, a specialist or general physician were positively associated with influenza vaccine uptake in a study of patients with rheumatoid arthritis receiving immunosuppressive drugs in Denmark [70].

A total of 64.2% of pregnant women in a study in Italy were aware that influenza is more dangerous during pregnancy, yet only 9.7% had received the vaccine and 21.4% of those unvaccinated were willing to receive it [81]. Factors including older age, high-risk pregnancy and tertiary education were associated with a higher likelihood of being knowledgeable about influenza vaccination [81]. The majority of respondents considered the vaccine not very useful during pregnancy [81]. In another Italian study, only 2.8% of pregnant women correctly identified all the vaccinations recommended during pregnancy and 1.4% had received seasonal influenza vaccination [82].

In a study of pregnant women in Germany, receiving a recommendation to vaccinate was strongly associated with influenza vaccine uptake, but only one-fifth of women reported receiving such a recommendation, and uptake of influenza vaccine was only 13% [83]. Raising healthcare professionals' awareness of vaccinating during pregnancy is needed to increase vaccine uptake in pregnant women.

Health literacy is a key determinant of vaccine decision-making [84]. A cross-sectional study in Spain showed that pregnant women with high health literacy were more likely to decline influenza vaccination; 24% of women who declined felt the vaccine was unnecessary and 23% claimed to have insufficient information [85]. In contrast, health literacy did not affect the likelihood of influenza vaccine uptake among high-risk groups in a study in Italy [86].

Less than half (42.3%) of participants in a study in Austria considered themselves to be sufficiently informed about national vaccination recommendations, with GPs the primary source of healthcare-related information for the majority (73.1%) of participants [87].

A survey of 1106 adults with non-communicable diseases in eight European countries showed that the main reasons for agreeing to have the influenza vaccine were disease prevention and healthcare practitioner recommendations [88]. Information provided to participants included how vaccines protect against infection, the risks of not getting vaccinated, possible side effects, duration of protection, and the need for a booster dose [88]. Unvaccinated individuals had received less information on these topics than those vaccinated [88]. A lack of belief in the need for an influenza vaccine, treating practitioners not recommending the vaccine, and experience of mild severity of influenza were the main barriers against the influenza vaccine [88]. The physician was the most preferred and tapped resource for information on influenza vaccines, followed by dedicated websites [88].

Considering these studies, differences in level of education and knowledge about influenza and influenza vaccination may contribute to the disparities in seasonal influenza vaccination in Europe. This observation provides a focus for reducing the disparity through a variety of educational and media initiatives.

1.2.2 Attitudes and Behaviours Towards Seasonal Influenza Vaccination

A cross-sectional survey in Greece showed that around half of eligible participants aged over 60 years received annual influenza vaccinations, and that behavioural (p < 0.001), normative (p < 0.001), and control (p < 0.001) beliefs and an increased intention score (p < 0.001) were associated with increased probability of vaccination [89]. In this context, these beliefs can be explained as follows. Behavioural beliefs include an individual's beliefs regarding the importance, necessity and safety of the influenza vaccination upon their own and their family's health [89]. In addition, normative beliefs refer to the influence of significant others (doctors, pharmacists, family, friends and other significant members of society) on an individual's behaviour and actions associated with influenza vaccination [89]. Lastly, control beliefs refer to an individual's beliefs about the presence of factors that may facilitate or hinder performance of a behaviour, in this case, the extent of control the individual has over the decision to get vaccinated [89].

Health behaviours, such as regular visits to the doctor, general health examinations and nonsmoking, have been reported to be significantly associated with higher odds of seasonal influenza vaccine uptake [64, 89].

Reasons for declining vaccination in a study of individuals aged over 65 years in Serbia included ‘they were in good health’ (33.5%) and ‘they did not believe that vaccine protects from flu’ (31.5%) [80]. Similarly, in a study in patients with rheumatoid arthritis in Denmark, reasons for not being vaccinated included fear of adverse effects, lack of information and recommendation, and perception of good health [70]. A total of 11.4% of participants in an Austrian cross-sectional survey deliberately refused vaccinations (including for influenza) [87]. The most common reasons for non-vaccination were fear of adverse effects (35.9%), doubt of effectiveness of vaccines (35.9%) and distrust of the pharmaceutical industry (23.1%) [87]. Lack of information, perception of own good health, and fear of adverse reactions were primary reasons for not being vaccinated among kidney transplant patients in a Danish study [73].

In a cross-sectional questionnaire-based study, only 4.5% of Polish teachers reported receiving influenza vaccine in the 2018–2019 season [90]. The main reasons for not being vaccinated were a lack of confidence in vaccine effectiveness (56.9%) and concerns related to adverse effects (30.6%) [90]. Although 43.8% of teachers believed that they were at risk of influenza infection, only 62.5% indicated vaccination as an effective method of preventing influenza [90]. This is an area that requires research as teachers are a potential target group for immunisation programmes against influenza infection [90].

In a 2009 study in a French teaching hospital, 22.3% of healthcare workers were vaccinated against seasonal influenza, with the immunisation coverage rate significantly higher among physicians [91]. The two most cited reasons for vaccination were ‘to protect the patient’ and ‘to avoid getting sick’; arguments against vaccination were ‘I never get the flu’ and ‘getting vaccinated is inconvenient and takes too long’ [91].

A study in Germany showed that the efficacy of seasonal influenza vaccination was misjudged by 77.8% of trainee midwives, possible adverse events were correctly estimated by only 35.2%, and 56.2% were not convinced of the safety of influenza vaccination during pregnancy [92]. Although 76.8% of trainees reported a positive attitude towards vaccinations in general, 73.3% complained of receiving too little information on complications due to vaccines [92].

Another example of inconsistent behaviour surrounding influenza vaccination is shown in a study in which a total of 51.5% of nurses and 90.6% of doctors in an Italian tertiary children's hospital believed that their risk of contracting influenza was greater than that of the general population, yet 77.8% of nurses and 45.8% of doctors were unvaccinated (p < 0.0001) [93].

Poor adherence to influenza vaccination guidelines by healthcare professionals, leading to low vaccination rates among their patients, has been reported in studies of community-dwelling individuals with spinal cord injury in Switzerland (10% in those aged < 45 years) [94], and asplenic patients (59%) [95] and obese patients (32.2%) [96] in France. In addition, adherence to guidelines by GPs in France varies across regions [97]. Adherence to guidelines for pregnant women has also been reported as suboptimal in studies in France (47.9%) [96] and Greece (16.2%) [98]. In the latter study, healthcare professionals appeared to be reluctant to recommend vaccination during pregnancy [98]. Missed opportunities to vaccinate have been reported to be a primary reason for suboptimal seasonal influenza vaccination coverage in older adults in Italy [99].

As shown, attitudes and behaviours towards influenza and seasonal influenza vaccination clearly influence whether healthcare professionals recommend vaccination to their patients and whether patients choose to accept it. Strategies to reduce disparities in seasonal influenza vaccination across Europe should include consideration of this important topic.

1.2.3 Vaccine Hesitancy and Vaccine Fatigue

Vaccine hesitancy refers to the refusal or delay in acceptance of vaccination despite the availability of vaccination services [100]. This hesitancy arises from reasons such as safety concerns about vaccination; beliefs that vaccines do not work, or that the risks of vaccination outweigh the benefits, and perceptions of low likelihood of contracting vaccine-preventable diseases [101]. In 2019, before the COVID-19 pandemic, vaccine hesitancy was considered by the WHO to be a top 10 threat to global health [102, 103]. Vaccine hesitancy is complex and is linked to human behaviours, including complacency, distrust, identity, beliefs and world views, and a fear of harm [103]. Understanding the reasons behind these behaviours is important when considering potential solutions that could lead to greater vaccine acceptance [103].

Vaccine hesitancy is common among healthcare professionals in Europe, necessitating shared strategies across all European countries to reduce disparities in this area [104]. Seventeen percent of primary healthcare workers in a study in Croatia were identified as vaccine hesitant [105]. Nurses and physicians differed (p < 0.001) in their general attitude, beliefs and behaviours towards vaccination, with nurses showing a higher level of hesitancy (adjusted OR 5.73) [105]. Personal hesitancy may negatively impact vaccination education and recommendations; therefore, interventions are needed to increase vaccination knowledge and confidence among primary healthcare workers, particularly nurses [105].

The COVID-19 pandemic has changed attitudes towards vaccinations, and vaccine fatigue (inertia or inaction towards vaccine information or instruction due to perceived burden and burnout [106] has been a large contributor [107]. In a study of the changes in attitudes and barriers to seasonal influenza vaccination from 2007 to 2023 in university students in the USA, time, convenience, and perceived risk continued to be significant barriers to vaccination, and although the students were getting more encouragement to vaccinate from their healthcare providers and parents, this encouragement was becoming less effective [107]. The study authors considered that effective and empathetic vaccine communications are needed to eliminate preventable vaccine fatigue across sectors in society [107].

Uncertainty and misinformation surrounding vaccines is well recognised; however, the growing popularity of the antivaccine movement, fuelled by social media as a major contributor to vaccine misinformation, is creating disparity, impacting vaccination coverage and affecting global epidemiology [107, 108].

1.2.4 Access to Vaccination

Influenza vaccination policies are vital to establishing a coordinated influenza vaccination programme, but do not guarantee equitable access or ensure vaccination coverage [109]. Reliable production of vaccine in relevant quantities and at an affordable price is critical for developing a successful vaccination policy; however, effective collaborations between private manufacturers, regulatory authorities and national and international public health services are also necessary to ensure optimal access and uptake [110].

National influenza vaccination policies vary by region, income, and immunisation system strength, and are less common in lower-income countries [109]. A lack of healthcare infrastructure hinders the internal distribution of and access to vaccines, particularly in remote communities, where motivating people to visit vaccination centres located far away may not be successful [111]. Barriers to adoption of influenza vaccination policy, such as cost, logistics, country priorities, need for yearly vaccination, and variations in seasonality, need to be addressed [109].

The relatively low level of influenza vaccination coverage in most European countries is largely due to patients having limited access to vaccinations [112]. All 120 maternity trusts included in a study in England were commissioned to deliver influenza vaccinations, although not all actually administered the vaccines and there was large between-trust variation in vaccination rates [113]. Reasons for these variations should be investigated to improve uptake and equity [113].

Currently, 15 European countries allow community pharmacists to dispense and administer influenza vaccines [114]. Therefore, this represents another disparity across Europe as many countries have not implemented the vaccinations in pharmacies. In some countries, implementing vaccinations in community pharmacies and by authorised pharmacists has significantly improved vaccination coverage rates and herd immunity [112].

Influenza co-infection was associated with severity and mortality in patients with COVID-19 during the pandemic [115]. Increased coverage of influenza vaccination was encouraged throughout this period to mitigate the transmission of influenza virus and reduce the risk of severe outcome and mortality [115]. Access to influenza vaccines, as well as COVID-19 vaccines, was a priority alongside COVID-19 testing and public health and social measures [116]. Efforts to increase influenza vaccine uptake during a respiratory virus pandemic should be consolidated in future clinical practice [117].

Access to vaccine is a crucial aspect of effective seasonal influenza vaccination programmes and is a key area to address to reduce disparity in vaccine uptake.