Interim 2022/23 influenza vaccine effectiveness: six European studies, October 2022 to January 2023

Real-life experience on COVID-19 and seasonal influenza vaccines co-administration in the vaccination hub of the University Hospital of Palermo, Italy

With the pandemic, there has been a global reduction in influenza virus circulation, with WHO reporting, during 2021/22 season, laboratory testing positivity rate for influenza of less than 3%. Influenza surveillance systems anticipated a peak of influenza cases in the Northern Hemisphere during 2022/2023 season and the Italian Ministry of Health recommended the routinary co-administration of influenza with bivalent COVID-19 vaccines for the 2022/2023 season. At the Vaccination Hub of the University Hospital (UH) of Palermo, more than 700 subjects received influenza and COVID-19 booster doses in co-administration, during the 2021/2022 season. A cross-sectional study analyzing attitudes and factors associated with adherence to influenza and COVID-19 seasonal vaccines co-administration was conducted at the Vaccination Hub of the UH of Palermo, from October to December 2022. Among the 1,263 respondents, 74.7% (n = 944) received the co-administration of seasonal influenza and COVID-19 vaccines. The main reason reported for accepting it was confidence in the recommendations of the Health Ministry (41.3%). At the multivariable analysis, subjects aged ≤ 59 y old (AdjOR: 2.48; CIs95%: 1.89-3.65), male (AdjOR: 1.51; CIs95%: 1.27-1.75), Health-care professionals (HCPs) (AdjOR: 1.66; CIs95%: 1.08-2.57) and those who received co-administration during 2021/2022 (AdjOR: 41.6; CIs95%: 25.5-67.9) were significantly more prone to receive co-administration during 2022/23 season. From data obtained, the role of HCPs in accepting and then promoting co-administration of COVID-19 and influenza vaccines is crucial, as well as receiving co-administration in the previous season that represented the main drive for accepting it in the following seasons, supporting safety and effectiveness of this procedure.

Rationale and design of NUDGE-FLU-CHRONIC and NUDGE-FLU-2: Two nationwide randomized trials of electronic nudges to increase influenza vaccination among patients with chronic diseases and older adults during the 2023/2024 influenza season

BACKGROUND

Yearly influenza vaccination is strongly recommended for older adults and patients with chronic diseases including cardiovascular disease (CVD); however, vaccination rates remain suboptimal, particularly among younger patients. Electronic letters incorporating behavioral nudges are highly scalable public health interventions which can potentially increase vaccination, but further research is needed to determine the most effective strategies and to assess effectiveness across different populations. The purpose of NUDGE-FLU-CHRONIC and NUDGE-FLU-2 are to evaluate the effectiveness of electronic nudges delivered via the Danish governmental electronic letter system in increasing influenza vaccination among patients with chronic diseases and older adults, respectively.

METHODS

Both trials are designed as pragmatic randomized implementation trials enrolling all Danish citizens in their respective target groups and conducted during the 2023/2024 influenza season. NUDGE-FLU-CHRONIC enrolls patients aged 18-64 years with chronic diseases. NUDGE-FLU-2 builds upon the NUDGE-FLU trial conducted in 2022/2023 and aims to expand the evidence by testing both previously successful and new nudges among adults ≥65 years during a subsequent influenza season. Persons with exemptions from the electronic letter system are excluded from both trials. In both trials, participants are randomized in a 2.45:1:1:1:1:1:1 ratio to either receive no electronic letter (usual care) or to receive one of 6 different behaviorally informed electronic letters. NUDGE-FLU-CHRONIC has randomized 299,881 participants with intervention letters delivered on September 24, 2023, while NUDGE-FLU-2 has randomized 881,373 participants and delivered intervention letters on September 13, 2023. Follow-up is currently ongoing. In both trials, the primary endpoint is receipt of influenza vaccination on or before January 1, 2024, and the secondary endpoint is time to vaccination. Clinical outcomes including respiratory and cardiovascular hospitalizations, all-cause hospitalization, and mortality are included as prespecified exploratory endpoints. Prespecified individual-level pooled analyses will be conducted across NUDGE-FLU, NUDGE-FLU-CHRONIC, and NUDGE-FLU-2.

DISCUSSION

NUDGE-FLU-CHRONIC is the first nationwide randomized trial of electronic nudges to increase influenza vaccination conducted among 18-64-year-old high-risk patients with chronic diseases. NUDGE-FLU-2 will provide further evidence on the effectiveness of electronic nudges among older adults ≥65 years. Collectively, the NUDGE-FLU trials will provide an extensive evidence base for future public health communications.

TRIAL REGISTRATION

NUDGE-FLU-CHRONIC: Clinicaltrials.gov: NCT06030739, registered September 11, 2023, https://clinicaltrials.gov/study/NCT06030739. NUDGE-FLU-2: Clinicaltrials.gov: NCT06030726, registered September 11, 2023, https://clinicaltrials.gov/study/NCT06030726.

Probable extinction of influenza B/Yamagata and its public health implications: a systematic literature review and assessment of global surveillance databases

Early after the start of the COVID-19 pandemic, the detection of influenza B/Yamagata cases decreased globally. Given the potential public health implications of this decline, in this Review, we systematically analysed data on influenza B/Yamagata virus circulation (for 2020-23) from multiple complementary sources of information. We identified relevant articles published in PubMed and Embase, and data from the FluNet, Global Initiative on Sharing All Influenza Data, and GenBank databases, webpages of respiratory virus surveillance systems from countries worldwide, and the Global Influenza Hospital Surveillance Network. A progressive decline of influenza B/Yamagata detections was reported across all sources, in absolute terms (total number of cases), as positivity rate, and as a proportion of influenza B detections. Sporadically reported influenza B/Yamagata cases since March, 2020 were mostly vaccine-derived, attributed to data entry errors, or have yet to be definitively confirmed. The likelihood of extinction necessitates a rapid response in terms of reassessing the composition of influenza vaccines, enhanced surveillance for B/Yamagata, and a possible change in the biosafety level when handling B/Yamagata viruses in laboratories.

End of 2022/23 Season Influenza Vaccine Effectiveness in Primary Care in Great Britain

BACKGROUND

The 2022/23 influenza season in the United Kingdom saw the return of influenza to prepandemic levels following two seasons with low influenza activity. The early season was dominated by A(H3N2), with cocirculation of A(H1N1), reaching a peak late December 2022, while influenza B circulated at low levels during the latter part of the season. From September to March 2022/23, influenza vaccines were offered, free of charge, to all aged 2-13 (and 14-15 in Scotland and Wales), adults up to 49 years of age with clinical risk conditions and adults aged 50 and above across the mainland United Kingdom.

METHODS

End-of-season adjusted vaccine effectiveness (VE) estimates against sentinel primary-care attendance for influenza-like illness, where influenza infection was laboratory confirmed, were calculated using the test negative design, adjusting for potential confounders.

METHODS

Results In the mainland United Kingdom, end-of-season VE against all laboratory-confirmed influenza for all those > 65 years of age, most of whom received adjuvanted quadrivalent vaccines, was 30% (95% CI: -6% to 54%). VE for those aged 18-64, who largely received cell-based vaccines, was 47% (95% CI: 37%-56%). Overall VE for 2-17 year olds, predominantly receiving live attenuated vaccines, was 66% (95% CI: 53%-76%).

CONCLUSION

The paper provides evidence of moderate influenza VE in 2022/23.

Cell-based influenza vaccines: An effective vaccine option for under 60-year-olds

Aim

Seasonal influenza poses a significant burden of disease, affecting not only older adults but also individuals under the age of 60. It carries a high economic burden, mainly driven by influenza-associated productivity losses in the working population. Conventional egg-based influenza vaccines may have reduced effectiveness due to antigen adaptation in eggs. In contrast, cell-based influenza vaccines are less likely to be affected by such antigen adaptation. This review aims to present real-world data (RWD) comparing the effectiveness of quadrivalent cell-based (QIVc) and egg-based (QIVe) influenza vaccines over three consecutive seasons.

Methods

A comprehensive review was conducted, analyzing RWD from retrospective cohort and case-control studies on the relative vaccine effectiveness (rVE) of QIVc versus QIVe during the 2017/18-2019/20 seasons.

Results

This study included six retrospective cohort studies and one case-control study, with a combined total of approximately 29 million participants. A cohort study involving people aged ≥4 years during the 2017/18 season showed a statistically significant rVE of QIVc compared to QIVe in preventing influenza-like illness, with a value of 36.2%. QIVc demonstrated statistically significant superiority over QIVe in preventing outpatient and inpatient medical encounters as observed in two cohort studies conducted during the 2018/19 and 2019/20 seasons. The rVE of QIVc compared to QIVe was found to be 7.6% in individuals aged ≥4 years and 9.5% in individuals aged ≥18 years. Three additional cohort studies conducted between 2017/18-2019/20 reported a statistically significant improvement in rVE (5.3-14.4%) of QIVc compared to QIVe in preventing influenza-related hospitalizations and emergency department visits due to influenza in individuals aged 4-64 years. In a case-control study across all three seasons, QIVc showed statistically significantly higher effectiveness compared to QIVe in preventing test-confirmed influenza, with rVEs of 10.0-14.8%.

Conclusions

RWD from the 2017/18-2019/20 seasons demonstrated that QIVc is more effective than QIVe in preventing influenza-related outcomes in individuals aged 4-64 years. Preferential use of cell-based influenza vaccines, as opposed to conventional egg-based vaccines, could reduce the burden of influenza-related symptoms on individuals and alleviate the economic impact on the German population under 60 years of age.

Influenza vaccine effectiveness against hospitalizations associated with influenza A(H3N2) in Hong Kong children aged 9 months to 17 years, June-November 2023

A test negative study was carried out from 13 June through to 15 November 2023 enrolling 3183 children hospitalized with acute respiratory illness in Hong Kong. Influenza A and B viruses were detected in 528 (16.6%) children, among which 419 (79.4%) were influenza A(H3N2). The overall vaccine effectiveness against hospitalization associated with any influenza virus infection was estimated as 22.4% (95% CI: -11.7%, 46.1%), and against influenza A(H3N2) specifically was 14.3% (95% CI: -29.2%, 43.2%). Despite the moderate to low VE estimated here, which could be a result of waning immunity and antigenic drift, influenza vaccination remains an important approach to reduce the impact of influenza in children.

Effectiveness of influenza vaccine in preventing severe cases of influenza: Season 2022/2023

INTRODUCTION

Influenza poses a significant burden in terms of morbidity and mortality, with vaccination being one of the most effective measures for its prevention. Therefore, the aim of this study is to determine the effectiveness of the influenza vaccine in preventing cases of severe influenza in patients admitted to a tertiary hospital during the 2022/23 season.

METHODS

Case-control study. All hospitalised patients with a positive result in an RT-PCR for influenza were included. Those who met the criteria for a severe case (pneumonia, sepsis, multi-organ failure, admission to ICU or exitus) were considered cases. Those who did not meet these criteria were considered controls. Vaccine effectiveness (VE) to prevent severe cases and its 95% confidence interval were calculated.

RESULTS

A total of 403 patients were admitted with confirmed influenza. Of these, 98 (24.3%) developed severe influenza. Of the total, 50.6% were men and 47.1% were over 65 years of age. VE adjusted for influenza type, age and certain comorbidities was 40.6% (-21.9 to 71.1). In a segmented analysis, influenza vaccine was effective in preventing severe cases in all categories. It was particularly relevant in the 65+ age group (VEa = 60.9%; -2.0 to 85.0) and in patients with influenza A (VEa = 56.7%; 1.5-80.9).

CONCLUSION

Influenza vaccination markedly reduced the occurrence of severe cases of influenza in hospitalised patients, therefore, it remains the main strategy to reduce morbidity and mortality and associated costs.

[Cell-based influenza vaccines: an effective vaccine option for under 60-year-olds]

The population < 60 years of age is also affected by a significant disease burden from seasonal influenza. It carries a high economic burden, mainly driven by influenza-associated productivity losses in the working population. Conventional egg-based influenza vaccines may experience reduced effectiveness due to antigen adaptation in eggs. In contrast, cell-based influenza vaccines are less likely to be affected by antigenic adaptations to the host system and showed better effectiveness in individuals 4-64 years old over several seasons compared to conventional egg-based influenza vaccines under real-world conditions. Preferential use of cell-based influenza vaccines, as opposed to conventional egg-based vaccines, could reduce the burden of influenza-related symptoms on individuals and alleviate the economic impact on the German population < 60 years of age.

Risk factors for hospitalization and pneumonia development of pediatric patients with seasonal influenza during February-April 2023

Objectives

In China influenza remains a low activity for continuous 3 years due to COVID-19 controls. We here sought to study the clinical characteristics and risk factors of the influenza infection among children after the mandatory COVID-19 restrictions were lifted.

Methods

We included 1,006 pediatric patients with influenza A virus (IAV) infection, enrolled in one tertiary hospital in Zhenjiang, Jiangsu Province, China, during February to April 2023. Patients were divided into the outpatient (n = 798) and inpatient (n = 208) groups, and their baseline characteristics were compared between two groups to conclude the risk factors for pediatric hospitalization. Separately, pediatric inpatients (n = 208) were further divided into the pneumonia and non-pneumonia groups with comparison of their clinical characteristics, including their laboratory test results and representative radiological features, to derive the key determinants for pneumonia development after hospitalization.

Results

Compared to outpatients, IAV-infected pediatric inpatients exhibited younger age, higher female: male ratio, more co-infection of influenza B virus (IBV) and hematological abnormality. Multivariate regression analysis determined the independent risk factors of hospitalization to be the clinical symptom of abdominal pain (OR = 2.63, [95% CI, 1.05-6.57], p = 0.039), co-infection of IBV (OR = 44.33, [95% CI, 25.10-78.30], p = 0.001), elevated levels of lymphocytes (OR = 2.24, [95% CI,1.65-3.05], p = 0.001) and c-reactive proteins (CRPs) (OR = 1.06, [95% CI, 1.03-1.08], p = 0.001) upon hospital admission. Furthermore, the cough symptom (OR = 17.39, [95% CI, 3.51-86.13], p = 0.001) and hospitalization length (OR = 1.36, [95% CI, 1.12-1.67], p = 0.002) were determined to be risk factors of pneumonia acquirement for pediatric inpatients.

Conclusion

While the abdominal pain, viral co-infection and some hematological abnormality mainly contribute to hospitalization of pediatric patients with IAV infection, the length of hospital stay and clinical sign of coughing upon hospital admission constitute the key determinants for nosocomial pneumonia development.

Influenza Vaccine Effectiveness Against Influenza-Associated Hospitalization in Hong Kong Children Aged 9 Months to 17 Years, March-June 2023

In March-June 2023, we conducted a test-negative study in 1671 children who were hospitalized with acute respiratory illness in Hong Kong. Two hundred and eighty-six children (17.2%) were tested positive for influenza virus including 188 with A(H1N1). We estimated influenza vaccine effectiveness against influenza-associated hospitalization as 69.6% (95% confidence interval: 49.3%, 81.7%).

Influenza Vaccine Effectiveness in Preventing Laboratory-Confirmed Influenza Cases and Hospitalizations in Navarre, Spain, 2022-2023

We estimated influenza vaccine effectiveness (IVE) in preventing outpatient and hospitalized cases in the 2022-2023 season. A test-negative design included a representative sample of outpatients and all hospitalized patients with influenza-like illness (ILI) from October 2022 to May 2023 in Navarre, Spain. ILI patients were tested by PCR for influenza virus. Influenza vaccination status was compared between confirmed influenza cases and test-negative controls. Among 3321 ILI patients tested, IVE to prevent influenza cases was 34% (95% confidence interval (CI): 16 to 48) overall, 85% (95%CI: 63 to 94) against influenza B, and 28% (95%CI: 3 to 46) against A(H3N2). Among 558 outpatients, 222 (40%) were confirmed for influenza: 55% A(H3N2), 11% A(H1N1), and 31% B. Overall, IVE to prevent outpatient cases was 48% (95%CI: 8 to 70), 88% (95%CI: 3 to 98) against influenza B, and 50% (95%CI: -4 to 76) against A(H3N2). Of 2763 hospitalized patients, 349 (13%) were positive for influenza: 64% A(H3N2), 17% A(H1N1), and 8% B. IVE to prevent hospitalization was 24% (95%CI: -1 to 42) overall, 82% (95%CI: 49 to 93) against influenza B, and 16% (95%CI: -17 to 40) against A(H3N2). No IVE was observed in preventing influenza A(H1N1). IVE was high to prevent influenza B, moderate against A(H3N2) and null against A(H1N1). A lower proportion of influenza B cases may explain the smaller IVE in hospitalized patients than in outpatients. The null IVE against A(H1N1) was consistent with the observed antigenic drift and supports the new composition of the 2023-2024 influenza vaccine.