The association between severity and aetiology of chronic liver disease and seasonal influenza vaccination uptake in adults: a retrospective cohort study using English primary care data (2019-2024)

This retrospective cohort study of English primary care data reveals that while chronic liver disease severity and specific aetiologies (alcohol- and viral-related) significantly increase seasonal influenza vaccine uptake among adults under 65 without other qualifying comorbidities, they are associated with a modest reduction in uptake among those who are already age-eligible for vaccination.

The Gist

Imagine your body is a bustling city, and your liver is the city's main water treatment plant. It filters toxins, manages waste, and keeps everything running smoothly. When this plant starts to fail (Chronic Liver Disease), the whole city is at risk.

Now, imagine a storm is coming: Seasonal Flu. For a healthy city, a flu storm is just a bit of rain. But for a city with a failing water treatment plant, that same storm can cause catastrophic flooding and collapse.

This study is like a detective report looking at why some of these "sick cities" (people with liver disease) are getting flu vaccines (storm shields) while others are not, even though they desperately need them.

Here is the breakdown of what the researchers found, using simple analogies:

1. The Big Problem: The "Shield Gap"

In England, people with liver disease are among the least likely to get the flu vaccine, even though they are in the "danger zone." It's like having a house with a leaking roof during a hurricane, but the owner refuses to buy an umbrella.

2. The Two Groups of People

The researchers split the people with liver disease into two main groups to see what changed their minds:

• Group A (The "Younger" Group): People under 65 who only have liver disease. They don't qualify for a free vaccine just because of their age.
• Group B (The "Older" Group): People over 65. In England, everyone over 65 gets a free flu shot automatically, regardless of their health.

3. The Surprising Findings

For Group A (Under 65, Liver Disease Only):

• The "Severity" Effect: The sicker the liver, the more likely they were to get the vaccine.
  • Analogy: Think of it like a car warning light. If the "Check Engine" light is just flickering (mild disease), the driver might ignore it. But if the engine is smoking and making loud noises (severe disease), the driver finally goes to the mechanic and gets the fix.
  • Why? When the disease gets severe, these patients see their doctors more often. They are in the "specialist zone," so they get reminded to get the shot.
• The "Cause" Effect: If the liver disease was caused by alcohol or a virus (like Hepatitis), they were also more likely to get vaccinated.
  • Analogy: It's like knowing your house was built with weak wood (alcohol) or has a termite problem (virus). You know exactly why you are fragile, so you take extra precautions.

For Group B (Over 65):

• The "Severity" Paradox: Here, the opposite happened. The sicker the liver, the less likely they were to get the vaccine.
  • Analogy: Imagine a very busy, chaotic hospital waiting room. If you are over 65, you are already in line for the vaccine. But if you are very sick with complex problems, you might get so overwhelmed by managing your other health issues (like diabetes or heart trouble) that the flu shot gets pushed to the bottom of the priority list. Or, perhaps they are too sick to easily travel to the clinic.
  • The "Comorbidity" Factor: If these older patients had other diseases (like diabetes) on top of their liver disease, the severity of the liver didn't matter as much. The other diseases acted as a "safety net," reminding them to get the shot.

4. The "Green Book" Clue

The study mentions the "Green Book" (a UK government guide for health professionals). It's like the rulebook for who gets a vaccine. The researchers found that if a patient's liver disease matched the specific rules in this book, they were more likely to get vaccinated. It's a bit like having a VIP pass; if your name is on the list, you get in.

The Bottom Line (The Takeaway)

• If you are young (under 65) with liver disease: The system works best when you are very sick. The doctors see you often and push the vaccine. But if you have "mild" liver disease, you might fall through the cracks and forget to get the shot.
• If you are old (over 65): Being very sick might actually make it harder to get the vaccine because your health is too complicated, and you might get lost in the shuffle of managing other conditions.

The Lesson: We can't just wait for people to get sicker to remind them to get vaccinated. We need to find better ways to make sure everyone with liver disease, especially those who are younger or have "mild" cases, knows that the flu is a serious threat to them and that the vaccine is their best shield.

Technical Summary

1. Problem Statement

Despite being at a significantly elevated risk of severe complications and mortality from seasonal influenza, individuals with Chronic Liver Disease (CLD) in England exhibit some of the lowest vaccination uptake rates compared to other high-risk groups (e.g., those with diabetes or chronic kidney disease).

• Knowledge Gap: The specific drivers behind this low uptake are poorly understood. Existing literature suggests inconsistent integration of vaccination advice into chronic disease management pathways and low patient-perceived risk.
• Hypothesis: The study posits that the severity (low, moderate, severe) and aetiology (alcohol-related, viral-related, or other Green Book guidelines) of CLD are critical, yet unexamined, determinants of vaccination behavior, particularly in relation to age-based eligibility and the presence of other comorbidities.

2. Methodology

• Study Design: Retrospective cohort study.
• Data Source: Clinical Practice Research Datalink (CPRD) Aurum, a representative primary care database in England.
• Population: Adults aged 18–115 years with a diagnosis of CLD.
• Timeframe: Five consecutive influenza seasons (2019/20 to 2023/24).
• Sample Size: Approximately 182,000 to 277,000 individuals with CLD per cohort season.
• Statistical Analysis:
  • Poisson Regression: Used to estimate vaccination uptake rates.
  • Key Variables:
    • Exposure: CLD severity (categorized as low, moderate, severe) and aetiology (alcohol-related, viral-related, other).
    • Outcome: Seasonal influenza vaccine uptake.
    • Stratification: Analyses were stratified by age eligibility (those under 65 vs. those 65+) and the presence of additional comorbidities (other qualifying clinical risk conditions).
  • Metrics: Incidence Rate Ratios (IRR) with 95% Confidence Intervals (CI).

3. Key Contributions

• Granular Stratification: Unlike previous studies that treated CLD as a monolithic risk group, this study dissects the population by disease severity and specific cause (aetiology).
• Contextual Nuance: It uniquely differentiates between patients who are eligible for vaccination solely due to CLD versus those eligible due to age or other comorbidities, revealing divergent behavioral patterns.
• Temporal Scope: The inclusion of five consecutive seasons provides robust longitudinal data, capturing trends across the pre-pandemic, pandemic, and post-pandemic periods.

4. Key Results

The study revealed a complex, non-linear relationship between disease severity and vaccination uptake, heavily dependent on age eligibility and comorbidity status.

A. Overall Uptake Rates

• Age-Eligible (65+): Uptake was high, ranging from 71.1% to 79.7%.
• Not Age-Eligible (<65): Uptake was significantly lower, ranging from 30.9% to 40.5%.

B. Impact of Severity and Aetiology (in those without additional comorbidities)

• Under Age Eligibility (<65):
  • Positive Association: Higher disease severity correlated with increased uptake.
    • Moderate Severity: IRR 1.80 (95% CI 1.69–1.90).
    • Severe Severity: IRR 1.95 (95% CI 1.84–2.08) in the 2023/24 season.
  • Aetiology: Alcohol-related and viral-related causes were also associated with increased uptake compared to other causes.
• Meeting Age Eligibility (65+):
  • Negative Association: Higher disease severity correlated with reduced uptake.
    • Moderate Severity: IRR 0.81 (95% CI 0.73–0.90).
    • Severe Severity: IRR 0.79 (95% CI 0.74–0.85).
  • Aetiology: Alcohol- and viral-related causes were similarly associated with reduced uptake in this group.

C. Impact of Additional Comorbidities

• The presence of at least one additional comorbidity attenuated the effects of severity.
• In the <65 group, the positive association between severity and uptake disappeared (IRR ~1.05).
• In the 65+ group, the negative association was largely neutralized (IRR ~0.99 for moderate, 0.91 for severe), suggesting that the "age" factor dominates the vaccination decision when other comorbidities are present.

5. Significance and Implications

• Targeted Interventions: The findings suggest that a "one-size-fits-all" approach to CLD vaccination is ineffective.
  • For <65 without comorbidities: The positive correlation with severity suggests that patients with advanced disease are already more engaged with healthcare systems (likely due to specialist care). Interventions here should focus on leveraging these specialist interactions to capture the remaining unvaccinated.
  • For 65+ with CLD: The counter-intuitive finding that more severe disease leads to lower uptake in the elderly suggests that complex health needs may create barriers (e.g., competing medical priorities, mobility issues, or fragmented care) that prevent vaccination despite high clinical risk.
• Aetiology Matters: The specific cause of liver disease (alcohol vs. viral) influences uptake, implying that stigma, patient-provider communication styles, or specific care pathways associated with these conditions play a role.
• Future Research Direction: The study calls for qualitative research to understand the specific facilitators and barriers for different subgroups, particularly those under 65 with severe disease who are not yet age-eligible, as this group remains highly vulnerable yet under-vaccinated.

Conclusion: CLD severity and aetiology are significant determinants of influenza vaccine uptake, but their impact is inverted depending on whether the patient is age-eligible. Addressing the low uptake in CLD requires moving beyond general awareness campaigns to tailored strategies that address the specific structural and perceptual barriers faced by patients at different stages of disease progression and age.