Seasonal Vs. Pandemic Influenza

  • Yujin Wang Master of Science – MSc, University of Sheffield, England

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Introduction

Influenza is commonly known as the flu, an infectious respiratory illness caused by influenza viruses. An infected patient usually has symptoms such as:

  • Fever
  • Cough
  • Sore throat
  • Muscle aches
  • Fatigue

Each year, millions of people worldwide are affected by influenza, leading to significant morbidity and mortality.

Two main types of influenza that pose distinct challenges to public health: seasonal influenza and pandemic influenza.

  • Seasonal influenza occurs annually and is typically caused by influenza A and B viruses. It follows predictable seasonal patterns, with peaks during the autumn and winter seasons. 
  • Pandemic influenza refers to the emergence of a new strain of influenza virus that spreads rapidly across multiple countries or regions, resulting in widespread illness and often causing severe societal disruption.

Understanding the differences between seasonal and pandemic influenza is crucial for public health and preparedness efforts. While both types of influenza share common symptoms and modes of transmission, they differ in terms of their epidemiology(the branch of medicine which deals with the incidence, distribution, and possible control of diseases and other factors relating to health.), severity, and impact on communities. Knowing these distinctions enables healthcare professionals and policymakers to implement appropriate prevention and control measures tailored to the specific challenges caused by each type of influenza.

In this article, we will look at the characteristics of seasonal and pandemic influenza, exploring their epidemiology, clinical features, public health responses, and prevention strategies. By gaining a deeper understanding of these two forms of influenza, you will be better equipped to navigate influenza prevention, and better protect yourself and your family during the peak seasons.

Seasonal influenza 

Seasonal influenza, as mentioned previously, refers to the annual occurrence of influenza viruses that follow predictable patterns of circulation. This type of influenza is characterised by its seasonal nature, often peaking during the colder months.

Definition and characteristics

Seasonal influenza is caused by various strains of influenza viruses, primarily influenza A and influenza B. These viruses can lead to respiratory illness with symptoms ranging from mild to severe. While most people recover within a week or two, seasonal influenza can sometimes result in complications, particularly in individuals with weakened immune systems or underlying health conditions. 

Epidemiology

Annual occurrence and seasonal patterns

Seasonal influenza typically occurs in annual cycles, with outbreaks occurring during specific times of the year. In temperate regions, such as the UK, seasonal influenza activity often peaks during the winter months, between December and February. However, the timing and intensity of influenza activity can vary from year to year.

Global impact and burden

Seasonal influenza has a significant global impact, causing millions of cases of respiratory illness each year. The burden of seasonal influenza extends beyond individual health, placing strain on healthcare systems and economies. In addition to healthcare costs associated with treatment and hospitalisation, seasonal influenza can also result in lost productivity due to illness-related absenteeism.4

Transmission dynamics 

Transmission of seasonal influenza primarily occurs through respiratory droplets when an infected person coughs, sneezes, or talks. These droplets can then be inhaled by nearby individuals, leading to infection. Seasonal influenza viruses can also survive on surfaces for a limited period, contributing to indirect transmission through contact with contaminated surfaces.

Certain populations are at increased risk of complications from seasonal influenza, including young children, elderly individuals, pregnant women, and those with underlying health conditions such as asthma, diabetes, or heart disease. Vaccination and other preventive measures are particularly important for protecting these vulnerable groups. 

Pandemic influenza

Now shifting our focus to pandemic influenza, we explore the defining characteristics and far-reaching consequences of this global health threat.

Definition and characteristics

 Pandemic influenza arises from the emergence of a novel influenza virus strain to which a large proportion of the population has little to no immunity. This novel virus can originate from various sources, including animals, and has the potential to spread rapidly across different regions and continents. 

The introduction of a novel influenza virus into the human population can occur through various pathways, including direct contact with infected animals, consumption of contaminated animal products, or exposure to environments where humans and animals interact closely, such as live animal markets.

Once introduced, pandemic influenza can spread rapidly due to factors such as international travel, population density, and interconnectedness. The global spread of the virus can occur within weeks or months, leading to widespread transmission and significant societal impact.6

Epidemiology

In the Dictionary of Epidemiology, pandemic influenza is described as “an epidemic occurring worldwide or over a very wide area, crossing international boundaries and usually affecting a large number of people”. According to the European Centre for Disease Prevention and Control and WHO, severity level should not be a measurement to distinguish seasonal and pandemic influenza. 

Societal impact 

The impact of pandemic influenza extends beyond individual health, it causes serious consequences for healthcare systems, economies, and social structures.

  • Healthcare System Strain: shortages of hospital beds, medical supplies, and healthcare personnel.
  • Economic Disruptions: absenteeism due to illness, reduced consumer spending, disruptions to supply chains, and impacts on trade and tourism.

Comparison of seasonal and pandemic influenza

Continuing our exploration, let's compare the key aspects of seasonal and pandemic influenza, highlighting the differences in transmission dynamics, clinical features, and public health response strategies.

Transmission dynamics

Seasonal influenza typically spreads gradually within communities, following predictable seasonal patterns. However, the unpredictable nature of pandemic influenza causes significant challenges for virus control, as the virus can quickly overwhelm healthcare systems and spread beyond borders.

Clinical features

Both seasonal and pandemic influenza can cause similar symptoms such as fever, cough, sore throat, and fatigue. The severity of symptoms may vary depending on factors such as the virulence of the virus strain and the individual's immune response.

Individuals of all ages can be affected by both seasonal and pandemic influenza, but vulnerable populations, including young children, elderly individuals, pregnant women, and those with underlying health conditions, are particularly in need of caution. 

Prevention and control measures

In the ongoing effort to prevent and control influenza outbreaks, various measures are employed, encompassing vaccination, non-pharmaceutical interventions (NPIs), and surveillance strategies.

Vaccination

Vaccination plays a crucial role in preventing influenza and reducing its severity and complications. Vaccination is recommended for individuals at high risk of complications, including elderly individuals, young children, pregnant women, and those with underlying health conditions.

However, developing and distributing vaccines for pandemic influenza presents unique challenges. Due to the unpredictability of the emergence of novel virus strains, pandemic vaccines must be rapidly developed and distributed globally. Other considerations also include cost, allocation, and deployment.13

Non-pharmaceutical interventions (NPIs)

In addition to vaccination, non-pharmaceutical interventions (NPIs) are essential for controlling the spread of influenza, particularly during pandemics.14

Keep social distancing: Examples include school closures, workplace measures such as remote working, and restrictions on public gatherings and events.

Hygiene Practices and Mask-Wearing: Frequent handwashing with soap and water, covering coughs and sneezes with a tissue or elbow. Additionally, wearing face masks can provide a physical barrier against respiratory droplet transmission. 

Surveillance and early detection

Surveillance systems monitor influenza activity by tracking trends in influenza-like illness (ILI), laboratory-confirmed cases, hospitalisations, and mortality rates. These systems provide valuable data to inform public health decisions, including vaccine strain selection, antiviral medication distribution, and implementation of control measures.

Pandemic alert systems, such as the World Health Organization (WHO) Global Influenza Surveillance and Response System (GISRS), provide early warning of potential pandemic threats by monitoring the emergence of novel influenza virus strains and assessing their pandemic potential. 

Challenges and considerations 

Antigenic variation and vaccine effectiveness

One of the key challenges in influenza control is the antigenic variation of the virus, which allows it to evade immunity and re-infect individuals. Seasonal influenza viruses undergo continuous antigenic drift, resulting in changes to the surface proteins (hemagglutinin and neuraminidase) recognised by the immune system. As a result, the effectiveness of seasonal influenza vaccines may vary each year, depending on how well they match the circulating virus strains. Similarly, pandemic influenza viruses may undergo antigenic shifts, leading to the emergence of novel strains to which the population has little to no immunity. Developing vaccines that provide broad protection against diverse influenza virus strains remains a significant challenge for researchers and vaccine manufacturers.

Healthcare system strain 

The burden of influenza on healthcare systems is a significant concern, particularly during severe influenza seasons and pandemics. Influenza-related hospitalisations and intensive care admissions can strain healthcare resources, leading to shortages of hospital beds, medical supplies, and personnel.

Vaccine access and resources

Achieving global cooperation and equity in access to influenza vaccines and resources remains a persistent challenge. Disparities in vaccine availability, distribution, and uptake between high-income countries and low- and middle-income countries can exacerbate global health inequities. Addressing these challenges requires collaborative efforts among governments, international organisations, and pharmaceutical companies to address barriers such as vaccine supply constraints, logistical challenges, and funding limitations.

Summary

Seasonal and pandemic influenza presents a significant public health challenge globally. Seasonal influenza recurs annually, following predictable patterns of transmission and affecting millions worldwide. On the other hand pandemic influenza arises from the emergence of novel virus strains with pandemic potential, leading to rapid and widespread outbreaks. Vulnerable populations and individuals with underlying health conditions are at heightened risk of severe outcomes during both seasonal and pandemic influenza outbreaks.

Prevention and control measures play a key role in mitigating the impact of influenza on population health. Vaccination, social distancing measures, hygiene practices, and surveillance efforts are key strategies for influenza control. However, challenges such as antigenic variation, strain-specific vaccine development, healthcare system strain, and global disparities in vaccine access and resources persist.

Addressing these challenges requires global cooperation, equitable access to vaccines and resources, and ongoing research and innovation in influenza prevention and control. By working together, we can reduce the burden of influenza on public health and global economics.

References 

  1. Maleki F, Welch V, Lopez SMC, Cane A, Langer J, Enstone A, et al. Understanding the global burden of influenza in adults aged 18–64 years: a systematic literature review from 2012 to 2022. Advances in Therapy. 2023;40(10): 4166–4188. https://doi.org/10.1007/s12325-023-02610-1.
  2. Social and Economic Impacts of the 1918 Influenza Epidemic | NBER [Internet]. [cited 2024 Apr 11]. Available from: https://www.nber.org/digest/may20/social-and-economic-impacts-1918-influenza-epidemic
  3. Warmath CR, Ortega-Sanchez IR, Duca LM, Porter RM, Usher MG, Bresee JS, et al. Comparisons in the health and economic assessments of using quadrivalent versus trivalent influenza vaccines: a systematic literature review. Value in Health. 2023;26(5): 768–779. https://doi.org/10.1016/j.jval.2022.11.008.
  4. Smith RD, Keogh-Brown MR, Barnett T, Tait J. The economy-wide impact of pandemic influenza on the UK: a computable general equilibrium modelling experiment. BMJ. 2009;339(nov19 1): b4571–b4571. https://doi.org/10.1136/bmj.b4571.
  5. Wouters OJ, Shadlen KC, Salcher-Konrad M, Pollard AJ, Larson HJ, Teerawattananon Y, et al. Challenges in ensuring global access to COVID-19 vaccines: production, affordability, allocation, and deployment. The Lancet. 2021;397(10278): 1023–1034. https://doi.org/10.1016/S0140-6736(21)00306-8.
  6. Imai N, Gaythorpe KAM, Abbott S, Bhatia S, van Elsland S, Prem K, et al. Adoption and impact of non-pharmaceutical interventions for COVID-19. Wellcome Open Research. 2020;5: 59. https://doi.org/10.12688/wellcomeopenres.15808.1.

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Yujin Wang

Master of Science – MSc, University of Sheffield, England

Yujin is a first-year master’s student in Health Technology Assessment and Reimbursement. She has several years of experience in medical and health reimbursement in public sectors. She is passionate in health related research and health promotions.

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