Influenza: A familiar term, yet its complexity extends far beyond common winter ailments. Healthcare professionals, researchers, and those in pharmaceuticals should understand the nature of influenza virus. But what exactly is this virus that so often challenges global health systems? And how do we navigate its diagnostic and therapeutic complexities?

In this article, we discuss the virology of influenza virus, a pathogen that demands our attention. Medical researchers and those in pharmaceutical development should not overlook influenza virus or allow it to be overshadowed by SARS-CoV-2.

Influenza virus: A Global Health Concern

Influenza, often known as the flu, is an infectious respiratory illness caused by the influenza viruses. Annually, it affects 5–15% of the global population, leading to significant health burdens.

Pandemics are rare, but influenza causes annual epidemics in communities. The impact of these epidemics varies.

Seasonal Patterns of Influenza Virus

In temperate regions, influenza mainly occurs as a seasonal illness, with infection rates peaking during winter months. This pattern is referred to as seasonal influenza, and distinct influenza seasons are observed in the Northern and Southern Hemispheres. The winter incidence increases due to enhanced virus transmission in low humidity and colder temperatures. In contrast, tropical regions experience more varied patterns of influenza cases, often occurring throughout the year.

Symptoms and Diagnostic Approaches

Influenza viruses infect the respiratory system. They target the nose, throat, and lungs and spread through droplets and aerosols. Those infected with influenza virus spread the infectious virus when coughing, sneezing, talking, or breathing. The incubation period lasts around 2 days. After that, symptoms appear. These symptoms include fever, cough, chills, headaches, body aches, fatigue, sore throat, and nasal congestion. Gastrointestinal symptoms, more common in children, may include nausea, vomiting, and diarrhoea.

These symptoms are not only found in influenza. Other viruses, such as respiratory syncytial virus, human metapneumovirus, adenoviruses, rhinoviruses, and coronaviruses, can also cause similar symptoms. Laboratory methods like reverse transcription PCR (RT-PCR) or viral culture provide accurate diagnosis and are considered the gold standard.

Alternative diagnostics include rapid antigen tests and digital immunoassays. However, these tests have lower sensitivity and specificity compared to RT-PCR.

Often, influenza is self-limiting, with recovery occurring within 2 to 8 days. So, many infected individuals do not seek professional healthcare. Even so, research suggests that during seasonal epidemics, most healthy adults with fever and respiratory symptoms have influenza.

Pandemic Influenza: A Historical Perspective

Influenza’s notoriety is partly due to its potential to cause severe illness and fatalities. This is especially true when novel strains trigger pandemics. In recent decades, seasonal influenza has caused hundreds of thousands of deaths annually.

The Spanish Flu of 1918–1920 was the most devastating influenza pandemic; it caused tens of millions of deaths. Pandemics in 1957–1958, 1968–1969, 1977–1979, and 2009–2010 have also had significant mortality impacts. Severe cases often lead to pneumonia caused by the influenza virus.

Pneumonia can also result from bacterial infections and become fatal. It can cause cardiovascular complications, ultimately leading to death. The risk of severe illness and death is higher in young children, the elderly, and individuals with chronic health conditions.

Mortality rates have declined due to healthcare advances, vaccination, and sanitation. However, influenza still has a global health impact and causes severe illness.

Vaccination and Treatment Strategies

Vaccination is one measure for preventing and controlling influenza virus. The available vaccines contain inactivated or live attenuated viruses, but they need to be changed every year. Antiviral medications like oseltamivir, zanamivir, and peramivir are also in part effective against influenza. They work best when given shortly after symptoms start. However, these are typically reserved for high-risk patients, aiming to prevent serious complications. Symptomatic treatments like anti-fever and pain-relieving medications can relieve discomfort but don’t affect virus clearance. And they can’t make you less contagious.

Diversity Among Influenza Viruses

Influenza viruses are categorized into four types: Influenza A (IAV), B (IBV), C (ICV), and D (IDV). IAV is primarily responsible for severe illnesses and seasonal epidemics in humans and also affects various animal species. IBV mainly affects children and contributes to seasonal epidemics, but it is not connected to pandemics. ICV and IDV, though less common in humans, have their unique epidemiological significance.

Viral Surface Proteins: Haemagglutinin and Neuraminidase

Influenza viruses have an envelope. This envelope has key glycoproteins – haemagglutinin (HA) and neuraminidase (NA). HA facilitates cell entry, while NA is involved in the release of new viruses. These proteins, particularly their antigenic regions, are primary targets for antibodies generated post-infection or vaccination. HA and NA exhibit significant antigenic variability, which is central to the virus’s ability to evade immunity.

Influenza Virus Evolution: Drift and Shift

Influenza viruses constantly evolve, with HA and NA surface molecules undergoing frequent adaptations. This process, known as antigenic drift, is why influenza infections recur and why regular vaccine updates are needed. In contrast, antigenic shift involves significant genetic changes, which can lead to pandemics. Only IAVs have been known to cause flu pandemics. Also, antigenic shift can cause animal-infecting viruses to infect humans. For example, this was seen in the 2009 swine flu pandemic.

Subtypes and Reassortment in Influenza Viruses

IAV subtypes, such as H1N1 and H3N2, are classified based on their HA and NA proteins. Historical pandemics have been linked to specific IAV subtypes. Unlike IAV, IBV is categorized into distinct lineages but lacks subtypes. Influenza viruses can exchange gene segments through reassortment. This process can lead to significant antigenic shifts.

Influenza Virus Research at VRS

At Virology Research Services (VRS) our influenza virus research is focused on IAV and IBV because these greatly impact human health.

We have many experimental capabilities, including antiviral and neutralization assays to industrial testing of liquid formulations, textiles, and non-porous surfaces. We can also design and conduct customized research projects to meet specific scientific goals.

Get in touch with our friendly team to learn how we can advance your influenza research project.

Further Reading

For those who would like to learn more about influenza virus and it’s epidemiology, we recommend the works by Spreeuwenberg. et al. (Reassessing the Global Mortality Burden of the 1918 Influenza Pandemic in American Journal of Epidemiology) and Caini et al. (The epidemiological signature of influenza B virus and its B/Victoria and B/Yamagata lineages in the 21st century, PLoS One).