Seasonal influenza is an infectious respiratory illness. It happens when a person gets infected with the influenza virus. It spreads quickly through various modes of direct transmission such as a droplet, contact, and aerosol methods. It can also indirectly be spread by touching surfaces, hands, or objects contaminated with the virus from a sick person. While the virus is highly contagious, influenza is still easily preventable.
Cases of symptomatic seasonal influenza clocks in at around 4-50 million per year in the EU/EEA. Deaths associated with the disease are at 15,000-70,000 European people annually.
Seasonal influenza globally occurs in epidemics at certain times of the year:
• Northern Hemisphere including Europe – Between November and April
• Southern Hemisphere – between June and October
Pandemics can also occur at irregular times across the globe. Continuous surveillance against seasonal influenza is being done throughout the world, including the EU.
Some illnesses present with influenza symptoms, although the influenza virus does not really cause them. These influenza-like illnesses (ILI) are often caused by other bacteria and viruses and thus cannot be classified as influenza.
Various types of influenza virus cause influenza. They all come from the same family of RNA viruses known as Orthomyxoviridae. The types of influenza virus are:
1. Influenza A – This group is known to infect humans the most. Type A viruses are further narrowed down into other subtypes depending on the virus’s two main glycoproteins lying on its surface:
• Haemagglutinin (HA) – H1-H18
• Neuraminidase (NA) – N1-N11
Influenza immunity lies with the human body’s production of antibodies against the glycoproteins.
2. Influenza B – This group also infects humans but causes less severe symptoms and complications. There are two antigen lineages belonging in this group, namely:
No other subtypes exist for Type B Influenza virus.
3. Influenza C – This group infects humans but only on rare occasions, causing only a few outbreaks.
It’s the types A and B viruses that we’ll focus upon on this post. These two types often cause influenza illness in humans.
There were times that humans got mixed types of influenza viruses, such as:
• A(H3N2) virus
• A(H1N1) virus
• Victoria and Yamagata Type B Influenza lineages
It was the A(H1N1) virus that spread and caused a worldwide pandemic of influenza last 2009.
Changes in the viruses are all due to a phenomenon known as antigenic drift. Here, influenza virus genomes gradually and spontaneously mutate. The RNA segments of both the influenza A and B genome can also be re-assorted (especially the Haemagglutinin of the Influenza A viruses), causing another change in the virus.
Influenza A viruses do not only spread in humans alone. Swine and birds can also catch the virus and get infected. The bad part is that the virus can be passed along from swine to humans and birds to humans, and it may also interchange. These are the virus strains that have affected swine, birds, mammals, and other animals:
• H1-H16 and N1-N9 – Seen in birds
• H17 and N10-11 – Seen in bats
Note that these viruses all belong to Influenza A only.
Given that the Influenza A viruses make animals their reservoir, this virus type is the only one known to cause pandemics. Pandemics occur as a result of more significant changes in the virus’s genes, known as antigenic shifts.
Antigenic shifts occur when HA and NA subtypes of the virus from infected birds or swine are included in a new virus through re-assortment. At least one RNA segment of the virus must be exchanged in order for a viable re-assortment to commence. The virus then gets the ability to infect humans as well as animals.
• Transmitting the influenza disease in humans
• Creating human-to-human transmission
• The inability of humans to develop antibodies or immunity against the viruses
Influenza virus infections are popularly known as “flu” in many parts of the world. However, people also use the term to describe mild illnesses that resemble influenza symptoms but are caused by other infectious agents.
Clinical Features of Seasonal Influenza
We’ll divide this section into two – for mild influenza and severe influenza.
Uncomplicated seasonal influenza disease
Mild cases of uncomplicated influenza start at a rapid onset, presenting with both respiratory and systemic symptoms. These symptoms include:
• Generalized feeling of illness
• Sore throat
• Runny nose
• Non-productive cough
• Muscle pains
• Fever or elevated body temperature
Vomiting and diarrhea can also be present, especially in infected children.
Most of the systemic symptoms such as aches and fever subside after a few days. Cough, weakness, sore throat, and runny nose may take a few more days to weeks before they completely disappear.
While many get symptoms of mild influenza, an estimated 75% of infected people may appear asymptomatic. This could happen during a known influenza season. You may have mild influenza but do not feel any symptoms at all.
Influenza that progresses to a severe case is typically due to a strong influenza virus infection or another infection of bacterial origin. Bacterial types such as Staphylococcus aureus and Streptococcus pneumoniae can infect a person already with influenza, causing aggravated illnesses such as pneumonia. Other possible diseases that can occur are:
• Encephalitis – Inflammation of the brain
• Myocarditis – Inflammation of the heart muscle
Both conditions are severe and can even be fatal.
If a person dies from these serious illnesses, it is often counted as a mortality due to that severe illness and not due to influenza. This is because sometimes, the initial influenza infection that led to those serious illnesses was not detected anymore. But the truth remains that an influenza infection can weaken a person to a point that other illnesses due to other causative factors set in. And influenza infection can also aggravate any underlying chronic disease a person may have.
• The strength of the virus itself
• The susceptibility of the host/person
• External factors such as access to adequate medical care
Severe seasonal influenza can easily infect people in these two main age-related risk groups:
• Young children less than 1 year old
• Older adults more than 65 years old
Such age groups often tend to get hospitalized more than other people who get influenza. Young children and older adults have weaker immune systems and are more predisposed to getting infected. Babies and toddlers haven’t fully developed their immune responses yet. Older adults often have underlying medical conditions that weaken their systems and make them vulnerable to influenza viral infections.
Apart from these age-related risk groups, people with underlying medical conditions can also get the disease. Such illnesses include the following:
• Chronic lung diseases such as bronchitis
• Chronic kidney illnesses like chronic renal failure
• Physical handicaps
• Long-term neurologic conditions
• Metabolic diseases such as diabetes mellitus/insipidus
• Cardiovascular diseases such as hypertension and coronary artery disease
• Hematologic problems
• Hepatic illnesses
• Morbid obesity with a body mass index of more than 40
• Genetic conditions
• Cancer and chemotherapy
Healthy pregnant women can also be infected by influenza and are at risk of becoming hospitalized as well. This often happens during known influenza seasons.
Note that even with all these risk factors present, influenza infection can strike anyone at any given time. Risk groups and factors may make certain people react more severely to the virus. Any underlying conditions can also be exacerbated due to the influenza infection.