Introduction 

Influenza is a viral illness, commonly known as flu, or seasonal flu as it occurs more often in winter seasons every year. One of the most common complications of influenza is pneumonia, which is a more serious chest infection where the lungs are inflamed and filled up with fluid. 

What is influenza?

Influenza is caused by the influenza virus. The most common symptoms of flu are runny nose, sore throat, fever and body aches. In most cases, the illness lasts up to 10 days and resolves on its own with home care and over-the-counter medication. 

However, flu is not a minor illness for some people- especially older adults (over 65 in particular), young children and those with underlying health conditions. The influenza virus can go down into the lungs causing a much more serious infection. The diagram below shows the lungs. In the cases of mild infections, the influenza virus will mainly be in the upper respiratory tract- the nose, throat and windpipe, visible between the lungs. In more severe cases, the virus travels down the smaller pipes in the lungs, and down into the small air sacs at the end of the pipes - known as alveoli. Over here, the virus can further damage the lungs and this can lead to complications. One of the most common complications of influenza is pneumonia, which can ultimately be fatal.²

What is pneumonia?

Pneumonia is the inflammation of the lungs. It is mainly caused by bacteria or viruses, which infect the lungs, replicate, and cause an immune response. White blood cells go to the area to destroy the invading bacteria viruses, and other inflammatory molecules present in the area. In trying to destroy the pathogens, the body can end up damaging the lungs. The alveoli then become damaged and fill up with fluid or pus, and the ability to breathe is compromised. This is accompanied by symptoms of flu including fever, sore throat, fatigue and body ache. Symptoms of pneumonia may include difficulty breathing, wheezing, productive cough (coughing up green or yellow mucus) and confusion usually in older adults.

Pneumonia can be classified according to the type of pathogen that causes it: bacterial, viral or fungal. Common bacteria that cause pneumonia include streptococcus pneumoniae, staphylococcus aureus and group A streptococcus. The most common viruses that cause pneumonia are the influenza virus and the common cold virus. SARS-CoV2 (the virus that causes COVID-19) is another common cause. Bacteria and viruses are the more common causes while fungal pneumonia tends to occur in those who are already immunocompromised such as AIDs.  

Pneumonia can also be classified as either community-acquired pneumonia or hospital-acquired pneumonia. Community-acquired pneumonia is the more common of the two.³

The mechanism by which influenza causes (viral) pneumonia is by the virus itself going down into the lungs, replicating, and damaging the alveoli.

However, an influenza infection can take a significant toll on a person's immune system (especially if they are high risk), and then the person can acquire a secondary infection by a pathogen that can cause pneumonia. In this case, it is not the influenza virus itself that goes into the lungs and causes pneumonia, but another pathogen (usually bacteria) that will cause pneumonia. This is sometimes also known as a coinfection if the person is infected with both pathogens at the same time.^3,4

Treatment of pneumonia 

The treatment depends on the cause. For bacterial pneumonia, antibiotics such as amoxicillin, azithromycin, co-amoxiclav or clarithromycin are recommended. For viral pneumonia, antivirals such as Tamiflu or ribavirin are often recommended. 

For those who have difficulty breathing, oxygen may also be given as part of treatment. Painkillers and other supportive measures such as fluids for hydration and cough medication can also be given to aid recovery. In hospitals, the fluid that is in the lungs can be drained.⁴ 

Complications of pneumonia 

If pneumonia is not treated, or if treatment is not working, a complication of the build-up of fluid is respiratory failure, where the person is unable to breathe. During respiratory failure, the lungs cannot oxygenate blood, nor can they remove the excess carbon dioxide from the body, which is life-threatening and can be fatal. Acute respiratory distress syndrome (ARDS) is another respiratory complication, which is similar to respiratory failure but it occurs suddenly. 

Another potential complication of pneumonia is bacterial sepsis, where the bacterial infection and subsequent inflammatory response spreads throughout the body rather than just to the lungs. This is also strongly associated with mortality.⁴

According to WHO in 2019, lower respiratory tract infections, including pneumonia ranked fourth in the top 10 causes of death worldwide. Most of these deaths were in adults over 70 and children under the age of 5. It is more common in lower-income countries, such as those in sub-Saharan Africa.¹ In high-income countries, respiratory complications of influenza are the leading cause of death from influenza, mostly in adults over 65 years old.² In the US, pneumonia caused by influenza was ranked eighth among the leading causes of death.  

Risk factors 

Age is one of the main risk factors. Particularly older adults over the age of 65 are at a high risk. Smoking is another one it damages the lungs, and if smoking continues for a long period of time a person will be more likely to have chronic lung disease such as COPD or fibrosis which can lead to worse outcomes if there is an infection such as influenza or pneumonia. Smoking lowers the immune response too, which exacerbates the risk of an adverse⁴ Exposure to indoor pollution, or living in a polluted area can have a similar damaging effect on the lungs as smoking. 

Obesity is a risk factor too since being obese puts the body in a chronic inflammatory state, which can make influenza a more severe disease in those who are obese compared to a normal weight. Obesity also increases the risk for cardiovascular disease and diabetes, which both lead to increased mortality.⁵

Those who live in care homes, and those who are in hospitals already are also at a higher risk. Around 30-40% of those in hospital with influenza also end up acquiring pneumonia.⁴

Prevention

The flu vaccine is the main protective mechanism against influenza. After vaccination, even if someone gets the flu, they are less likely to have a severe outcome. For high-risk groups vaccinations can be life-saving. Vaccination is also important for those who live with people who are high-risk, as it can reduce the viral load and therefore increase the chance of transmission. The influenza virus has many different strains, and every year, there is a new strain that causes the pandemic, and every year the vaccine is developed to immunise against the new strain, therefore it is important to be vaccinated each year. Quitting smoking and maintaining a healthy BMI  also significantly lowers the risk of severe outcomes of influenza, pneumonia and many other health conditions.^4,6 

Summary 

Influenza is a viral illness that usually affects the upper respiratory tract (throat, nose and sinuses). It is caused by the influenza virus. Pneumonia is one of the most common complications of flu. 

Pneumonia can be caused by the influenza virus itself when it travels down into the lower respiratory tract (into the lungs) and can inflame the alveoli and cause the lungs to fill up with fluid.  A person infected with influenza can also be infected by bacteria(Pneumonia) referred to as a coinfection. This is because the immune system is already weakened by the influenza infection, and the bacteria are then able to cause an infection. 

Treatment of pneumonia requires medical attention and usually involves antibiotics (for bacterial infections) or antivirals (for viral infections), along with painkillers and oxygen if there is difficulty in breathing. 

Complications of pneumonia include respiratory failure (or acute respiratory distress syndrome), where the fluid in the lungs impedes the ability to breathe and the ability to take in oxygen and release carbon dioxide. Sepsis is another complication, where the infection from the lungs spreads throughout the body. Complications are serious and life threatening and in many cases, fatal. 

Getting the flu vaccine every year is one of the best ways to lower your risk of serious outcomes of influenza.