Introduction 

Influenza and sinusitis are both conditions that affect the respiratory tract. They have some common features, but overall, they are separate conditions with different causes and effects. 

This article will aim to give an overview of both conditions, highlighting their similarities and differences. 

What is influenza?

Influenza, commonly known as ‘flu’, is caused by influenza viruses. There are four categories of influenza viruses - A, B, C, and D. Types A and B cause annual influenza outbreaks. 

Influenza viruses can also affect animals - such as avian flu in birds, or swine flu in pigs. There are many different strains within each category of virus - each strain is slightly structurally and genetically different to the others. 

One of the most commonly known strains is H1N1 which is a type A influenza virus. It caused the 1918 Spanish Flu pandemic and the 2009 swine flu outbreak. 

Influenza is a contagious disease - meaning it can be passed on from one person to another. It is spread via droplet infections - i.e if someone is infected, the air that they breathe out (or cough or sneeze) will contain the influenza virus. If you then breathe this air in, you can get it yourself. However, it will depend on how much you breathe in, known as ‘viral load’. Small amounts can be dealt with by the immune system without causing any symptoms, but larger amounts can cause illness.¹ 

Influenza vs the common cold or COVID-19

A cold is not caused by an influenza virus, even though the illnesses may have similar symptoms. Colds are caused by different viruses such as rhinovirus. Fever and body aches are very uncommon in the case of a common cold, but are often present in flu. 

Influenza is also different from COVID-19. Symptoms of flu and COVID-19 can be very similar, such as fatigue, sore throat, or even respiratory distress or failure in severe cases. COVID-19 is caused by a virus called SARS-CoV-2, and a test such as rapid antigen testing or PCR is the only way to confirm a diagnosis.²

Influenza usually causes epidemics of varying severity (dependent on the region) in the winter seasons annually. 

What is sinusitis?

There are air-filled cavities in some of the facial bones - these are known as sinuses. These are behind the forehead, eyes and cheeks. The sinuses aid in breathing and in producing mucus which can trap dirt that is breathed in. 

Sinusitis is the inflammation of the lining of the sinuses and is a very common condition. 

Sinusitis itself is not a contagious disease - you will not get sinusitis alone from being around someone who has it. However, if the person has a cold for example, which has caused sinusitis, the cold is contagious.³ 

There are four different classifications of sinusitis, depending on how the symptoms start and for how long they last:³

• Acute sinusitis: Sudden onset, lasting less than 4 weeks with complete resolution
• Subacute sinusitis: When symptoms of acute sinusitis continue for more than 4 weeks but less than 12 weeks
• Recurrent acute sinusitis: Four or more episodes of acute sinusitis, lasting at least 7 days each, within a 1-year period
• Chronic sinusitis: Symptoms persist 12 weeks or longer

Similarities and differences between influenza and sinusitis 

Causes

Influenza in humans is caused by types A and B of the influenza virus. Whereas sinusitis can have a variety of causes:⁴ 

• Viral cause - usually when someone has the common cold, they will have some symptoms of sinusitis as well 
• Allergens such as animal hair, pollen, or polluted air - all of which can irritate and inflame the lining of the sinuses
• It can be caused by bacteria or fungi in rare cases

Symptoms

Influenza is an infection that usually affects the upper respiratory tract - meaning the nose, throat, and trachea (windpipe). (Note - this means that the virus has not spread all the way down into the lungs at this point).

It is often accompanied by fever, headache, malaise (a general feeling of unwellness), and muscle pain. This would be a common presentation of flu, and in these cases, symptoms usually resolve within 7 days with home treatment and rest. It is often like a bad cold.¹ 

However, flu in some people is more than just a mild infection. Those who are more susceptible to severe symptoms are younger children, older adults, and those with underlying conditions that compromise their immune system.¹ 

Statistics vary between countries but in 2022-2023, the US reported the following:⁵

• There were around 31 million cases of influenza
• 360,000 hospitalisations
• 21,000 deaths
• Older adults accounted for 72% of these deaths 

Sinusitis is most commonly a localised infection, meaning it stays in the sinuses without spreading to other parts of the body:⁴

• The most common symptoms are pain in the face around the areas of the sinus, or a headache
• Nasal congestion and the production of extra mucus (which can be green or yellow) can also be present
• Usually, influenza symptoms appear relatively quickly, but sinusitis can come on more gradually
• There may also be a sore throat or a cough

Prevention 

Vaccination is one of the most effective ways to reduce the morbidity and mortality rates from influenza, especially for high-risk groups. The influenza viruses mutate often and so have many strains, and each one is genetically and structurally slightly different. 

There is usually a new strain every year which ends up causing annual outbreaks. Vaccine production usually takes place each year to keep up with the new strains, although it can start even before strain recommendations have been released, due to the time pressures to prepare for “flu season”. This can mean that the vaccine is not always 100% effective for preventing the newest strain, but can help to a significant extent, based on similar and previous viruses.^5,6

There is no vaccine for sinusitis. Preventative measures include:² 

• Avoiding allergens 
• Avoiding close contact with those who may have a cold or flu
• Maintaining hand hygiene
• Not smoking (or being around people who do smoke)

Treatment 

• Influenza in mild cases usually resolves by itself with home care, rest, and over-the-counter medication
• Painkillers such as paracetamol and ibuprofen can help with body aches and pain
• Nasal decongestants can help with clearing the airways
• Drinking plenty of fluids will help with loosening the mucus to avoid it becoming thick and difficult to clear
• Throat lozenges and/or cough syrup can help with a sore throat¹

Sinusitis treatment depends on the cause. For viral causes, perhaps if the sinusitis comes with a cold, symptom management may be all that is necessary as the infection is self-limiting - this usually will involve a nasal decongestant. 

The vast majority of cases of sinusitis are caused by viral infections. For sinusitis caused by allergens, antihistamines would help to resolve symptoms. Only around 0.5% of sinusitis cases are caused by bacteria. It is often difficult to determine which cases are bacterial as testing is not routinely done.

On the whole, prescribing antibiotics for cases as a precautionary measure does not usually result in any particular benefit to the patient.²

Complications

Influenza in an older adult (over the age of 65), younger child, or someone who is immunocompromised, should be taken more seriously. Severe cases of influenza are not uncommon, with hundreds of thousands of deaths occurring every year across the world. 

Respiratory symptoms in these cases of flu start when the infection spreads into the lower respiratory tract (i.e., goes below the throat and into the lungs, causing complications like pneumonia). It can even spread to the kidneys, causing kidney failure, or to the heart, and can cause heart attacks. In these cases, influenza is life threatening and can ultimately be fatal.¹

Sinusitis complications are however nowhere near as common as influenza complications. These are usually from acute sinusitis rather than chronic. Severe infections spreading to the back of the eyes, the brain or even the rest of the body, resulting in sepsis, although life threatening, are very, very rare.⁷

Summary

Sinusitis and influenza are two illnesses that can both affect the upper respiratory tract. They can both cause symptoms of nasal congestion, sore throat, and excess mucus production. 

Influenza is caused by a virus, and sinusitis in the majority of cases is also caused by viruses, although sinusitis can have other causes, mainly from allergens like dust, animal fur, or polluted air. 

Both influenza and sinusitis are usually self-limiting conditions, which resolve with home care within a week or two. 

Sinusitis is inflammation of the lining of the sinuses, which are air-filled cavities in the bones of the face. Usually, symptoms are limited to the sinuses themselves. 

Influenza can start off affecting the throat and nose but often affects the rest of the body too - causing fever, fatigue, and body aches. Influenza is life-threatening for some people, where the infection can spread to the lungs and can cause pneumonia and respiratory failure.