Introduction

The 21st century has witnessed several epidemics and pandemics of airborne viruses causing respiratory infections. The most recent one, COVID-19, had already led to over 7 million deaths globally up to August 2024. The mortality rate of hospitalised COVID-19 patients was around 16% on average.¹ Yet the rate was 1.44 higher for immunocompromised patients.² 

If you are over 80, experiencing cancer, using immunosuppressant medications following surgery, or diagnosed with immunodeficiency disorders, be aware of the augmented risks of severe symptoms or even death from acute viral respiratory infections. A better understanding of why you are more vulnerable in front of these diseases is useful for getting prepared for the next flu or pandemic.

Your immune system and how it gets weakened

The immune system

Imagine your body as a wealthy city with invaders coveting the resources within. The immune system is then the defence system that keeps the invaders out. This includes: 

• Your skin - acting like a wall impenetrable to pathogens
• Your mucus - acting like traps behind the walls
• Most importantly, your immune cells - acting like an army of defence attempting to kill pathogens by all means possible

The innate immune system

In an infection, the innate immune system first tries to kill the pathogens. This system involves all defence mechanisms that you are born with and is always prepared to bring down pathogens. For example, when encountering pathogens, neutrophils circulating in your blood engulf them or release toxins to kill them. 

The acquired immune system

In many cases, when the pathogen count is overwhelming or they have mechanisms to evade the innate immune cells, the innate immunity is insufficient to stop the infection. Rather, another system, known as acquired immunity, comes into play. 

With the help of innate immune cells, T-helper cells that can recognise this type of pathogens are activated. They in turn stimulate T-killer cells and B cells that are specific to the pathogen. In a few days, the former can kill infected body cells and pathogens within; the latter can release proteins called antibodies, which can bind to the pathogens. This binding can tag pathogens for attack by other cells like neutrophils, aggregate pathogens to prevent them from invading body cells, and even neutralise toxins secreted by pathogens. 

Typically, an infection is suppressed after the release of antibodies. After the first infection, the pathogens are “remembered” by the acquired immune system, and specific antibodies are released against them immediately if they invade again, suppressing second or third infections at once.

Immunodeficiency

However, not everyone has a perfectly functioning immune system that effectively fights off pathogens. There are many situations where people have weakened immune systems. Some people may be born with an imperfect immune system, which is known as primary immunodeficiency. It is caused by any inherited genetic mutations that affect the functioning of any parts of the immune system mentioned above. 

The immune system may also be compromised by non-congenital diseases. This is known as acquired immunodeficiency. Among them, the most common one is Acquired Immunodeficiency Syndrome (AIDS). HIV viruses enter T-helper cells, replicate in them, and eventually kill them.³ Without T-helper cells, B cells cannot be activated, so antibodies cannot be secreted, aggravating infections of all kinds of pathogens. 

Disease-unrelated factors can also contribute to acquired immunodeficiency. Age, for example, is associated with a reduced number of T cells and thus weakened acquired immunity. The use of immunosuppressant medications, like after a transplantation surgery, also hampers immunity.

People with weakened immune systems easily suffer from all kinds of infections that lead to a variety of symptoms. For example, infections in your digestive tract can lead to diarrhoea or constipation. In terms of acute viral respiratory infections, much more severe symptoms can take place in immunodeficient people than in people with normal immunity as it will be explained below.

The impact of acute viral respiratory infection

Acute viral respiratory infection

In the past five years, we have witnessed an unprecedented global pandemic—COVID-19. Along with other notorious diseases like SARS and MERS, it is classified as an acute viral respiratory infection. This refers to a sudden (with a rapid onset) infection in the upper (from your nostrils to your voicebox) or lower respiratory tract (from your windpipe to your lungs) by viruses such as the coronaviruses.⁴

The immune system plays a vital role in fighting off acute viral respiratory infections. After the viruses enter your body, they sneak into your body cells, like those lining your respiratory tracts. They use their resources to replicate themselves without being detected by your immune cells. This period is known as the incubation period. For COVID-19, this period typically ranges between 2 and 14 days, with an average of around 6.⁵ When the dose of viruses achieves a certain level, innate immune cells can detect them. 

Once the innate immune system detects the viruses, it acts to not only control viral replication by killing infected cells but also create an environment hostile for viruses yet advantageous for immune cells, which actually leads to the symptom of fever.⁶ More importantly, it also presents dissected parts of viruses to the adaptive immune system in order to selectively activate specific T-helper, T-killer, and B cells. This typically takes between 6 and 10 days to occur for COVID-19.⁷ During this period, the viruses induce your body to cough and sneeze so that they can spread. With the proliferation of adaptive immune cells and the production of sufficient antibodies, viruses are gradually eliminated.

Severe symptoms and complications

For healthy people, COVID-19 and other acute viral respiratory infections are overwhelming. Yet for people with compromised immunity, they can be even life-threatening. They cannot efficiently eliminate viruses from their body and experience more adverse immune reactions.

If your innate immunity is not fully functional, the viruses have more time to replicate and thus expand to a great number.⁷ They can infect a larger number of body cells and disturb the functioning of several body organs, such as your gut and your kidneys.⁸ Additionally, because innate immune cells are responsible for stimulating adaptive immunity, their malfunctioning also leads to a delay in the adaptive immune response, providing more time for viruses to destroy more body cells. 

When adaptive immunity is defective, the innate immunity remains normal (the case for aged patients and HIV carriers). This means the work that antibodies and T-killer cells are meant to do is left for the innate immune system to do. This means that inflammation and fever are prolonged. Since inflammation are beneficial for immune cell functioning, but not optimal for body cells, an extended period of inflammation is harmful. 

For COVID-19, this excessive inflammation occurs primarily in the lungs, which is known as COVID pneumonia. Air sacs in the lungs, known as alveoli, which are responsible for gas exchange, can no longer function properly. This is due to fluid accumulation caused by the inflammation. 

COVID pneumonia is observed in severe cases of COVID-19, and there is an elevated risk for immunodeficient patients. If it further progresses, it can develop into acute respiratory distress syndrome, in which case you need a ventilator to survive. If the excessive inflammation leads to the failure of other body organs like the kidneys, the illness becomes life-threatening.

The greater risks of serious breathing difficulties and organ failures mean that immunocompromised patients need more medical attention.  reported that these immunocompromised patients had larger chances of being admitted into the intensive care unit (ICU) or failing to survive COVID-19.⁹

Longer recovery and lasting effects

As mentioned above, compromised innate immunity delays the acquired immune response, and deficient acquired immunity cannot wield full power against viruses. Hence, the recovery of immunocompromised patients from the infection is much slower than healthy individuals. 

These delays also give viruses more time to replicate in body cells and lead to a reduced ability to clear viruses out from the body. Therefore, the viral load in immunodeficient patients can remain at a high level for months or longer.⁷ This is probably the cause of long COVID these patients are likely to experience, and is characterised by persistent fatigue and shortness of breath.⁹

Preventive measures

Considering the unpredictable adverse outcomes of such an infection, it’s worth the effort to avoid catching it and prevent the complications if you unfortunately catch it.

There are several ways you can prevent getting infected by airborne respiratory viruses:

• Getting vaccinated: various vaccination technologies are mature nowadays. Consult your doctors for which one is the most suitable for your immune conditions
• Social distancing: the fewer people you meet, the lower the risk of getting infected. If certain social events are necessary, wear a mask
• Keeping hygiene: wash hands regularly using soaps. Disinfect surfaces using detergents

If you are unfortunately infected, seek help and advice from your doctors at once for antiviral treatment available and other necessary measures. Once you turn negative, avoid too much physical activity and inform your consultants of any discomforts if there are any because it takes longer for you to fully recover.

Summary

To conclude, since the immune system is the primary player in fighting against acute viral respiratory infections.

Deficiencies in either the innate or the acquired immune system can result in serious undesirable outcomes, including severe pneumonia, multiple organ failures, and incomplete recovery. 

Therefore, if you are aware that you are, by some means immunocompromised, and there is an airborne disease rife in your area, take all preventative measures and obtain professional advice when you feel necessary.