Upper respiratory vs. lower respiratory tract infection: Which COVID-19 variant affects what and how serious can it get

In just over two years, multiple SARs-COV-2 variants have emerged, affecting our lives in the most grievous ways. We have battled three coronavirus waves and are still recovering from the impact of the most recent Omicron variant. While the heavily mutated strain is believed to be highly transmissible, the Delta variant, which was one of the driving forces of the second COVID-19 wave, is still the most dominant strain across the globe. However, both these WHO-declared 'variants of concern', although very contagious, are said to affect the body in a different manner. Hence, experts have noted differences in the kind of symptoms triggered by each variant. Let us find out how and why.

Also read: Coronavirus: If you had the original Omicron infection, can you still catch 'stealth' Omicron?

Our respiratory system is divided into two main parts: Upper and lower respiratory systems. The upper respiratory tract consists of the nose, nasal cavity and the pharynx, whereas the lower respiratory tract is associated with the larynx, trachea, bronchi and the lungs.

Therefore, an infection of the upper respiratory tract most often includes the common cold, tonsillitis, a sinus infection, laryngitis and the flu, which cause mild symptoms such as a sore throat, runny nose, sneezing, headache, muscle pain, etc.

Also read: Coronavirus: Reasons why COVID-19 cases are rising around the world, as per the WHO

On the other hand, lower respiratory tract infection involves bronchitis, pneumonia, bronchiolitis, tuberculosis and sometimes even the flu.

The SARs-COV-2 virus has also been identified to cause infections of both upper and lower respiratory tract, depending on the kind of COVID-19 variant one gets infected with.

While upper respiratory tract infections affect the throat and sinuses, leading to mild cold, headache, sore throat and other cold-like symptoms, lower respiratory infections usually last longer and are more serious. The latter may cause severe damage to the airways and lungs, increasing the risk of complications.

The Delta variant took over and replaced other globally dominant variants such as Alpha, Beta, Gamma along with three other variants, Eta, Iota and Kappa. It not only drove the second wave of COVID-19 in India, but also led to a sudden spike in the number of COVID cases in the UK and the US.

But what was and is still concerning about the Delta variant is that it is highly contagious and can also cause severe illness, leading to hospitalization and death. Experts believe this is because the Delta affects the lower respiratory system, and directly attacks the lungs. It not only infects the lungs but also causes inflammation, leading to the accumulation of fluid in the air sacs in the lungs. This then restricts the normal flow of oxygen to the blood, leading to shortness of breath, breathlessness, chest pain, low blood oxygen levels in the body, etc.

Unlike the Delta variant, Omicron appears to cause less damage to the lungs and is said to primarily affect the upper respiratory tract. “It’s fair to say that the idea of a disease that manifests itself primarily in the upper respiratory system is emerging,” Roland Eils, PhD, a biologist at the Berlin Institute of Health, told the New York Times.

Initially when the variant emerged, experts were concerned about its heavy mutations and its ability to dodge vaccine-induced immunity. However, while the Omicron variant proved to be highly infectious, researchers noted that it caused mild illness and cold-like symptoms.

According to Tim Spector, Head of UK's ZOE symptoms study app, some of the most common symptoms include sore throat, runny nose, fatigue, headache, body pain, mild fever, cough and digestive issues in some.