The SARS-CoV-2 virus can affect both the upper and lower respiratory tracts. The upper respiratory tract consists of the nose, sinuses, and throat, whereas the lower respiratory tract includes the trachea and the lungs.
Mild illness or early SARS-CoV-2 infections are likely to involve upper respiratory tract symptoms, such as a runny nose and sore throat.
Severe illness due to the wild-type SARS-CoV-2 and the previous variants often involves the infection and inflammation of the lungs.
Inflammation can cause fluid to accumulate in the air sacs, or alveoli, in the lungs, reducing the capacity of the lungs to transfer oxygen to the blood.
Scientists have conducted experiments using animal models and laboratory cultures of lung cells to characterize the ability of Omicron to infect the respiratory tract and cause severe illness.
This includes a study carried out at the University of Hong Kong that used human lung cells cultured in the laboratory, to analyze the ability of the Omicron variant to infect the lungs. These cells were cultured from lung tissue removed during treatment of the lung. Typically, this tissue is discarded.
In the study, Omicron replicated 70 times faster than Delta in the human bronchi, which are the tubes connecting the trachea with the lungs. However, it was less efficient at replicating in the lung tissue than Delta and the wild-type SARS-CoV-2.
Other research groups have compared the ability of the Omicron, Delta, and other SARS-CoV-2 variants to cause illness in animal models, such as hamsters and mice.
There is a link between infection with the Delta and other variants and weight loss in hamsters and mice after 1 week, with some data suggesting a correlation between increased virus levels in the respiratory tract and weight loss.
However, various research groups have independently shown an absence of such weight loss in hamsters and mice after an Omicron infection.
Moreover, these studies found that the hamsters with an Omicron infection showed higher or similar levels of virus, compared with the wild-type SARS-CoV-2 and the Delta variant in the upper respiratory tract. By contrast, the researchers observed lower levels of the virus in the lower respiratory tract after infection with the Omicron variant, compared with the Delta variant.
In sum, these studies show that Omicron may be less efficient at infecting the lungs. Significantly, these animal studies show that an Omicron infection results in lower levels of inflammation and injury to the lungs.
Consistent with this, there is growing evidence suggesting that people with an Omicron infection are less likely to be hospitalized or to require intensive care unit admission or mechanical ventilation than individuals with the Delta variant.
Medical News Today spoke with Dr. Scott Roberts, a professor of infectious diseases at the Yale School of Medicine in New Haven, CT. He said,
“A number of laboratory-based studies have now shown that the Omicron variant is less able to infect the lungs as well as other variants and, as a result, is leading to less patients being admitted with pneumonia who require oxygen and ventilators.”
“We are seeing [that] the majority of patients infected with Omicron have mild disease that is more localized to the upper respiratory tract and that hospitalizations are not rising as fast as they have with prior variants,” Dr. Roberts added.
“However, hospitalizations and deaths lag behind overall case counts, and as our cases are continuing to rise and set daily records, we ultimately need to wait several more weeks to get a full picture of disease severity here, in the United States.”