Study Explains the Silent Spread of Covid-19

Posted on by Jesse Santiano, M.D.

A study published at Oxford Academic gives a reason why millions have Covid-19 in more than 180 countries.

In this study, the scientist compared the growth of the SARS virus and SARS-CoV-2 and the damage they cause in human lung tissues. They also measured the immune response of the lungs to both diseases.

The SARS coronavirus virus is an epidemic in 2003 that killed 744 people and had a high mortality rate of 9.19%.

The SARS-CoV-2 causes Covid-19. It is called SARS-CoV-2 due to its many similarities with the SARS virus.

How did they test the viral growth and immune response?

The scientists obtained bits of healthy lung tissues from six lung cancer patients who had lung biopsies. Then they put the SARS and SARS-CoV-2 viruses separately in the lung tissues. 

Finally, they looked at three things:

  1. How fast both viruses multiplied
  2. What lung cells get infected
  3. The immune reaction of the lung tissues to both viruses.
1024px-SARS-CoV-2_(yellow)
SARS-CoV-2

Study Results

A. The Covid-19 virus multiplies 3.2 times faster than the SARS virus.

B. The Covid-19 and the SARS viruses infect the lung cells equally.

Type I lung cells are thin and flat. They have to be flat so that oxygen and carbon dioxide can pass quickly and get exchanged. If infected, these cells swell, and the gas exchange becomes harder. This leads to low oxygen levels throughout the body.

Type II lung cells produce surfactant. Surfactants are secreted in the lungs and make it easier for the lungs to expand. When Type II cells are infected, the lungs become stiff, and breathing is much harder. Type II cells can also multiply and replace any damaged Type I cells.

Lung macrophages are a type of white blood cell. They protect the lungs against infections. Once infected, they become useless.

Overall, The Covid-19 and SARS virus prevent the lungs from providing enough oxygen to the body.

C. The Covid-19 infection elicits a much lower inflammatory response than the SARS virus.

In this study, the degree of the immune response was measured by checking the levels of 13 inflammatory factors.

Inflammatory factors increase as a response to an infection. When they rise, symptoms like chills, fever, sore throat and cough begin. If they don’t go up, no symptoms appear.

SARS-CoV-2 activated only 5 of the 13 inflammatory factors. They include IL6, MCP1, CXCL1, CXCL5, and CXCL10 (IP10). Plus, SARS-CoV-2 elicited less expression of genes that make pro-inflammatory cytokines and chemokines. This explains why Covid-19 has mild symptoms in the majority of cases.

In contrast, The SARS virus can activate 11 out of 13 inflammatory factors and explains why SARS produces more symptoms and have a higher death rate. They include the following interferons Type I (IFNβ), II (IFNγ), and III (IFNλ1, IFNλ2, and IFNλ3) and other chemokines and cytokines.

Limitation of the Study

The study was done on lung tissues and is only on a minimal number (6). The lung tissues only last for several days. The immune response of the whole body may be different. Although the results are helpful, more studies are needed.

The results reveal the reason behind the asymptomatic transmission and long incubation period.

To appreciate the infectiousness of Covid-19, let us look at SARS.

The symptoms of SARS patients usually start with a high fever, followed by a rapid course of pneumonia. That is why it is called Severe Acute Respiratory Syndrome or SARS.

In SARS, the patients are only infectious when symptoms start. The presence of symptoms makes it easier to identify them for isolation and testing.  This lessens the spread of SARS.

In contrast, in Covid-19, the immune response is not fully triggered. That is why infected people have no symptoms. The decreased immune response allows the SARS-CoV-2 to multiply rapidly.  At the same time, these asymptomatic carriers can freely move about for several days and infect others.

That is why there are more Covid-19 patients.

Hand-washing, social distancing, and wearing a mask can prevent the spread of Covid-19.

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Reference:

Hin Chu, Jasper Fuk-Woo Chan, Yixin Wang, Terrence Tsz-Tai Yuen, Yue Chai, Yuxin Hou, Huiping Shuai, Dong Yang, Binjie Hu, Xiner Huang, Xi Zhang, Jian-Piao Cai, Jie Zhou, Shuofeng Yuan, Kin-Hang Kok, Kelvin Kai-Wang To, Ivy Hau-Yee Chan, Anna Jinxia Zhang, Ko-Yung Sit, Wing-Kuk Au, Kwok-Yung Yuen, Comparative replication and immune activation profiles of SARS-CoV-2 and SARS-CoV in human lungs: an ex vivo study with implications for the pathogenesis of COVID-19, Clinical Infectious Diseases, ciaa410, https://doi.org/10.1093/cid/ciaa410

Image Credit:

SARS-CoV-2 By NIAID – https://www.flickr.com/photos/niaid/49531042877, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=87014916

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