Jul 4, 20207 min read

A Closer Look at COVID-19 and its Spread

What you need to know about COVID-19's structure, spread, and our immune system.

Facing the New World: A COVID-19 Reality Part 3

Written by: Akshaya, Kavya, Haritha

So far, we have talked about viruses generally. In this post, we will be looking at COVID-19 in more depth.

COVID-19 Structure

Let’s start with the structure of the COVID-19. COVID-19 is a spherically enveloped viral particle with spike-like projections on the outside. The capsid inside the sphere contains positive, single-stranded RNA (2). The information from the RNA strand (in the form of sequences as discussed in the last blog post) is what is responsible for the viral replication. A notable feature of COVID-19 is the spike protein surrounding the outer membrane (2). The spike protein structure is made from three to four distinct proteins. One such protein is the S protein which plays an important role in silencing the detection of a foreign body by the antibodies (more on antibodies towards the end), making it harder for our immune system to detect its presence. The delay in the detection of the foreign body is what makes someone pre-symptomatic and delays the onset of the symptoms found in COVID-19 patients.

The cover picture: This is a 3D model of one of the spike proteins of COVID-19. As you can see it is made from multiple proteins (represented by the different colors). One of these proteins is the S protein.

More about the viral RNA content:

First of all, have you ever wondered why we get a fever when we are infected? Fevers raise the temperature of our body. When we are infected with a foreign body (bacteria or virus) the rise in temperature makes it harder for the pathogens to survive in our bodies. So, getting a fever helps fight against the pathogen and further activates our immune system. However, getting a fever is not enough to kill COVID-19. Here’s why:

Let’s start with this fact - Coronaviruses have several G-C sequences varying between 32% and 43% (2). So what is the significance of that statement? As we discussed in the last blog, genomes can either be made of RNA or DNA. DNA and RNA are made of four repeating bases (A, T/U, C, and G). If you remember from your general biology class, there is a pairing rule between the bases such that the A and T form a pair and C and G form another pair. These pairings are made due to the compatibility of their chemical structures. The pairing is also important for the structure of DNA and RNA. A and T have two hydrogen bonds between them whereas G and C have three hydrogen bonds between them.

If you think about it, the more bonds or connections there are, the harder it is to break the connection. In other words, it requires more energy to break the bonds between G and C than A and T. Therefore, the more G-C content in the genome the more bonds you have, and the more energy it takes to degrade it and make it non-functional. Generally, raising the temperature will aid in degrading the viral genome and deactivate its replication. However, as COVID-19 has a higher temperature tolerance due to the high G-C content in its genome, a fever of a really high temperature is required to degrade its genome. But if we were to get a fever of that temperature, our own cells would start to die. Therefore, a fever is not sufficient enough to kill off COVID-19. This is one of the reasons why COVID-19 is so dangerous.

Now how exactly did the novel coronavirus spread that quickly and how was it discovered? The first known case of the virus was in Wuhan in December 2019 presented as a severe case of pneumonia. In January, this new virus was identified as COVID-19 and it was discovered to have more than 95% similarity with the bat coronavirus and more than 70% similarity with the SARS-CoV (3). On January 23, the several million people living in Wuhan were under lockdown. Due to the mass migration of people from China to the USA (during and after Chinese New Year), cases started occurring in the United States as well. However, when Chinese migration to other countries ceased, there were still newer cases in those countries. Investigations concluded that human to human transmission of COVID-19 had taken place.

Epidemiology

“The virus is viable on surfaces for days if the conditions are favorable.”

When we look at the epidemiology of COVID-19, we can see that it affects all ages. The virus can be spread through large droplets that come from coughing or sneezing of those who are symptomatic, as well as individuals who are pre-symptomatic (before their symptoms show), and also asymptomatic (those who are affected but do not show any symptoms). The virus is viable on surfaces for days if the conditions are favorable. Although this is true, the virus can be destroyed in less than a minute by using common disinfectants, for example, sodium hypochlorite and hydrogen peroxide. To discuss in more depth the way the virus is spread, the infection can be contracted by inhaling the droplets, or by coming into contact and then touching one’s nose, mouth or eyes, with the surfaces that have been contaminated. More specifically, the virus enters the respiratory mucosa through a receptor in the mucosal lining (angiotensin receptor 2). The incubation period of COVID-19 varies from 2 to 14 days (3). The incubation period refers to the amount of time it takes for us to show symptoms.

**Ro (*R-naught***)

Is there a mathematical way to characterize the spread of COVID-19? This is where Ro comes into the picture, commonly referenced in a lot of articles. Ro is an epidemiological metric used to represent the reproduction rate of the disease in an established population. It is also known as the basic reproduction rate. Ro is not constant and can change over time or from organism to organism (7). So how is Ro interpreted? When Ro>1, it means that the disease is an epidemic, when Ro<1, the disease will die out, and when Ro = 1, the disease will sustain or replace itself (8). Earlier studies conducted around six months ago stated the Ro of COVID-19 to be around 5.7 (9). Ro does not indicate anything about how fast the disease will spread or how severe the disease will be. Ro portrays the worst case scenario of how the pathogen can spread across the population without any intervention (12). Recent research predicts that the Ro of COVID-19 is somewhere around 1.5 to 3.5 (12). Also, Ro decreases as people die from the disease or obtain immunity against it. It is also important to realize that while calculating Ro, there are a lot of assumptions that are made accounting for human behavior (11). Therefore, Ro should be seen as a mathematical measure that relies on specific assumptions of human behavior, which as we can predict, can vary.

Immune System

Our bodies have ways of fighting diseases using our immune systems. The immune system aids the body to become resistant to diseases by fighting off the pathogens with a group of specialized cells that defend against these foreign bodies.

There are two types of immune systems in our body: innate and adaptive immune systems. The innate immune system is faster than the adaptive immune system in response but is more general in its attack (10). The innate immune system consists of the mucosal layers and other body fluids. When a foreign body is detected, the innate immune system kills off the pathogen in a short time span. The adaptive/acquired immune system is triggered when the pathogen is not killed off by the innate immune system (10). Although the adaptive immune system requires time to be activated, it can specifically attack the foreign body and remembers the pathogen for quicker response in the future. The immunity that we in turn get is given through the use of antibodies. Antibodies are protein molecules that are produced by the immune system (6). They are created in response to antigens (markers that the immune system recognizes as being foreign), which come from viruses or bacteria, as a means of fighting against them through the function of the immune system.

Antibody: Here you can see that the antibody is specific to an antigen. In the picture you can see that only only the orange antigen fits in to to the Antigen-binding site. This is part of the adaptive immune system, where there is a targeted attack of the pathogen.

Antibodies are produced by B cells, or B lymphocytes, in the bone marrow and found in the blood and lymph. The structure and shape of the antibody allow it to bind to antigens in order to destroy them. They have a Y shape, where the sites at each end bind to the matching site on the antigen. Antibodies are also known as immunoglobulins. Understanding the basics of the immune system and of defense through antibodies is important during this global pandemic because we can learn how to target for the treatment of the disease. In order to explain how this relates to COVID-19, we can start by understanding what serological tests can do. Firstly, serology is the science of dealing with the immunological properties and actions of serum (a fluid component of the blood), which serologists test for antibodies. Serological tests for antibodies to COVID-19 are important because these tests determine if someone has detectable antibodies against a specific antigen (6). If one’s serodiagnosis is seropositive, it means that the serum is positive for antibodies against the antigen that have been found, and the opposite if one is diagnosed as seronegative (6). Performing these tests aid in our understanding of the immune response to COVID-19 and allow those who do have the antibodies to possibly have the chance of becoming donors of convalescent plasma. Therefore, it is important to understand our immune system while considering the different types of treatment we will discuss in the next post. We will talk more about the adaptive immune system in a later post.

Thanks for reading! Please send us any questions and feedback you have. And as always, stay informed, stay tuned, and stay safe!

References:

1. Features, Evaluation and Treatment Coronavirus (COVID-19) - StatPearls

2. Genotype and phenotype of COVID-19: Their roles in pathogenesis

3. A Review of Coronavirus Disease-2019 (COVID-19)

4. https://www.webmd.com/lung/coronavirus-covid-19-affects-body#1

5. Learn about COVID-19

6. https://www.dictionary.com/e/what-are-antibodies-and-do-they-kill-viruses/

7. Complexity of the Basic Reproduction Number R0

8. Introduction to Disease Ecology: Measuring the Impacts of Diseases and their Control

9. High Contagiousness and Rapid Spread of Severe Acute Respiratory Syndrome Coronavirus 2

10.The innate and adaptive immune systems - InformedHealth.org