SciMoms COVID FAQs: What is COVID-19?

In this series of COVID FAQs, the SciMoms answer frequently asked questions about the coronavirus or COVID-19 pandemic. In this first post, we focus on the basics: what is COVID-19? How do viruses work? How did this particular virus become a pandemic? How infectious is COVID-19?

How do viruses work and how they make us sick?

Viruses are infectious agents that can only replicate inside a living host cell. They consist of genetic material wrapped in protein and sometimes lipid membranes. This genetic material can be RNA or DNA, depending on the type of virus. A virus enters a host cell and uses the machinery in that cell to create more copies of itself. New viruses leave the host cell by either pushing through the host membrane, which leaves the new virus with a lipid membrane (known as an envelope), or when the host cell dies and ruptures. In fact, viruses aren’t even considered living organisms, because they rely so heavily on their hosts for survival.

Viruses can cause symptoms by stimulating the immune system or damaging tissues in your body. It is usually a combination of these processes that produce disease pathology and symptoms. For example, fever is a symptom that the immune system uses to fight an infectious agent. However, in some individuals the immune response can get out of control and be harmful to the person.

Coronaviruses are a family of viruses known to cause upper respiratory infections. Coronaviruses use RNA as their genetic material and are enveloped (they have the lipid membrane mentioned above). Their protein coat has “spikes” that give this family of viruses their name.

A rendered image of a coronavirus with its “spikes” (image from Pixabay)

Two coronaviruses that caused recent outbreaks are the SARS coronavirus (SARS-CoV), which caused severe acute respiratory syndrome (SARS), and the MERS coronavirus (MERS-CoV), which caused Middle East respiratory syndrome (MERS). The current pandemic is caused by the coronavirus SARS-CoV2, which causes the disease COVID-19. COVID-19 received its name because it was first discovered in 2019. 


Viruses (Updated) (by the Amoeba Sisters)

Coronaviruses: An Overview of Their Replication and Pathogenesis



Why did the SARS-CoV2 virus become a pandemic?

Generally speaking, SARS-CoV2 became a pandemic because of three main factors:

  • It is a respiratory virus that is easily transmitted through coughing or sneezing (see FAQ on how SARS-CoV2 spreads). 
  • It also appears to usually produce a mild disease, with some people not realizing they are infected so they can pass the virus on without realizing they are sick (see FAQ about asymptomatic spread). 
  • Finally, SARS-CoV2 is a new virus for humans, meaning that before the present pandemic, no one had immunity to it from a previous infection. 


Profile of a killer: the complex biology powering the coronavirus pandemic

How related is COVID-19 to the flu?

SARS-CoV2 is not closely related to the influenza viruses that cause the flu, even though both are respiratory viruses with lipid membranes (envelopes) that use RNA. The influenza and coronavirus belong to different groups and families of viruses. They are both RNA viruses, but their genetic code is read differently.  


Types of Influenza Viruses

Coronavirus | Human Coronavirus Types

Q&A: Influenza and COVID-19 – similarities and differences (Explains differences and similarities between the 2 diseases) 

How are Viruses Classified?

How infectious is COVID-19? What is the “R-value” and why is it important?

When compared to more familiar viruses, SARS-CoV2 is much less contagious than measles, but more contagious than seasonal flu. This comparison is based on R0 (R-nought), or basic reproduction number

R0 reflects the power of the virus itself – it’s potential to spread. While this number is largely based on properties of the virus itself, it also depends on people and R0 can vary from place to place. For example, R0 is higher in dense cities than sparse rural areas. 

We can look at R0 values to see how infectious SARS-CoV2 is compared to more familiar viruses. R0 for seasonal flu is about 1.28, estimates from R0 for SARS-CoV2 range from 2.2 to 5.7. Measles has an R0 of 18. This means that SARS-CoV2 is much less contagious than measles, but more contagious than seasonal flu.

Rt stands for effective (or current) reproduction number and is a measure of how much a disease is currently spreading. Rt is the average number of people a person with the disease infects before they recover or die. For example, an Rt of 5 means that, on average, each person with a disease infects five other people. If the Rt is below 1, the number of new cases of the disease should drop. If the Rt is above 1 the number of new cases of disease will increase. Roughly speaking, R0 is what Rt is at the start of an outbreak, before immunity or interventions. R0 and Rt are different but have been conflated in most reporting.

How does “Rt” change?

Both the biological properties of a virus and the behavior of people in an affected area influence the Rt. Thus, it is possible for the exact same virus to have different Rt values in different areas depending on how people interact with each other. In addition, Rt can change over time as people gain immunity and interventions change. This is important with COVID-19 as practices such as scrupulous hygiene, mask wearing, and physical distancing can and have lowered the R-value.

When you watch the stats for how your area is doing, an R less than 1 is a good indicator that your region is doing well. R less than 1 indicates that each infected person infects less than 1 person and cases will be decreasing and eventually stop.


What Happens Next? COVID-19 Futures, Explained With Playable Simulations

Rt: Effective Reproduction Number (Tracks R values in the US by state)


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