A Useful Model

Scientists create mental models to help understand the world around us.  Not that these models are reality; the real world is much, much more complicated.  Instead, these models are useful ways to think about reality.  Today I would like to create a mental model of the SARS-CoV-2 virus that will be useful as we learn more about the virus in coming weeks and months.

Think of the virus as an egg with spikes driven into the shell.  Now focus on two parts of this model: the spikes and the yolk inside the egg.  The spikes correspond to the S-proteins on the outside of the virus.  That’s easy to remember—S for spike.  The yolk corresponds to the nucleocapsid that covers the genetic material on the inside of the virus.  Let’s call the nucleocapsid “N” for short, which will also help us remember that N-proteins are inside the virus.  Now in our mind’s eye we see a virus shell with S-proteins on the outside and N-proteins on the inside.

When foreign proteins show up in your body, your immune system responds by making antibodies.  Therefore, when infected by SARS-CoV-2, your body will make at least two different types of antibodies: S-antibodies and N-antibodies. Blood tests are now available that can detect both kinds of antibodies, and both should be detected in someone who has been infected by SARS-CoV-2 in the past.

Vaccines expose the body to S-proteins only.  N-proteins are not part of vaccines.  Therefore, someone who has received a vaccine but has never been infected will have S-antibodies but not N-antibodies.  S-antibodies may come from vaccine or infection.  N-antibodies come from infection only.

Now we can use our model to predict antibody test results from four different groups of people, represented in the table below:

	Previously Infected	Never Infected
Vaccinated	S+ N+	S+ N-
Unvaccinated	S+ N+	S- N-

A simple model, but does it work?  To find out, I collected the results of antibody tests from four groups of people: people infected but not vaccinated, people vaccinated but not infected, people vaccinated and infected, and people never infected nor vaccinated.  Next time, we will review the results.