SARS-CoV-2, the coronavirus that causes COVID-19, emerged into the human population late in 2019. COVID-19 is an airway infection much like the common cold. One in six colds are caused by coronaviruses, although these are other types of coronaviruses. Why, then, did COVID-19 become such a devastating pandemic that has led to the deaths of 5 million people and has infected at least 400 million more people worldwide as of early 2022?

The first reason is that the virus was new. No human had ever encountered SARS-CoV-2 before, and therefore, no one was immune. The immune system has two major parts: the innate immune system and the adaptive immune system. The adaptive immune system—the part of our immune system that gets rid of viruses and other pathogens that it recognizes as things that will make us sick—is the one people were lacking. When the immune system encounters a virus, special antibody-producing cells are activated. These cells remember their target (the virus) for a long time after recovery. This is why people who have had COVID-19 or who have been vaccinated often have a much milder illness—or no illness at all—if they are infected with the virus a second time.

SARS-CoV-2 is an RNA virus like all coronaviruses, which means it uses RNA to store its genetic information instead of DNA, which is where our genetic material is stored. This is part of the reason SARS-CoV-2 can produce so many variants so quickly—RNA is more prone to mutations than DNA. But it also makes the virus sensitive to special RNA-binding proteins that activate enzymes that specifically break down viral RNA. People who have a certain form of a RNA-sensing protein are much less likely to get COVID-19, and if they do get it, they’re less likely to become severely ill. This is part of the innate immune system that is present in all cells in the body. (Learn more about the immune system in the I Spy Physiology post “Spotlight On: The Immune System.”)

During the COVID-19 pandemic, scientists have been able to study SARS-CoV-2 in detail and have identified some key findings, including how a strong antibody response can prevent severe illness and how vaccination can help tremendously. Researchers are also looking at why a strong activation of the innate immune system causes damage to other organs besides the lungs. Because the virus is so new, the research has moved quickly, but in a sense, is only beginning. As time goes by, scientists hope to learn much more about SARS-CoV-2.

Michael Hultström, MD, PhD, is a critical care anesthesiologist and associate professor of physiology at Uppsala University in Sweden. His research group works with circulatory function and the development of acute and chronic kidney damage both in experimental models and in patients. During the pandemic, Hultström’s research group has worked extensively with critically ill COVID-19 patients.