Why do some people become seriously ill and even die after infection by SARS-CoV-2 and other people have no symptoms at all? Why do some infected people get COVID-19 and others do not? These are vexing questions that do not have satisfying answers. We will learn much about COVID and the virus that causes it in coming months and years, but today we will consider what is currently known about the penetrance of COVID-19.
Penetrance is a medical term used to describe the relationship between the number of people with a disease and the number of people with the condition causing that disease. If most people with the condition develop disease, the disease has high penetrance. Incomplete penetrance is the term used to account for the fact that not everybody with a condition suffers from the disease caused by that condition. Although these terms have roots in clinical genetics, we may apply these concepts to viral infections. For example, without treatment, most people infected by HIV develops AIDS, but only 30% of people infected by the Hepatitis B Virus (HBV) develop acute hepatitis. (Of course, I must add that with current anti-retroviral therapy, many fewer people with HIV infection develop AIDS.) We can say that among people infected by the causative virus, AIDS has higher penetrance than Hepatitis B. I am sure any number of researchers who can tell you why this is. I cannot. But I can tell you that it happens.
In the case of COVID-19, it seems that penetrance is relatively low. In fact, according to the CDC’s best estimates, the rate of asymptomatic infections is about 40%, meaning that COVID-19 penetrance is about 60%. But that is an overall rate for all patients. Can we identify who is at higher risk for disease after infection?
Although we cannot accurately predict which individuals will get severe disease after infection, we can identify populations who are at greater risk. For example, advanced age is clearly associated with risk of severe disease, and, therefore, higher penetrance. Using the best CDC estimates, about one in twenty infected people over 70 dies of disease, compared with one in 200 aged 50-69, one in 5,000 adults 20-49, and one in 35,000 young people under 20.
There are other conditions that predict serious symptoms after infection. Obesity, diabetes, COPD, heart disease, pregnancy, cancer, sickle cell disease, high blood pressure, smoking and immunodeficiency are all associated with more severe disease, and therefore, higher disease penetrance. But it’s not just the elderly or chronically ill who get sick; young, healthy individuals can and do get seriously ill. Even if you are in a group that has a low disease penetrance, if you get seriously ill, the penetrance for you is 100%.
Because there are still so many unknowns, it is more prudent to avoid infection than “get it over with.” Wear your mask in public, avoid social gatherings, and keep your distance. Stay safe, and help keep others safe too.
