COVID-19 and Infectious Dose

The COVID-19 disease is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). For the COVID-19 disease to occur from exposure to the SARS-CoV-2 virus, a sufficient amount or “infectious dose” of virus needs to enter the body. This means enough virus would need to enter the respiratory tract through inhalation, or into the mouth, nose or eyes from surface contact with your hands. 

A dose-response curve is used to show the relationship between the amount of virus a person is exposed to, the dose, and the potential for infection or response. Figure 1 below shows a dose-response curve for SARS-CoV-1, a dose-response curve for SARS-CoV-2 is not available at this time. This dose-response relationship shows us that very low doses, the left side of the curve, are below the infectious dose and are not expected to result in illness. Controls from masks to social distancing are intended to move your level of potential exposure to the left and keep it at the non-infectious level.

Figure 1 – SARS1 Dose-Response Curve

While we don’t know exactly how much of the COVID-19 virus will cause the disease, there is reliable data that the dilution effect of distance reduces the risk of illness. Therefore, it’s reasonable to assume that this same type of dose-response curve is valid for COVID-19.

As the source of the virus is an infected person, the closer we are to the person or source, the higher the amount of virus we may be exposed to. With social distancing as the distance increases the risk decreases. Masks substantially reduce the generation and travel distance of both droplets and aerosols from an infected person, and masks may also provide some beneficial filtration for the user. Most building ventilation systems, or even just opening a window for fresh air, will dilute airborne virus reducing the risk of transmission. Cleaning of surfaces in public areas, hand washing or sanitizing, and avoiding hand contact with your face after contacting surfaces all reduce the amount or dose of virus that may potentially get into your body.

Time is also a factor in dose. When walking past someone in the supermarket you are potentially exposed to much less of their exhaled aerosols than in a shared office space, restaurant or bar. Think of it as walking past a smoker, or sitting across the table from them, it’s the same concept.  Reopening and increasing the time we are in contact with other people does increase risk, which makes our other controls even more important.