Question: Per yesterday’s post, does this research suggest low exposure to the virus can provoke antibodies and some immunity to the virus?

Answer: Hmmmmm… maybe. Two big outstanding questions about COVID-19 remain — 1) How much virus does it take to make a person sick?; 2) Is the size of the infecting dose associated with the severity of disease? As discussed yesterday, some early research has shown that low exposure to the virus (e.g. exposure while wearing personal protective equipment) is associated with asymptomatic infection. People who are asymptomatically infected have similar viral loads as those who are symptomatically infected (see Q&A of 7/4 #Asymptomatic Transmission), which could mean that they go on to develop antibodies and other immune system responses that could confer future immunity. Unfortunately, it also means that they can unknowingly transmit the virus to others. Related, we have recent evidence showing that exposure to other types of coronaviruses does not confer any immunity to SARS-CoV-2 (note: this study is not yet peer reviewed).

Then we have the issue of defining “low exposure” since we still don’t know just how much SARS-CoV-2 virus it takes to make a person sick (e.g. the minimum infectious dose). Based on studies of other coronaviruses and influenzas, coupled with how quickly COVID spreads, experts think the minimum infectious dose is relatively small — perhaps 1,000 virons (e.g. 1,000 of the tiny SARS-CoV-2 virus particles). To put that in perspective, scientists estimate that one infected host cell can produce up to 1000 SARS-CoV-2 virons over the course of infection (Image 1). As reported in Science, “estimates using an average sputum viral load for SARS-CoV-2 indicate that 1 min of loud speaking could generate >1000 virion-containing aerosols. Assuming viral titers for infected super-emitters (with 100-fold higher viral load than average) yields an increase to more than 100,000 virions in emitted droplets per minute of speaking.” Masks keep the majority of these droplets from reaching your airway (Figure 1)! Again, this is why mask wearing, social distancing, and good ventilation are so important.

Image 1. SARS-CoV-2 (in blue) emerging from infected cell (from NIAID)

Figure 1. Masks reduce transmission (from Science)