Question: What’s the latest research on dose response for exposure to COVID-19 and what can it tell us about our risk choices?
Answer: We know that the more virus a person is exposed to, the higher their risk of being infected (for more background, see Q&A of 5/20 #Viral Load, Q&A of 5/12 #Risk). This is a big reason why public health experts recommend keeping interactions where social distancing isn’t possible as brief as possible (<15 minutes). Findings published last week in CDC’s MMWR also indicate that multiple brief exposures over 24 hours that cumulate in exposure time of 15+ minutes can result in infection, hence CDC’s recent “close contact” definition change (discussed in Q&A of 10/22 #Incubation). It’s also why mask wearing is so key! And why outdoors is safer than indoors.
When it comes to the relationship between initial exposure dose and disease outcome, we are far less certain as the science is still emerging. Some scientists have hypothesized that lower initial exposure dose could be an important factor for asymptomatic infection. And this perspective, “Facial Masking for Covid-19 — Potential for “Variolation” as We Await a Vaccine,” published in the New England Journal of Medicine last month hypothesizes that by limiting initial exposure dose, mask wearing could “reduce the severity of disease and ensure that a greater proportion of new infections are asymptomatic.” Here are a few interesting findings, which point to the potential of a relationship between exposure dose and disease outcome:
- This article, “Inoculum at the time of SARS-CoV-2 exposure and risk of disease severity” published in August in the International Journal of Infectious Disease concluded that “a greater viral inoculum at the time of SARS-CoV-2 exposure might determine a higher risk of severe COVID-19.”
- This article, “Factors associated with asymptomatic infection in health-care workers with severe acute respiratory syndrome coronavirus 2 infection in Wuhan” published last month in Clinical Microbiology and Infection concluded that healthcare workers “who performed tracheal intubation or extubation were most likely to develop related symptoms, whereas those taking aggressive measures, including consistent use of N95 masks and eye protection, tended to be asymptomatic cases.” These findings indicate that one reason for asymptomatic infection is that the infecting dose is smaller.
- This article, “SARS-CoV-2 viral load predicts COVID-19 mortality,” published in Lancet in August, found a significant independent relationship between higher viral load and increased risk of mortality. Here, viral load does not necessarily equate with initial exposure dose, so we can’t draw conclusions on that front, but nonetheless, it is in keeping with other pieces of evidence we’re seeing on the dose-response relationship.