A few days ago, the WHO’s technical lead on COVID-19 Maria Van Kerkhove, caused quite a kerfuffle by suggesting that infected, asymptomatic people with COVID-19 very rarely spread the infection. The WHO and Dr. Van Kerkhove fairly quickly walked back those comments but not before her words raced through the internet and caused heated discussion everywhere.I thought it worth digging into a bit. What is the current science on asymptomatic spread?
First it’s important to understand that asymptomatic can mean at least two things with respect to COVID-19 and I think this is where some of the confusion arose with Dr. Van Kerkhove’s remarks. The first type of asymptomatic patient is the one who is infected and will never get sick with COVID-19. The second is the person who is infected, has not YET shown symptoms of the disease but will eventually become sick. We know a lot more about the second person than we do about the first. The second person is probably better described as pre-symptomatic. Studies indicate that the incubation period for COVID-19 is probably between 2 and 14 days with the average being 5 days. Studies largely out of Asia indicate that pre-symptomatic spread during those 5 days (2-14 days) almost certainly occurs enough to be a significant driver of the disease. Likewise, the a study from April confirmed that viral spread can occur well before symptoms and is likely responsible for up to 44% of secondary case infections. Overall, the data seems to clearly point to pre-symptomatic spread being an important factor in COVID-19 disease worldwide.
The amount of spread from truly asymptomatic individuals, those that never display signs or symptoms of the disease, is less well known but is likely to be less of a factor than for symptomatic and pre-symptomatic people. One preprint study (not yet peer reviewed) out of Australia attempts to quantify the percentage of individuals with COVID-19 who are asymptomatic and found from reviewing over 900 studies that this number is likely around 15%. Of all those studies only four looked at the forward transmission of virus from asymptomatic individuals and hand sufficient numbers of participants to draw conclusions. Those papers found that the rate of transmission ranged from 0-2.2% suggesting that asymptomatic spread occurs quite rarely.
Another study, however, out of Scripps published in the last week looked at specific closed cohorts from around the world (USS Theodore Roosevelt, Diamond Princess, Rutgers University Students, various nursing facilities and prisons to name a few) to examine the rates of asymptomatic spread. Three of the cohorts (from Iceland and Indiana) had representative samples of the population so they represent a better model for estimating true asymptomatic spread. In those cases the researchers found that the asymptomatic rate of infection was 40-45%. Numbers that high clearly indicated that asymptomatic spread could indeed be a very important driver of disease worldwide.
All of these studies have their problems. Many studies looking at asymptomatic spread have too few participants to draw strong conclusions. Others were unable to clearly distinguish between asymptomatic, pre-symptomatic and mildly symptomatic. It is clearly too soon to draw definitive conclusions from these studies or as scientists are wont to say, “more studies are needed.” That’s actually part of what Dr. Van Kerkhove said after discussing the possibility of low asymptomatic transmission rates in her media briefing several days ago but that tends to get lost in stories by the lay-press. When reporters hear a soundbite they know will draw in readers, viewers, listeners and clickers they are excited to report it because they know it’s a hot-button issue. They rarely focus on the cautionary parts of the scientist’s comments because, frankly, they’re a buzz-kill. But science doesn’t work that way. Simply put, it’s too early for us to know the answer to this question. Making policy decisions now with the evidence we have about asymptomatic spread would be utterly foolish. Science is a process that gradually and with methodical verification and re-verification, works slowly but surely closer to understanding the natural world around us.
These nuances are often lost on the lay-press when they report on science and medicine. They’re terrible at it because they want no part of the gray; they want only the black and the white. In the history of science there are only a relatively small handful of topics for which science has gotten anywhere close to the black and the white of it all. Even then, scientists work to reexamine and refine our knowledge of the best understood concepts in science. You can always know more and things can always change, even if incrementally. But when the press reports an informal statement by one very good scientist as potential gospel, only to have to report something different the next day because the first was off the mark, it does nothing but erode the public’s trust in science, medicine and public health.
I urge you to take stories from the lay-press with a gigantic grain of salt. The best studied concepts and theories in science have, in some cases, been examined and reexamined for hundreds of years. We’ve known about SARS-CoV-2 for six months. Think about that. Things will change; our understanding will change. That’s not a fault of science and medicine, the absence of dogma and science’s ability to change is what makes it the best tool humans have ever devised to understand our world and the threats to it.
CHRIS DAEL 