You have probably been reading more and more about the asymptomatic spread of COVID-19, particularly as we lurch more closely to re-opening businesses and society. It should worry you – but not unduly so. A recent headline from Business Insider and other press reports seem alarming – such as this one: “Nearly 400 employees at a Missouri pork plant tested positive for COVID-19. None of them had symptoms.”

How do you reconcile these headlines with the fact that airports and medical offices will screen your temperature first, if asymptomatic spread of COVID-19 is such an issue? Even the venerable Dr. Fauci has commented that “somewhere between 25% and 50%” of people who have COVID-19 don’t show symptoms. But these asymptomatic or presymptomatic people can still spread the illness to others.

So here is the first key distinction – being truly asymptomatic (having the infection and never ever showing symptoms) versus being presymptomatic (having the infection but not yet showing symptoms). Unfortunately, these two words are used interchangeably or are combined together as a single entity – both of which are misleading.

Data of course are limited but this exact distinction was evaluated in the COVID-19 nursing home outbreak in Kirkland, Washington. There, a health care provider who was working while symptomatic tested positive for infection on March 1, 2020. All residents were then tested. Among 76 residents, 48 (63%) had positive results, with 27 (56%) “essentially asymptomatic.” However, classic symptoms subsequently developed in 24 of these 27 residents on average within 4 days. So although half were asymptomatic at testing, only 3 (3.9%) remained so in the days that followed.

This is not to suppose that transmission of COVID-19 among patients without obvious illness is unimportant. The opposite is true. A high viral load (a measure of how infectious an individual might be) soon after infection is a unique feature of COVID-19. When compared to SARS (also a coronavirus) viral loads peak a median of 5 days earlier in COVID-19, which makes symptom-based detection of infection challenging. In spite of claims that COVID-19 is ‘just like’ the flu, it functions quite differently. With influenza, people with asymptomatic disease generally have lower quantitative viral loads and a shorter duration of viral shedding than those with symptoms. This phenomenon then decreases the risk of transmission from people with few symptoms.

There is additional evidence of a 4% prevalence of truly asymptomatic individuals with COVID-19 infection. In a survey of 328 adults in Shanghai, 13 (4%) patients were asymptomatic. Among 262 hospitalized in Bejing, 13 (5.0%) were asymptomatic cases. However, children may be more often asymptomatic – one very small study of 36 children in Zhejiang, China found that 10 children (28%) had no symptoms.

So where does Dr. Fauci get the 25-50% asymptomatic estimate? That’s from Iceland and it’s probably an incorrect interpretation of their data (sorry Dr. Fauci). For two weeks in March, 10401 individuals were tested on a voluntary basis among the general population of Iceland. A total of 84 infections were discovered by the study – a prevalence rate in the tested sample of 0.81%. However, there was also a targeted testing group – namely those that had traveled recently in high risk areas within that study. Here’s where the data on ‘symptoms’ get interesting – among the participants with positive results, symptoms of Covid-19 were reported by 93% of those in the targeted-testing group but only 57% of those in the population-screening group. But wait, there’s more – 29% of participants who tested negative in the overall population-screening group also reported having symptoms. There was not documented follow-up for any development of symptoms down the line.

So it is better to rephrase the blaring headline below. “Nearly 400 employees at a Missouri pork plant tested positive for COVID-19. None of them had symptoms – yet. But 386 will develop symptoms, on average in 4 days.”

But that’s a terrible headline, and definitely will not get your attention. But headlines are meant to get your attention – accuracy is sometimes secondary.