The science behind COVID-19 asymptomatic spread
Pictured: Still necessary, unfortunately. Source: Pexels
There are so many myths about COVID-19 that sometimes it is really quite challenging to keep up. Despite spending the end of 2020 writing about numerous myths, and why the were wrong, they still pop up with alarming regularity.
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Case in point: the new idea that if you don’t have current symptoms of COVID-19, there is no reason to worry that you could spread the disease. While this is a very popular thought, it is, unfortunately, just not true.
Pictured: Not a conspiracy. Sorry! Source: Pexels
This is something we’ve known for quite a while, and is actually completely expected given what we know about how viruses transmit and the epidemiology of such infections.
So let’s talk COVID-19, symptoms, and why we are still worried about people spreading the disease even if they feel fine.
Isolating Issues
Why is this all important? Well, the main point of contention is that people who test positive for COVID-19 generally have to isolate, to stop them from spreading the disease on to other people. This is a basic public health measure that has been in place since the start of the pandemic, and is one of the most obvious and easy tools we have in our repertoire to prevent COVID-19 from spreading.
But, goes the myth, if most people are asymptomatic and thus can’t spread the disease, then we don’t need to isolate at all! If a positive test is meaningless in terms of pandemic control, then we’ve all been wasting our time all year with these measures.
Unfortunately, that is just wishful thinking, a fantasy concocted to avoid the frustrating reality that is this global catastrophe.
Specifying Symptoms
The first thing that’s important is to define what we mean by asymptomatic. This is something that is rarely done when people discuss symptoms in news headlines or social media, but it turns out to be extremely important as part of the conversation.
You see, there are three kinds of people who have a COVID-19 infection and are currently not experiencing symptoms — those who never experience symptoms (‘asymptomatic’), those who will go on to experience symptoms in the future (‘presymptomatic’), and those who have recovered but may still test positive on PCR (‘postsymptomatic’).
COVID-19 stock photos are…odd. This one is titled “Woman in white hijab sitting on brown wooden chair”. Source: Pexels
The problem is, when people discuss ‘asymptomatic’, they almost always are referring to the first group of people, those who never experience any symptoms during the course of their disease. And it’s true to say that this group probably does transmit the virus a bit less than people who develop symptoms, if only because they cough and sneeze less and thus get less of their virus all over everyone else.
That being said, we know that they definitely do transmit the disease sometimes, although the precise risk is hard to quantify, which means that transmission from asymptomatic people is definitely a worry. The current best estimate is that asymptomatic people have a 40% reduced relative risk* of passing on the disease to other people compared to those who do experience symptoms, which is substantial but not overwhelming. We also know that truly asymptomatic people — remember, those who never experience any symptoms of COVID-19 during their infection — are a fairly small minority of all infections, making up less than 20% of those infected.
On the other hand, we have presymptomatic people. These are people who are currently not experiencing any symptoms, but go on to develop them in the future. This is very common, because people usually take a few days to develop symptoms for COVID-19 but can test positive to the disease before these start to appear.
Unlike asymptomatic people, presymptomatic people are a big worry. There is some evidence that we are most infectious just before and just after starting to feel sick, which means that if you are presymptomatic you might be the most infectious that you will ever be. The challenge is that it is basically impossible to predict the future, and so very hard to tell is someone who is currently asymptomatic will remain so or is doomed to go on to develop symptoms. So while they may currently not be as infectious as other people, this may not be true forever.
Time is the only difference between “currently asymptomatic” and “presymptomatic” unfortunately. Source: Pexels
Finally, we have postsymptomatic people. This group is the least worrying, because they are easy to identify — they are people who had symptoms, were sick, and got better. While they do sometimes test positive for some time after being infected, it’s a much smaller issue because this can be discussed with a healthcare provider and a plan worked out to see what happens next.
The Science
And this, in a nutshell, is why we use isolation as a useful way to control COVID-19. Yes, asymptomatic people may spread the disease a bit less, but they do still spread it, and more importantly it’s impossible to tell at face value someone who is currently asymptomatic but is really presymptomatic from a truly asymptomatic infection. Even worse, asymptomatic infections are a small minority of all cases, so it’s most likely that someone who is currently not experiencing symptoms will eventually go on to have them and thus is at high risk of passing on COVID-19.
Yes, there are some people who have already recovered but will still test positive, but this is generally a much smaller issue than you might imagine. If you have had symptoms, but are recovered, you can have a few tests over a week to see what’s happening, or consult with your doctor to discuss next steps. The majority of people quarantine for their infection anyway, so adding a few extra days just to be sure you won’t kill your grandparents isn’t quite as worrying as social media might have led you to believe.
Like I said, the stock photos are odd. Source: Pexels
So where do all these myths come from anyway? Well, the recent bout of nonsense appears to have originated from a systematic review and meta-analysis that looked at household transmission of COVID-19, and found that in the few studies that looked into the question that asymptomatic cases spread the disease significantly less to their household than symptomatic ones.
Unfortunately, this study has been completely misconstrued by denialists to mean something that it does not. But rather than listen to me, you could read this excellent twitter thread by A/Prof Natalie Dean, one of the authors on the paper:
Overall, the evidence is clear that asymptomatic spread does happen, even for people who never go on to experience symptoms of COVID-19. Even worse, someone who currently is not feeling unwell might still go on to become very sick, and we really can’t tell that person apart from a truly asymptomatic individual.
Unfortunately, isolation is still an important part of our toolkit to prevent COVID-19 spreading. While there is a light at the end of this dank, dark tunnel, with vaccines already being given across the world, for now we have to remain vigilant and that means isolating ourselves when we get a positive test.
COVID-19 can indeed be spread even if you are not currently experiencing symptoms.
Sorry to be the bearer of bad news.
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*Note: the absolute risk difference here will vary on the attack rates in the population, but you can calculate the very crude number for different situations by multiplying the proportion infected by symptomatic people by ~0.6 if you’re interested.
