Shhh: What is a Silent Spreader?
Another week of social distancing and staying at home draws to a close. After over a month of being in my house, I am definitely beginning to settle in to this “new normal”, but that doesn’t stop me from really missing those friends and family who are isolating in their own homes who I am unable to see. However, maintaining distance from those outside your household is still crucial to prevent further spread of the coronavirus. The main reason for this is because we do not know who is infected and who is not. So even if someone appears healthy, they could in fact be carrying and spreading the virus. These individuals are known as ‘silent spreaders’ and are believed to have been one of the major reasons this coronavirus has managed to touch almost every continent on Earth.
In order to control an epidemic (local outbreak) and even more importantly for a pandemic (global outbreak), scientists need to know a few vital figures in order to model the disease. These include:
How the disease spreads: what is the mechanism and how can we stop it?
How many people have the disease: Who has it and where are they located?
How long does is the interval between an individual catching the virus and getting sick: what is the incubation period?
How many individuals are immune to the virus: Who has already had the disease and how long does this immunity last?
With the current outbreak of coronavirus (specifically known as SARS-CoV-2) which causes COVID-19, scientists are trying to answer the above questions in order to fight and contain the spread of disease. While how the disease spreads is believed to be somewhat understood (although there is still debate regarding how far the virus can travel in the air amongst other theories), the major piece of the COVID-19 puzzle we need is to know how many people actually have the disease. If we knew this exact number, we could determine accurate figures for parameters like infection rate, death rate and the number of people who should be immune.
But, identifying exactly who has been infected with SARS-CoV-2 is a tough task. You may have heard in the news that is it predicted around 80% of individuals who are infected with SARS-CoV-2 have mild to no symptoms. Although this number has come from a small study on 166 individuals in China and more testing is needed to get a real figure for these cases, the study suggests a larger proportion of those infected may not show symptoms for a while if at all. Individuals who carry the virus but do not show symptoms can be defined as ‘asymptomatic carriers’. These individuals may represent people who will go on to show symptoms, people who get mild symptoms or people who never get symptoms. Which of these paths asymptomatic carriers follow depends on how well the virus is able to replicate in their body. This can be impacted by factors like genetics and immune response.
Asymptomatic carriers, although all infected with the virus, are not all able to spread the disease. However, if a person shows no symptoms and is actively transmitting the virus to others, these individuals are described as ’silent spreaders’. Of the asymptomatic carriers, those most likely to silently spread the disease are the individuals who go on to develop symptoms. This is due to the likelihood that these people will have a higher “viral load” (aka more individual virus particles) in their body as they have enough to make themselves sick, meaning there is more chance of this viral particles escaping from this type of asymptomatic individual. Even though the main method of viral transmission is believed to be through coughing, those who do not show this persistent symptoms are believed to be able to spread the disease through touching their mouth followed by other surfaces, the small coughs & sneezes a healthy person has on a daily basis, talking loudly or increased respiration through exercise. All of these methods produce respiratory droplets, which can contain the virus, and so could cause spread. For people who never develop symptoms, these individuals will most likely have a reduced viral load as the virus will have not had the chance to replicate and cause illness, meaning they will not have produced enough particles to escape through respiratory droplets.
With SARS-CoV-2, the virus is believed to incubate in the body for approximately 5 days before first symptoms show, meaning most people who contract the virus are asymptomatic carriers for the best part of a week. It has been reported the highest viral load (when a person contains the most viral particles) in the throats of infected people is at symptom onset, suggesting patients have a significant viral load just before they show signs of the disease. This mechanism is one of the reasons researcher think SARS-CoV-2 has spread so quickly – because people do not realise they have the virus as they feel well enough to go out, but are very likely transmitting to other people. Another study from China reported ~79% of documented cases contracted the disease from non-documented individuals, implying a large majority of patients catch the disease from someone who isn’t presenting or has very early symptoms. These studies, although small, are extremely important because they paint a picture of how this disease spreads and makes modelling this pandemic more accurate; allowing us to pick the most appropriate interventions for containment & treatment.
Together, the data coming out of these early COVID-19 patient studies suggests silent spread is playing a huge role in the transmission of this virus. When a disease utilises healthy-appearing individuals to spread, the best interventions are to make everyone isolate at home and test the majority of the population to pinpoint those asymptomatic carriers. Although the latter is tougher to manage, staying inside a single dwelling is something which many of us can do. Even if our lives are put on hold for a little while and we miss our loved ones, staying in is one of the most important collective acts we can do with this silent spreading disease.
Julia xoxox (@Julia.ravey.science)