Not the ‘Usual Suspects’: Silent carriers of the SARS-CoV-2
The COVID-19 disease has enveloped the world and driven us into our homes. Although physically distanced, never before has the world been so united and focussed on a single scientific quest as it is today. What made this virus behind this disease, the SARS-CoV-2, take over the human population in just a matter of weeks? The answer to this question is rooted in our understanding of how this infection spreads from one person to another, a.k.a. disease transmission. This feature story will focus on a subgroup of asymptomatic individuals who may inadvertently be transmitting the virus to others.
Individuals infected by SARS-CoV-2 are most likely to transmit the disease during their symptomatic phase, i.e., when they actively exhibit symptoms such as cough, fever or sore throat. A second mode of contracting the disease from infected people is during the presymptomatic phase, a time between infection and actual manifestation of disease symptoms. The length of this presymptomatic phase depends on a number of factors such as the nature of the virus, how many viral particles initially infect the individual, how fast the virus multiplies and reaches the threshold required to show symptoms or the individual’s immune response. In case of the COVID-19 disease, this interim symptom-free period, also known as the incubation period, can last about a couple of days to two weeks. This is quite a long period, when compared to say, the influenza virus (that causes flu) whose incubation period is about 1-3 days. However, the more important question is whether or not an individual is contagious during this period. Much to our tribulation, the SARS-CoV-2 infected individual is.
A third stealthy mode of transmission which is now under scrutiny is viral spread from individuals who are asymptomatic. Asymptomatic individuals are people who are infected by the virus, but do not develop or show symptoms at any point during the infection. The subtle difference between asymptomatic and presymptomatic individuals is that during the course of infection, disease symptoms will eventually become apparent in presymptomatics, but are completely absent in asymptomatics.
The downside to having such contagious asymptomatic individuals is that they seem healthy and hence may not come under the testing radar. These ‘naive carriers’ of the virus could silently enter and exit the infected population, possibly accelerating the pandemic. Therefore, the first challenge is in identifying them.
Identifying the unsuspected
Identification of asymptomatic individuals strongly depends on testing methods. For instance, a screening strategy solely based on symptoms would either miss or highly underestimate the number of asymptomatics. On the other hand, a random and extensive screening strategy will pick them. Another approach that involves tracing and testing close contacts of symptomatic individuals (irrespective of symptom manifestation) will be able to report asymptomatic cases.
China and South Korea, the two countries which have set a benchmark for testing, reported that a third of their tested population was asymptomatic. The Centers for Disease Control and Prevention (CDC) estimate this percentage to be ~25% of the infected population. However, individual reports from various pockets show a variable proportion of asymptomatic individuals among those tested. This is likely an artefact of testing strategies, number of people tested, their age range and how every country classifies infectious patients.
Nonetheless, the take home message here is that a substantial proportion of the infected population could be asymptomatic. This is both good and bad news. Good because a few of us may wade through an infection unscathed. However, it is bad news if the unwitting carriers are contagious and go unnoticed. This would then change the dynamics of disease transmission and in fact add more pressure to the healthcare system.
Evidence for asymptomatic transmission
By carefully studying the timeline of events such as onset of symptoms and travel history, asymptomatic transmissions are coming to light in small familial clusters. One particular report from China identified a family of six, amongst whom five developed notable symptoms of the coronavirus infection. The single remaining asymptomatic member, who travelled back from Wuhan, is suspected to have seeded the infection within the household.
Another instance is from a long-term care skilled nursing facility in Washington. A healthcare worker first tested positive for the virus. Subsequent screening of over 70 individuals within the facility identified ~20% of them to be asymptomatic. Scientists believe that a lack of isolation of these individuals could have set off a wider spread of the virus within the facility.
The question then is if we can find out how prevalent asymptomatic transmission is. The short answer is that we do not know. However, mathematical modelling of the disease outbreak in China estimates that undocumented cases (infected, but symptom-free) were responsible for over 70% of the symptomatic cases, thereby fuelling the global pandemic.
The ‘how’ of asymptomatic transmission
With asymptomatic transmission being quite likely, we might want to understand how it occurs. If infected individuals do not cough or sneeze, how could they possibly infect others? The answer lies in recent findings that indicate that the SARS-CoV-2 may spread through aerosols. Meaning, the disease may be airborne.
Aerosols are fine particles (like mist), measuring about 20 times smaller than the width of a hair. Virus encapsulated aerosols can linger in air for up to half an hour and may then settle down on objects. An asymptomatic individual can passively release virus encapsulated aerosols while talking or even breathing. Thus an exchange of conversation or sharing the same air space with such an individual can likely infect us, provided the number of live viral particles is high enough to trigger a full-fledged infection.
Airborne transmission of the virus is of particular concern in hospitals where healthcare workers and hospital staff may not be equipped with proper personal protective equipment. For example, a few reports have shown the presence of viral particles in hospital vents and in air samples from patient rooms and corridors. However, evidence for aerosol based transmission of the virus is still in its nascent stages and hence one needs to exercise caution, but not necessarily enter into a fit of panic.
Why are some individuals symptom-free?
With asymptomatic cases just beginning to get identified, it is not clear why certain infected people may fail to develop symptoms. At this point, we can only speculate. It is possible that these people, to begin with, were infected with very few, but detectable levels of viral particles. Such a small viral ‘load’ may be easy for the immune system to handle and bring under its reigns. On the hand, in case of a significant viral infection, it is possible that these individuals’ immune response kicked in rather swiftly, prevented the infection from blowing out of proportion and thereby suppressed the symptoms.
Tackling the silent spread
Finally, as our understanding of the disease transmission evolves, so must our precautions. The CDC has revised its guidelines and now recommends that everyone wear a cloth mask when venturing into public places. This would hopefully prevent the asymptomatic individuals from unintentionally spreading the virus and healthy individuals from catching it.
It is however important to remember that this recommendation does not underscore the importance of continuing to socially distance ourselves. Until we reach the safe end of this pandemic, it is best that we stay home and maintain safe distance from each other. One never knows if our unsuspecting friend is in fact the virus in disguise.
About the author: Preethi Ravi is a researcher turned science communicator. She completed her PhD in Neurogenetics from the National Centre for Biological Sciences, Bangalore. She is currently a freelance popular science writer, braiding her appetite for science and passion for writing. Twitter: @catchpreethir