Routes of transmission
According to WHO at present we are aware of three ways of transmission of the novel coronavirus:
Symptomatic transmission- which happens from a person is experiencing symptoms
Pre-symptomatic- the incubation period or the time between exposure and onset of symptoms of for COVID-19 is on an average of 5-6 days but can also take 14 days. The person is still capable of infecting others before symptom onset
Asymptomatic transmission- which is the transmission of the virus from a person, who does not develop symptoms.
WHO clarifies that though there are’ few reports of laboratory-confirmed cases who are truly asymptomatic, and to date, there has been no documented asymptomatic transmission’ but this does not exclude the possibility that it may occur.
Asymptomatic carrier
A study published in the International Journal of Infectious Diseases about five family clusters from Wuhan and Luzhou, China showed how asymptomatic carriers can be an important link in the spread of the virus. A 50-year-old woman after returning from a family reunion was admitted to hospital in February after testing positive for coronavirus. Soon after four other family members also tested positive and showed symptoms of varying seriousness. Through research, it is deduced that some family members were asymptomatic leading to the spread of the infection without their knowledge. Over days while few showed mild symptoms there was still one case among the five who showed no symptom and had no adverse impact. The researchers say that the study indicates that ‘asymptomatic carriers can be a latent source of COVID-19’
Possible causes
An article published in The New England Journal of Medicine (NEJM), says while in the case of SARS-CoV-1 or the virus that causes SARS, replication of the pathogen happened in the lower respiratory tract, SARS-CoV-2 may stay on the upper respiratory tract in large amounts even in pre-symptomatic patients. Also, while viral loads (quantity of virus in a particular amount of body fluid) of SARS-CoV-1 peak around 5 days after the onset of symptoms. This is not the case of the novel coronavirus that may be present in high numbers even before symptoms show. Research published in the medical journal The Lancet that had tested saliva samples and other body fluids of patients from two hospitals in Hong Kong suggested that ‘SARS-CoV-2 can be transmitted easily, even when symptoms are relatively mild. This finding could account for the efficient person-to-person transmission noted in community and health-care settings.’
Another study on the nursing facility in King County, Washington, United States showed that most cases who tested positive for the virus later were asymptomatic at the time of testing. While researchers still argue if such cases should be considered Pre-symptomatic, the one big thing even when a person is not showing any symptoms, they are highly contagious and symptom-based testing is not enough.
Importance of Face masks
Widespread tests in areas with a high concentration of cases can help which includes those that currently do not show any symptoms that can be helpful. According to a report by Times of India, ICMR is also looking at the door to door surveillance just like what was used in for eradicating polio to ensure that asymptomatic cases are also accounted for. Although such methods have their own restrictions given that the low number of test kits available in the country and the expenses that it may incur.
Meanwhile, simple methods such as wearing face masks and maintaining social distancing could be an effective tool in preventing infection. According to the article in NEJM, the prevalence of asymptomatic cases support use of face masks in crowded outdoor or indoor spaces.