Transmission potential of pre-symptomatic persons appears to be low
Dr. G. V. S. Murthy (Source: By Arrangement)
There is fear and anxiety among people ever since outbreak of COVID-19 which has become a pandemic leading to lockdowns till May 7 with much uncertainty ahead. In this interview, Director of Hyderabad Chapter of the Indian Institute of Public Health G.V.S. Murthy and his wife medical microbiologist P. Vishalakshi explain about the virus spread and bust a few myths.
What are common ways of getting infected?
Major pathway for infection is through close contact and through large droplets (>5 μm in size) that fall rapidly to the ground and do not travel more than one meter. They can reach a person in close contact or a surface like table tops, door knobs, cardboard, cutlery etc. within that distance range. Direct face-to-face contact with an infected COVID-19 patient for a long time increases the risk of infection or sustained contact of more than 15 minutes can be dangerous. This does not discount other potential modes of transmission but they account for only a small proportion.
What about persons not showing any clinical features of infection?
World Health Organization (WHO) states that to date there is no documented evidence about transmission of COVID-19 from people not having any clinical signs of the disease. We know that for every 10 people coming in close contact with a positive patient, there would be 7-8 asymptomatic, while 2-3 people develop symptoms.
Is asymptomatic transmission a good thing?
Yes, because these people are unlikely to get infected again in the present cycle and therefore, add to ‘herd immunity’. It simply means the proportion of people protected from future infection, either through vaccination or from having the disease. Once a critical proportion of the population is ‘protected’, it reduces the number of people who can get infected. The level of ‘herd immunity’ required to break further transmission varies for each disease.
Can you give us an example?
For measles, it could be as high as 90-95% and for diphtheria it is 80-85%. Experts believe for COVID-19, if herd immunity is 55-60% it can help in breaking the transmission chain. Since, there is no vaccine available at the moment, it is only those infected and recovered who are our line of defence. Hence, recovered patients and those asymptomatic, are critical elements.
It is not necessary for every ‘exposed’ person to get infected?
Higher the dose of exposure to the virus, higher is the risk of infection. A fleeting exposure to a small amount of virus may not result in an infection. Clinical signs like cough, fever, sore throat etc., show up in 3-4 days and can continue for 14 days but maximal viral shedding occurs early therefore early identification is crucial. So, tracking such persons and isolating them can contain the spread.
Who should be put in isolation?
Tracing those who have come in contact with an infected person should be kept under close supervision and isolation till they are free of any clinical manifestations. However, the transmission potential of such pre-symptomatic persons appears to be low. This again is a good sign for us.
Are lockdowns going to halt transmission?
Holding back a large proportion of population is a good short-term strategy but it does not prevent the susceptible from getting infected in the future. Sudden opening up can expose a large number of susceptible people, so opening up gradually helps the government monitor how the situation is panning out and if necessary, the ‘sluice gates’ can be closed down again. But, hand washing, personal hygiene and protection practices should be ingrained in our minds forever as we will soon be grappling with diarrhoea, malaria and dengue.
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