Understanding the spread of the SARS-CoV-19 virus

Padmashree Dr Ravi Kannan

Director, Cachar Cancer Hospital and Research Centre, Silchar

PART - I

The SARS-CoV-19 virus spreads from one individual to the other through droplet infection in aerosols generated during coughing, sneezing or spitting. Since there is no definitive therapy or preventive vaccine, the only and most important strategy that governments and communities have is to slow down the spread of the infection.

The rapidity of spread of the virus or its infectious capability depends on understanding 3 parameters. The first one is the incubation period of the virus which ranges from 1 to 14 days, typically about 4 or 5 days. The incubation period (pre-symptomatic) is the time taken for an infected individual to manifest symptoms. An unknown number of individuals may remain asymptomatic during the entire period of infection. Of those who become asymptomatic, 80% will have very mild symptoms.

The second parameter to understand is the serial interval. This is the time between the onset of infection in the index (primary or first) patient (also called infector)to that in the contact (secondary also called infectee). If the serial interval is less than the incubation period, then spread of infection will happen in the pre-symptomatic period (that is before symptoms develop in the infector). The serial interval of the SARS-CoV-19 virus has been estimated to be 3.96 days. This is in comparison to 8.4 days for SARS and 14.6 days for the MERS viruses. This means that in the current pandemic, transmission of infection will occur in the incubation period in a significant number of cases and that the contact (infected) could develop symptoms before the infector. The public policy implication of this is that containment via case isolation would not be enough. Aggressive testing and rapid contact tracing strategies and isolating primary and secondary contacts as well is important.

The third parameter is the reproduction number or R0. This is the average or expected number of people that an infected person could spread the virus to in his/her 'infected' period assuming a homogenous transmission and constant recovery rate. This also presumes no one has been infected or vaccinated earlier. The R0 for this virus has been estimated from 2.3 to 6.6. The R0 gives us an idea when herd immunity could be reached. Herd immunity is the resistance a community develops to an infectious agent when a certain proportion of the population develops immunity from being infected and cured or through vaccination. The fraction of individuals in the population who need to be infected to achieve herd immunity is calculated as (R0 – 1)/R0. If R0 is 1.66, for example, 40% of the population needs to be infected to achieve herd immunity. If R0 is 5, more than 80% of the population would need to be infected.

In reality not all individuals spread the infections equally. Some infected persons will infect many more than the predicted R0 (super spreaders) while many others will infect fewer or none.

Achieving herd immunity through vaccination is safe while achieving the same through infections will be accompanied mortality (deaths) and morbidity (complications). The death rate from the SARS-CoV-19 is estimated to vary from 1% to 3%.

In addition to ongoing government directed measures of testing, contact tracing and quarantining/ isolation five community-owned and driven measures are important to reduce spread of the SARS-CoV-2 virus in the community. These include wearing masks every time anyone is outside home, meticulously practising social distancing, scrupulously observing hand hygiene and cough etiquette and voluntary testing and quarantining every individual with suspected infection. All of these will help reduce spread of infected droplets from one individual to the other.

Masks should be worn properly covering the nose and mouth. They help break the force of the expectorated plume of the cough reducing the area of spread of the virus. In addition, properly made masks also help filter aerosol particles. While standard 3 ply surgical masks or the N95 or equivalent masks are recommended, home-made masks can be equally effective. It is recommended to use 4 layers of material consisting of 2 different materials (2 layers of high thread count tightly woven cotton and 2 layers of silk) providing for mechanical and electrostatic barriers with an 80 to 99% filtration efficacy. It is important for masks (including N 95 masks) to fit tightly without allowing space between the facial skin and the mask. Even a 1 cm. gap will reduce the effectiveness of a mask significantly. Elastic bands can be devised to achieve this. It is important for everybody to understand that wearing masks is recommended to protect others around them. It is thus every citizen's right to demand others to wear proper masks.

Following social distancing norms of 3 feet between individuals, along with wearing masks will help reduce transmission of the virus between people. This is because, when infected individuals wear masks properly, they limit the spread of infected droplets to a small area of a couple of feet around them, thus preventing others who are further away from inhaling them. The masks also help filter out the aerosol particles containing the virus. It is important to avoid large congregations of any kind. Celebrations of all kinds requiring individuals to be in close proximity are best deferred till a vaccine or remedy becomes available. These should include group celebration of festivals and observing marriage, birth and death ceremonies. Academic conferences are fast adapting to the online mode supported by technology. Social distancing is important while shopping for day to day needs including medicines, grocery, fish, vegetables and poultry and dairy produce. Transaction times must be kept short (definitely under 15 minutes) since transmissions increase as this increases.

The third important behavioural change needed is for everyone to develop a habit of frequently washing one's own hands with soap and water or at least use 70% alcohol-based hand sanitizers when hands look clean. One must definitely wash before and after touching his/ her nose or mouth or eyes.

The fourth important behaviour to adopt is cough etiquette. One should always cover one's mouth and nose with a cloth or tissue paper while coughing or sneezing and discard the piece of cloth or tissue safely. If such cloth is unavailable, it is recommended to cough onto the sleeve of one's dress covering the shoulder. It is not appropriate to cough or sneeze into bare hands or hollowed fists. Spitting in public should be a strict NO and should be strongly disapproved of by the community since this is an effective way of transmitting the infection to many individuals. This becomes important especially considering the high prevalence of tobacco and areca nut chewing.

The last important measure to implement is to show up voluntarily for testing in case one has been exposed to anyone with known infection or has travelled outside of the home district to areas with high prevalence of infection or has unexplained symptoms of cough, fever, body ache, malaise (weakness), diarrhoea and loss of taste or smell. One could have picked up the infection from an asymptomatic but infected individual. Understandably, except for the last two, these symptoms are non-specific in someone with history of travel or contact. If testing is not an option but a suspicion is there, it is imperative to self-quarantine at home for a period of 2 to 3 weeks. Remember that quarantine at home mandates the entire family living in the same dwelling should be quarantined for the same period.

It is important to bear in mind that getting the SARS CoV2 infection is not a shame or an indication of guilt. It is not something to be hidden and kept secret. As the phone message goes, "We are fighting the ailment not the ailing." Being quarantined is not a punishment. The infection is not a punishment from God. The virus does not infect any particular community or group or gender or age selectively. Immense harm will happen if the pandemic is not handled collectively and rationally. We have to work collectively now more than ever before to keep all of us safe. The bard of the Northeast Bhupen Hazarika sang "We're on the same boat brother. If you tip one end, (You) gonna rock the other, it's the same boat brother." Together we sink or float.

(To be concluded) 

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