Basics of Basic Reproduction Number of SARS-CoV2

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The ongoing outbreak of emerging severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) disease (COVID-19) from Wuhan to almost all 200 countries over the globe, has brought global attention and was declared a pandemic by the World Health Organization (WHO) on March 11, 2020. The very initial cluster of patients with pneumonia-like symptoms was linked to a local Huanan seafood market in Wuhan in December 2019. Unfortunately, there is no specific therapeutics and vaccines yet available for disease control and thus the COVID-19 epidemic has posed a great threat for global public health. Widespread lockdown of almost half of the human population in their homes, altered geopolitics leading to immense economic loss, and so on so forth, has clearly indicated the urgent need of understanding the epidemiology of currently ongoing novel infectious human disease. Because of the ongoing COVID-19 pandemic worldwide, most of countries are suffering with a rapidly increasing number of positive cases and unfortunate deaths (almost 5 million corona positive cases and unfortunate 325,000 human deaths till 20 May 2020). Scientific advancements since the pandemic of SARS in 2002-03 and the Middle East respiratory syndrome (MERS) in 2012, have accelerated our understanding of the epidemiology and pathogenesis of COVID-19 to some extent, if not fully.

SARS-CoV2 is the causative agent of this fatal human disease respiratory system. Extensive measures to reduce person-to-person transmission of COVID-19 have been implemented to control the current outbreak. Special attention and efforts to protect or reduce transmission to the susceptible populations including co-morbid patients, children, health care providers, and elderly people have been practiced. Amidst the COVID-19 pandemic, the free and fair flow of news, guidelines, and information is more essential than ever, ensuring an open dialogue and the exchange of vital information between Government officials and the public. It has been observed that even a common man, who never would have given attention to disease epidemiology, but is now a day’s better versed with dozens of medical terminology, FAQs, and a litany of dos and don’ts. One of the most common words buzzing around is basic reproduction number or R naught (R0). Is the R0 so crucial to comprehend today?

The basic reproduction number is also being called as R value, R0 or R naught. It is a way of rating an infectious disease’s ability to spread in the society, thus an important metric of epidemiology. The basic reproduction number of an epidemic is defined as the average number of secondary cases that an infected subject produces over its infectious period in a susceptible and uninfected population. This way, the R0 is being used to measure the transmission potential of a disease. As per Prof. Paul L. Delamater and his team at Washington University “R0 is affected by numerous biological, socio-behavioral, and environmental factors that govern pathogen transmission and, therefore, is usually estimated with various types of complex mathematical models, which make R0 easily misrepresented, misinterpreted, and misapplied”. Among the aerosol-based transmissible diseases, the R0 value for measles is the highest as up to 18, common cold to 2-3, and influenza to 1.5-2.8. Similarly R0 for HIV/AIDS is 2-5 with the route of body fluids and for polio is 5-7 through the fecal-oral route.

For example, if the R0 for SARS-CoV2 in a geographical population is 3.0, then we would expect that each new case of SARS-CoV2 shall produce 3 new secondary cases (assuming everyone around the case was susceptible or without vaccination). However, the R0 excludes new cases produced by the secondary cases and the value of R0 does vary from one population to another and similarly from one geographical area to another. The measles disease has one of the highest basic reproduction number of 18 and thus may cause explosive outbreaks in human population. The ongoing novel coronavirus has a basic reproduction number of about 2.5-3.0.

For any given infectious microorganism, the scientific literature might be found with numerous values of R0. Estimations of the R0 is generally often calculated over 3 parameters i.e. (a) the duration of contagiousness after a person becomes infected, (b) the likelihood of infection per contact from an infectious person to a susceptible person, and (c) the contact rate, along with additional parameters that can be added to describe more complex cycles of transmission.

A population will rarely be totally susceptible to an infection in the real world. Some contacts will be immune, for example due to prior infection which has conferred a life-long immunity, or as a result of previous immunization. The R0 is not a biological constant for an infectious agent as it is also affected by other factors such as environmental conditions (cold and hot weather), geographical location, age of the population, and the behavior of the infected population (say immunity of human individuals). The R0 values are estimated from mathematical models, and the estimated values are dependent on the model used and the values of other parameters. Thus values given in the literature only make sense in the given context and it is recommended not to use obsolete values or compare values based on different models. The value of R0 does not alone by itself give an estimate of how fast an infection spreads in the population.

The most pertinent uses of R0 value is to determine how much an emerging infectious disease may spread in a population and also to determine what proportion of the population should be either immunized through vaccination to eradicate a disease or taken for another arrangement if a vaccine is not available (as in the case of COVID19 disease)? If the value of the R0 is more than one then the infection will be able to start spreading rapidly in a population, but if the value of R0 is less than one then the disease will eventually dilute out as not enough new people are being infected to sustain the outbreak. Thus, the larger the value of R0, the harder it is to control the epidemic.

Let’s understand an example where the value of R0 for COVID-19 is 4 in a human population, where half of the population seems healthy, then the effective basic reproduction number shall be 4 x 0.5 = 2. In such a circumstance, a single positive case of COVID-19 would produce an average of 2 new secondary cases. The consideration of R0 is very important to impose and lift the lockdown in an infected area and thus the aim of the Government should be to keep the reproduction number below one to allow the public to go out home for usual chores. The main aim of the declaration of lockdown by local governments is to keep people inside their homes to minimize and push back the exponential growth of basic reproduction number to one or even less than it.

In heterogeneous human populations, the estimation of R0 is not straight forward and the same is true to our country, which proudly boasts “Unity in Diversity” due to lots of variety in whole country states-wise. The small countries, especially Europe, have not much heterogeneous populations and thus have a narrow range of R0 value. Everywhere the Government arrangements want to force the reproduction number of COVID-19 down from about three to below one. To successfully eliminate a disease from a population, R0 needs to be less than 1. This is the only reason we are unable to move out of homes nowadays, have had to work from home and the children are not visiting schools. Thus, we are trying to stop the people coming into contact with each other to cut the novel coronavirus ability to further spread.

Sr. No. Disease Transmission Route R0
1- COVID-19 Respiratory droplets 1.94–5.7
2- Chickenpox (varicella) Aerosol 10–12
3- Mumps Respiratory droplets 10–12
4- Polio Fecal–oral route 5–7
5- Pertussis Respiratory droplets 5.5
6- Smallpox Respiratory droplets 3.5–6
7- Measles Aerosol 12–18
8- HIV/AIDS Body fluids 2–5
9- SARS (Severe acute respiratory system) Respiratory droplets 3.1–4.2
10- Common cold Respiratory droplets 2–3
11- Ebola Body fluids 1.5–1.9
12- Influenza Respiratory droplets 1.4–1.6
13- Influenza (2009 pandemic strain) Respiratory droplets 1.4-1.6
14- MERS (Middle East respiratory syndrome) Respiratory droplets 0.3–0.8

Table 1: Common infectious human diseases, their transmission route and value of their R naught

The ongoing COVID-19 pandemic is a common challenge faced by mankind in the age of expanding globalization in the 21st century. Presently, the sharing of resources, experiences, and lessons, regardless of who you are and your nationality, seems an inevitable must-to-do for us to overcome this so-called, 3rd world war. The real remedy to win over this pandemic is not isolation, but the cooperation of mankind keeping aside the geopolitics and personal grudge of countries.

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