**1. Introduction**

SARS-CoV-2 or COVID-19 is a newly discovered coronavirus representing the third coronavirus associated epidemic to emerge from a species leap i.e. from the wild animals to the humans, after severe acute respiratory syndrome (SARS) in 2003, and the Middle East Respiratory Syndrome (MERS) in 2012 [1]. COVID-19 transmission dynamics is yet to be understood thoroughly. Basic Reproduction Rate (R0) is a term which is often used in an epidemic. It is the average number of secondary cases of an infectious disease that one case would generate in a completely susceptible population within its longest incubation period [2]. If *R*0 > 1, then the number of infected cases is likely to rise, while if *R*0 < 1 then the transmission will probably die out. The basic reproduction number is an important measurement

in infectious disease epidemiology, indicating the risk of an infectious agent with respect to epidemic spread. Estimated basic reproductive number of COVID-19, declared by the WHO (dated January 23, 2020) ranges within 1.4–2.5 [3].

A recent review by Liu et al. found the average R0 of COVID 19 infection to be 3.28 and median to be 2.79, which exceed the WHO estimates [4]. It was observed in SARS that a few of the infected people in the community spread most of the infection, whereas most people, although infected, spread it to only a handful [5–12]. In addition to R0, scientists use a value called dispersion factor (K), a number that indicates the likelihood of a certain disease to spread in clusters [13]. Lower the K value, more is the transmission that occurs from a small number of people. In 2005, an article in Nature estimated that SARS had a K value of 0.16 [13]. In a recent publication, K value for COVID-19 was estimated to be as low as 0.1 [14] suggesting that probably about 10% of the total number of infected cases will be spreading to about 80% of the cases.

Apart from Basic Reproduction Rate, certain other terminologies are just as important to understand the transmission pattern of infected cases in a community or in a defined geographical area. The term epidemic is used when there is occurrence of large number of infected cases in a community or in a defined geographic area and in a particular period, clearly in excess than the expected number of cases during the same time of previous years. When the occurrence of cases is less than that of the expected number of cases compared to previous years, it is called endemic. Epidemic may be common source epidemic (all cases occurring simultaneously from a single source such as food poisoning following consumption of a meal at a marriage party) or propagated epidemic (when few cases occur from a source and then they further infect some other people, who in turn spread to another group). Certain factors are associated with transmission of infection such as virulence of organism, organisms' density in infectious material, lack of health seeking behaviour, population density, population movement, lack of awareness, lack of hygienic practices, etc. The infectiousness of an infectious disease is measured by Secondary Attack Rate (SAR) - defined as the occurrence of new cases among the susceptible population from the primary case within the longest incubation period of that disease. Usually it is expressed in percentage. Secondary Attack Rate depends on population density, population mobility, virulence of organism, organism density, behaviour of the individuals concerned, etc. in absence of any intervention such as vaccination or medication.

A review work was carried out by a group of researchers that raised several questions. COVID-19 via person-to-person contact had had spread like a wildfire, affecting almost every country in the world. In the past 100 years, the world never experienced a pandemic as cataclysmic as COVID-19. It is easily understood that both previous outbreaks of other members of the coronavirus family (severe acute respiratory syndrome (SARS-CoV) and middle east respiratory syndrome (MERS-CoV)) did not produce even 2% of the global harm which has already been inflicted by COVID-19. There are also four other CoVs capable of infecting humans (HCoVs), which circulate continuously in the human population, but their phenotypes are generally mild, and these HCoVs have received little attention. These dramatic differences between infection with HCoVs, SARS-CoV, MERS-CoV, and SARS-CoV-2 raise many questions, such as: How quick transmission occurs in COVID-19? Does viral structure play any role in it? Any specific human (host) factors are involved? Any environmental factors involved? A review work was done by a group of researchers with the aim of having possible logical answers to above questions [15].

Data collected in above mentioned review clearly indicated that SARS-CoV-2 uses multiple ways for efficient transmission. The virion structure is optimised to survive various environmental conditions, allowing this virus to use both respiratory and faecal-oral transmission modes. Its S protein has an amended structure

*Control of an Epidemic of SARS-CoV-2 by Assessing Transmissibility of Its Infected Cases... DOI: http://dx.doi.org/10.5772/intechopen.96201*

for efficient interaction with the ACE2 receptor and is optimised for furin cleavage. Moreover, S protein could be primed with activation by TMPRSS2, furin, and multiple non-furin proteases (e.g., plasmin). In addition to ACE2, SARS-CoV-2 can interact with other cellular peptidase receptors, such as ANPEP and DPP4, and also can utilise non-peptidase receptors, such as DC-SIGN1, CLEC4G, and CLEC4M. SARS-CoV-2 utilises multiple ways for cellular entry (both non-endosomal and endosomal) and potentially uses various means of epigenetic control to inhibit the initiation of the host innate immune response. During pandemic period continuous genetic rearrangements occurs within the virus cell genetic structure, which enable the virus particles for immunological escape. SARS-CoV-2 is associated with intricate interplay involving various host genetic factors and pathways. Cytokine storm is the result of above interactions, which promotes cellular death programme of various cells, such as pyroptosis, apoptosis, and necrosis, which might contribute to the COVID-19 pathogenesis. This remarkably broad spectrum of means for the efficient SARS-CoV-2 transmission indicates that it is very unlikely that COVID-19 can be cured by targeting just one segment of this complex mosaic [15].

### **2. Recent experience from India**

India had had witnessed a tremendous rise in the daily number of COVID-19 infected cases during its epidemic period. It started from January 2020 with single detected case in Kerala that reached its plateau in mid-September 2020 with almost 98,000 cases in a day during that period [16]. First Covid-related death was reported in March 2020 and on September 18 of the same year, 1195 reportedly died due to the very reason. Till September there was so sign of decline in the daily number of new cases. Reaching the plateau and subsequent decline then seemed to be a far-fetched dream in the absence of an effective vaccine or drug. On January 4th, 2021, a total of 16278 new cases were detected while 200 deaths were reported country wide. Various models of COVID-19 were put forward to predict the trend yet no estimate had turned out to be even close to the reality.

Officials had claimed that increasing the number of tests would have had helped control the epidemic [17, 18]. Yet they could not provide with any effective strategy for reducing new infections which would have had helped flattening of the national Covid-19 epidemic curve. Several survey results established that a small percentage of the people in the country have had developed herd immunity even though they were unaware of their infection. Thus controlling the epidemic through reaching an adequate amount of herd immunity is a remote possibility. Taking these situations into consideration, public health experts of the country opined and rooted for effective public health measures including evidence-based epidemic control policy to be undertaken unless some safe and effective vaccines were available. As per the suggestions of the authority, testing was increased manifold, resulting in detection of more cases and, increase in treatment and quarantine. But the problem was that all the tests were concentrated in the metropolitan cities and the urban areas. Also a sizable number of cases were asymptomatic or pre-symptomatic; they perhaps unknowingly had spread the infection to others.

An epidemiological study was conducted by ICMR-National Institute of Occupational Health, Ahmedabad, in western India, to assess the distribution of infected cases in the community and whether initial viral load of COVID-19-infected case indicated by cycle threshold (Ct) value of reverse transcription polymerase chain reaction (RT-PCR) could predict about transmission pattern in the community apart from population mobility and its density. The study revealed that only 7% of the infected ones carried high viral load, while another 9% of the infected

population had moderate viral load and rest 84% were carrying low viral loads as per community distribution was concerned [19]. Viral load was categorised as high when cycle threshold value = <24, moderate = 24 to <31 and low = 31 & above (**Figure 1**).

Interestingly, most of the Covid-19 infected cases' clustering happened around the houses of cases, infected with with high viral load. Also, the number of secondary cases was directly related to the increase in viral load. Higher the viral load, more were the secondary cases (**Figure 2**). On an average, each index case with high viral load spread to 6.2 secondary cases, case with moderate viral load spread to 2.7 secondary cases and same with low vial load spread to 0.8 secondary case. Conclusion of the study was that viral load is an important determinant for transmission of Covid-19 infection in the community. It also advised, viral load based segregation of infected cases, with higher (high & moderate) viral load being quarantined away from their families along with contact tracing of all of them for previous 5 days and subsequent screening of the contacts, believing to be an effective strategy to combat the epidemic.

When the country was grappling in the dark, trying to come up with a suitable strategy to contain the epidemic, Ahmedabad COVID Control Model was developed based on previous study findings. The said model appears to be biologically plausible as the same holds true for other infectious diseases too, such as HIV, malaria, leprosy. Tuberculosis etc. Standing up to the expectations, execution of the Ahmedabad Model of Covid Control exhibited a reasonable reduction in the daily number of new cases within weeks of implementation in June 2020. This model of management did not add any extra cost to the existing health care delivery practices for managing COVID 19 cases. In it, Ct value obtained from the RT-PCR machine needed to be mentioned routinely on the all RT-PCR test reports; thereby indicating high, moderate & low viral load, that helped the healthcare personnel assess the transmissibility of the detected cases. It was assumed that on an average 50% reduction would happen which is largely dependent on dedication & motivation of grass root level community health workers, supportive & effective supervision, timely logistic supply of test facilities including contact tracing, timely referral for institutional quarantine etc. Fortunately, the model concerned had some extra added benefits. For instance, if the health care personnel are aware about who all the patients with high viral load are, they would take some extra precaution while dealing with the latter - thereby reducing the chances of infection among the health care providers. The following **Figures 3** and **4** depict the decline & ascent of daily cases in intervention as well as non-intervention areas following initiation of Ahmedabad Model for COVID 19 Control, which was published as an original

**Figure 1.** *Distribution of cases with viral load (n = 138) in the studied community.*

*Control of an Epidemic of SARS-CoV-2 by Assessing Transmissibility of Its Infected Cases... DOI: http://dx.doi.org/10.5772/intechopen.96201*

#### **Figure 2.**

*Secondary cases observed in relation to index cases of high, moderate and low viral load with trend line.*

#### **Figure 3.**

*Daily reported COVID-19 cases in Ahmedabad district 10 June till 07 July.*

article titled COVID control strategy – is there any light at the end of the tunnel? in the journal of Family Medicine & Primary Care, 2020; 9:5502-5.

Because of continuous influx of people from UK to India during December 2020 & January 2021, resulting possible emergence of newly detected highly infectious viral strain of Britain in India, Indian Council of Medical Research (ICMR) has instructed various zone-wise Indian laboratories to search for the said infectious Covid 19 strain using cycle threshold (Ct) value of 30 or less considering the fact of their higher infectivity [20]. This again indicates that cycle threshold (Ct) value may be used as a marker of infectiousness or infectivity of Covid 19 transmission. If an appropriate measure is undertaken on time, it can reduce the transmission of cases as was observed in Ahmedabad based study (**Figure 3**), where it was implemented as a method of control.

**Figure 4.**

*Daily reported COVID-19 cases in rest of Gujarat State barring Ahmedabad district 10 June till 07 July.*

Another study conducted by ICMR-National Institute of Occupational Health, India, showed that viral load is also an important determinant for fomites based transmission too [21]. The study was done on common surfaces such as packaging cardboard and stainless steel surfaces that were smeared with COVID 19 infected materials with known viral load. It showed that fomites contaminated with higher viral load remained infected for a longer period compared to that contaminated with low viral load material. So, viral load is a key factor for the presence of the infectious virus on the surfaces and possibly contributing to the transmission, even after a considerable duration. The viral RNA has higher chances of being identified post-90-min observation period on surfaces contaminated with higher viral load. Therefore surfaces with higher viral load are potentially contagious for longer period as compared to those with lower viral load. The study identified a positive relationship between the viral load of samples used for contaminating the surfaces and viral load of the surfaces post-90-min observation period. The relation was stronger among cardboard surfaces than stainless steel surfaces. A probable explanation can be acquired from the fact that the moist surface of the absorbent cardboard could provide a better harbouring site for viral particles than a non-absorbent surface like stainless steel. The results however partly corroborate with a similar study, where the cultured viral titre was measured over a duration on various surfaces [22]. This study could not suggest the viability of viral particles on the surface but it only assessed the presence of amplifiable viral RNA for specific genes (ORF1ab, in this case). The mentioned study, suggested the viability of these viral particles for over 3 days on these surfaces [21]. The said study was perhaps the earliest from India, to document the relationship between viral load and their detectability on common surfaces. In addition, surfaces with contaminated with relatively higher viral load and with higher absorbability (cardboard) are independently associated with higher risk of COVID-19 retention and transmission. Considering the rapidly evolving literature and experimental procedures, this study was limited by a single sample collection (post 90-minutes observation duration) and did not indicate the viability of viral particle/ virion. Further, the results might be extended to emphasise the need for sterilising such fomite surfaces to prevent viral transmission. Considering the positive relation between viral load and the disease contagiousness [19, 23, 24], the sources

*Control of an Epidemic of SARS-CoV-2 by Assessing Transmissibility of Its Infected Cases... DOI: http://dx.doi.org/10.5772/intechopen.96201*

(spreader/positive subjects) with high viral load should be treated with great care, i.e., health care facility with possibly high viral load should adopt maximum precautionary measures. The study indicates that fomites could play an important role in the disease transmission in addition to human contact, particularly at COVID-19 care facilities, market places etc. Awareness on fomite-based COVID-19 transmission and the persistence of virion on these surfaces among the health care workers could reduce their risk of contracting COVID-19. Viral load on fomites and the potential role in disease transmission have potential implications in limiting transmission of the recent viral infectious respiratory disease. The above finding has far-reaching public health implications for educating the public in adopting safer behaviour to avoid transmission through fomites. About a fraction of the infected population harbour high viral load and are designated as super spreader which is a matter of great concern [25]. Apart from person to person transmission, the above population would also spread the infection at a much higher rate through fomites unless effective public health controls are undertaken. Similarly, infected cases with moderate vial load would spread the said infection at a moderate rate both by person to person as well as through fomites. Consequently, there should be an effective mass awareness programme using suitable mass awareness education tools by some experienced health care workers. This is more important in places like business areas, shopping malls, market places, tours and travelling, etc., where large gathering occurs with high population mobility and there is every possibility of transmission through fomites apart from person to person spread. So, respective authorities must pay adequate attention to minimise the spread in the above areas as mentioned already.

### **3. Other factors related to transmission**

In Wuhan, China, a novel and alarmingly contagious primary atypical (viral) pneumonia broke out in December 2019. It had since been identified as a zoonotic coronavirus, similar to SARS coronavirus and MERS coronavirus and named COVID-19. As of 8 February 2020, 33738 confirmed cases and 811 deaths were reported in China. Scientists investigated the basic reproduction number (*R*0) of the COVID-19 virus considering the fact that *R*0 is an indication of the transmissibility of a virus among adjacent population, who are coming in contact to the primary case/s. This investigation found that the estimated mean *R*0 for COVID-19 is around 3.28, with a median of 2.79 and IQR of 1.16, which is considerably higher than the WHO estimate of 1.95. The study concluded that the reproductive number of COVID-19 was higher compared to SARS coronavirus. These estimates of *R*0 depended on the estimation method used as well as the validity of the underlying assumptions. Due to insufficient data and short onset time as mentioned by the authors, estimates of *R*<sup>0</sup> for COVID-19 were possibly biased as mentioned in a study [4].

Another study investigated the aerosol and surface stability of HCoV-19 and compared it with SARS-CoV-1, the most closely related human coronavirus. They also looked for the stability of HCoV-19 and SARS-CoV-1 in aerosols and on different surfaces and estimated their decay rates using a Bayesian regression model. The study found that the stability of SARS-CoV-2 was similar to that of SARS-CoV-1 under the experimental circumstances tested. The study concluded saying differences in the epidemiologic characteristics of these viruses probably arise from other factors, including high viral loads in the upper respiratory tract and the potential for persons infected with SARS-CoV-2 to shed and transmit the virus while asymptomatic. The study results indicated that aerosol and fomite transmission of SARS-CoV-2 is plausible, since the virus can remain viable and infectious in aerosols for hours and on surfaces perhaps for days (depending on the inoculum shed).

These findings echo those with SARS-CoV-1, in which these forms of transmission were associated with nosocomial spread and super-spreading events, and they provide information for pandemic mitigation efforts [26].
