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\def\RunningAuthor{Bhushan Tekam.}
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\title{Modes of Transmission of Covid-19}
\author{Bhushan~Tekam\textsuperscript{*}~\\[5pt]{Department of Community Medicine, Jawaharlal Nehru Medical College, Datta Meghe Institute of Medical Sciences, Sawangi (Meghe), Wardha, Maharashtra {\textendash} 442001, India}}

\begin{abstract}
The Coronavirus disease (COVID-19) was declared as a pandemic as by the world health organization (WHO) in the mid-March of 2020. It is stated to be first recognized in china and has affected millions of people and many thousands are dead across several countries, affecting a majority of earth's population. Exhaling activities like talking, (also coughing, and sneezing) of an infected patient is thought to be the cause of the spread of this disease transmitted commonly via respiratory droplets formed from these activities. As many cases affecting major areas have been identified with or without genuine travel history, a root for community transmission might have been conceivable. Broadly speaking transmission of COVID-19 exist by two modes{\textemdash}Direct and indirect. The direct mode incorporates transmission through aerosols formed by surgical and dental procedures or in the form of respiratory droplet nuclei, other body fluids and discharges, for instance, excreta, saliva, urine, semen, and tears; and from maternal-to-fetal. The indirect form of transmission includes: surfaces (e.g., furniture) also termed as fomites present within the prompt environment of an infected patient and, objects utilized on the infected individual (e.g. thermometer). A considerable lot of these modes discovered or undiscovered with evidence are overlooked, henceforth is requires to specify and delineate them. Although the literature may or may not mention these routes, it is important to emphasize them as it may help in prevention and treatment. 
\end{abstract}\def\keywordstitle{Keywords}

\begin{keywords}COVID-19,\newline Modes of Transmission,\newline Aerosol,\newline Droplet Nuceli
\end{keywords}
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\section{Introduction}
CORONAVIRUS DISEASE (COVDI-19) is an infectious disease affecting mainly respiratory system along with other systems of the body, caused by a virus discovered as SARS Cov-2 (severe acute respiratory syndrome -2). It was first reported in China Wuhan province in December 2019 as pneumoniae of unknown etiology. The WHO declared the disease as emerging pandemic on 11 March 2020 after it posed a threat to public health. Innumerous cases have been reported since it was first discovered and reported. As of now the deceased all over the world has reached a mark of around 56 million with India reporting its cases around 8.85 million and deaths around 130 thousand.

Coronaviruses are a group of viruses that belong to the Coronaviridae family, infecting both animals and humans. The virus affecting humans causes mild to severe disease. Signs and symptoms of the disease include fever, cough and respiratory symptoms like dyspnea, sore throat, dry cough. Other symptoms are running nose, anosmia, tastelessness, and myalgia. In severe cases, it causes pneumonia, severe acute respiratory syndrome and in uneventful cases, death.

The tests available for COVID-19 are serological and molecular tests. The molecular test includes: Rapid antigen test, Reverse-transcriptase polymerase chain reaction (RT-PCR) and serological test include an antibody test for the virus. RT-PCR is a diagnostic test. Screening of the disease is done through swab testing for these tests. Investigations include Chest X-ray, HRCT. Prevention includes social distancing in public places, frequent handwashing with alcohol-based hand rub or soap water, use of masks especially surgical masks and layered masks with maximum efficiency that allows minimum air to flow out while sneezing, avoid close contact with anybody having fever, cough or other symptoms.

Treatment depends upon the severity of the disease. Earlier hydroxychloroquine, an anti-malarial drug was making rounds but is not included in the treatment. Anti-viral drugs such as remedesevir, favipiravir, oseltamivir have been in use around the world. Patients whose infection has reached the lungs are started upon high-dose antibiotics of ceftriaxone etc. Remedesevir and dexamethasone are also used for patients whose infection has reached the lungs. In those whose Spo2 levels are drastically decreasing and are having respiratory effort can be put on additional oxygen therapy. Severe patients also might require ventilator support.

Keeping above-mentioned points in mind, it is important to course through the routes by which the virus is shed. The main route being respiratory droplets created when an infected person coughs or sneezes. Other routes, although posing high or low risks, are also possible and needs to be illustrated for it may help identify the possible transmission and preventing the disease.

\textbf{Direct Contact}

The fundamental transmission courses for the individual-to-individual spread of SARS-CoV-2 are respiratory droplets and contact transmission. Aerosol and fecal-oral transmissions are other potential routes that incorporate which have not yet been affirmed. 

\textbf{Contact Transmission}

Since the virus was discovered to endure inside the climate of infected people in China (Surfaces, entryway handles, phones, and other possible fomites), it isn't genuinely sure that the virus (SARS-CoV 2) can likewise be sent through both contacts: immediate and aberrant with respect to virions coming in contact. The virus can be sent through the nasal cavity, oral cavity, \& different mucosal linings, when vulnerable people get infected by coming into contact with infected people or creatures by the latter's body liquids (sputum, spit, feces). Contaminated body fluids cause indirect transmission of the disease when vulnerable individuals come in contact with it. This very well explains the transmission of virus particles through fomites which is an example of indirect contact transmission.

 \textbf{Airborne Transmission }

Principle route for transmission of the virus suspected to be occurring through aerosols generation is the airborne transmission. The particles of aerosols are under 100 \ensuremath{\mu }m in diameter \unskip~\citep{971768:21118973}. Hence, their miniature size and suspension noticeable all around in the air and in their immediate environment may easy direct acquisition of the infection. Various surgical and dental procedures and droplet nuclei that are formed by an infected patient while talking, coughing, and sneezing may be the main reason for aerosols formation and its transmission. In study 8, health care workers and 5 post-operational patients tested positive for the coronavirus because they were in close contact with a contaminated patient. This proposes that an important mode for human-to-human transmission can be considered as droplet formation. Thus, the risk of virus transmission and the amount of exposure is increased. Medical fields involving procedures of aerosol generation are at much higher risk. Thus, appropriate protective equipment should be a priority while performing aerosol-generating procedures \unskip~\citep{971768:21118974}.

\textbf{Aerosol-Generating Procedures }

Various examinations have indicated a relationship between AGPs and medical services specialist disease during the scourge of SARS-CoV-1. These studies are review accomplice and case arrangement having numerous jumbling factors that incorporate: review inclination from review polls; hand washing and preparing; assortment in PPE; inadequate follow-up; and little examination sizes. Significantly, airborne levels were never estimated in any of the examinations. The creators of an efficient audit of AGPs recognized 10 investigations reasonable for consideration, which were all esteemed 'very loquacity proof' according to reviewing of proposals, appraisal, improvement and assessments (GRADE) measures. Reviewing of proposals, appraisal, improvement and assessments recommend alert when deciphering these outcomes as 'any gauge of impact is dubious'. 

An affiliation is seen between medical care worker contaminations and nearness to fundamentally unwell patients who required critical care. The relative dangers of the AGPs were: Tracheal intubation with a more relative danger of medical care laborer contamination of 4.2 (1.5{\textendash}11.4) than control of a breathing apparatus and urinary catheter addition. This may suggest that actual vicinity and amount of time within sight of a basic patient is high risk as opposed to the system as such \unskip~\citep{971768:21118972}. 

Scarcely any investigations have estimated terminated microbe load according to AGPs. During flow volume breathing, particles that contained viral RNA were found noticeable all around H1N1 affected patients in the emergency unit. Except for in-line airway route suctioning and bronchoscopy, The AGPs as defined by The World health organization were not related with a huge ascent in airborne viral substance. Airborne viral substance diminished with expanding term of disease and with expanding relative stickiness.

\textbf{Particle exhalation and deposition}

Activities involving exhalation (e.g. coughing) creates a jet. A turbulent phasic gas cloud is formed during coughing and sneezing that can frame a gas cloud around the droplets which is protective from evaporation which may expand is life expectancy permitting it to move further. This gas cloud can make a trip up to 8 m, conveying a dispersed range of droplets. The cloud loses its mass and velocity eventually and the excess droplets evaporate and remain suspended for a quite a long time forming droplet nuclei gaining the capacity to cause wide-range infectious transmission \unskip~\citep{971768:21119067}

The particle deposition mechanism of the environment and airways partly depend on the diameter of the particle. Due to the impact of humidity diameter is a constantly changing variable. There is a swift decrease in the diameter of a particle of 25{\textendash}50\% and the relative humidity decreases as the particle leaves the respiratory tract. This cycle is switched on particle inhalation.

The site for deposition of a particle in the air route may rely upon (1) (size) diameter, structure, speed, arrangement, density, temperature and moisture; and (2) subject-definite factor and disease. Both expanded temperature and humidity have appeared to build the pace of respiratory viral decay. In respiratory infections, seasonal and regional differences are likely a factor. Also, the heat generated from the patient and medical worker will change the flow of air due to warm air-currents \unskip~\citep{971768:21118971}

In human flu models, inoculation of the aerosol is related with expanded sickness seriousness and (instead of upper) lower respiratory tract disease and may communicate contamination even in lower inoculum size. 

When straightforwardly construing particulars of transmission starting with a respiratory infection onto next caution is required as each has its own infective inoculum and airborne attributes. The Severe acute respiratory syndrome coronavirus (SARS CoV-2) infection utilizes the S-spike protein to attach to the ACE-2 receptor. In contrast to bronchial epithelial cells, angiotensin-converting enzyme-2 has altogether more prominent articulation to alveolar type-2 epithelial cells. Because of a thinner respiratory tract lining fluid, the alveolar epithelium has less protection, giving more straightforward admittance to the angiotensin-converting enzyme-2 receptor, perhaps encouraging disease. In artificially generated aerosols (\textless\ 5 \ensuremath{\mu }m) the disease remains fixed for up to 3 hours whilst maintaining an infectious titre.

The viral RNA was disengaged from a roof fan in a patient's room having negative pressure even when no AGPs were accounted for. Viral SARS Cov-2 RNA with a particle size less than 100\ensuremath{\mu}m which is also the size for aerosol particle out in the open, staff and clinical zones have been accounted for (Liu et al. unpublished perceptions). Levels were quite raised in the protective equipment removal (doffing) room and patient latrines. Because of expanded ventilation, levels were lower in the emergency unit, and the pinnacle particles size was in the sub-micron range (0.25{\textendash}1 \ensuremath{\mu }m) (Liu et al. unpublished perceptions).

Experimental studies uphold the likelihood that SARS-CoV-2 might spread through aerosols (supposed airborne transmission) even without aerosol-generating procedures (for example, intubation or noninvasive positive pressure ventilation). Researchers have exhibited that talking and coughing produce a combination of the two droplets and aerosols in a no of sizes, that these discharges can travel together for up to 27 feet, that it is attainable for SARS-CoV-2 to stay suspended all around and reasonable for quite a long time, that SARS-CoV-2 RNA can be recuperated from air tests in clinics, and that poor ventilation delays the measure of time that aerosols remain airborne. 

It might demonstrate hard to set up that SARS-CoV-2 is infectious when the virus is airborne because of technical challenges related to air examining of reasonable viral particles, and study of human-to-human transmission being unethical.

\textbf{Droplet Transmission }

Droplet transmission refers to the spread of infections which happens when the droplets that are formed by respiratory manifestations like coughing or sneezing are transmitted to an individual who comes in close contact (within 1 m) with the infectious person displaying the symptoms and also to susceptible individuals who is at risk of having his/her mucosae or conjunctiva exposed to possibly infective droplets. Droplet spread may likewise happen from the prompt surrounding of the infected person, i.e. through fomites.  In this manner, the transmission of the CORONA virus can happen by directly when in contact with contaminated people and indirectly from contact with surfaces in the prompt environment or with objects \& materials utilized on the infected person (e.g., stethoscope)

Droplets are classically depicted as particles ({\textgreater}5 \ensuremath{\mu }m) that are bigger and quickly drop to the ground due to gravity, ordinarily inside 3 to 6 feet of the source individual. Aerosols are more modest particles (\ensuremath{\leq }5 \ensuremath{\mu }m) that quickly vanish noticeable all around, abandoning droplet cores that are sufficiently little and light enough to stay suspended noticeable all around for quite a long time (undifferentiated from dust). 

Deciding if aerosols or droplets prevail in the transmission of the virus (SARS-CoV-2) has basic ramifications. On the off chance that the virus is basically spread by respiratory droplets, wearing an approved face mask, shield, or keeping a safe distance of 6 feet from others ought to be sufficient to forestall transmission. Assuming, nonetheless, the virus is contracted by aerosols concentrates that stays suspended all around for delayed periods, clinical masks would be lacking (on the grounds that aerosols can both infiltrate and bypass covers), face shields would give just incomplete security (on the grounds that there are opening in the shields), and distance of 6 feet would not give assurance from aerosols that stay suspended in surrounding air or are conveyed by its flow.

\textbf{Vertical Transmission }

There are barely any instances of pregnancies with novel Coronavirus, the greater part of them with a gentle sickness course. There is a limited proof about in utero disease. 

As of late, reports suggesting infection positive IgM antibodies in neonatal blood following birth, with a negative test of RT-PCR of the nasopharyngeal swab in children with SARS-CoV-2 infected mothers are there \unskip~\citep{971768:21118975}. Given that IgM doesn't generally cross the barrier of the placenta because of its contributing structure from mother, it is possible that IgM was delivered in the neonate because of vertical transmission of the infection. Anyway, this isn't indisputable proof, and may likewise be because of placental changes permitting the entry of IgM, or false positive testing.

Vertical transmission can be forestalled by delivering the fetus in negative pressure isolation rooms. Post-delivery contamination of neonates may also occur by means of breastfeeding or via droplet inhalation created by infected parents and/or medical professionals. Protective measures like wearing protective equipment and masks consistently while present around the newborns may mitigate transmission. It is of utmost importance to inculcate appropriate measures like hygiene, transfer of the newborns to the neonatology department after birth and avoiding exposure of the neonate to other family members.

\textbf{Faecooral Transmission }

Fecal-oral transmission refers to the cycle where the disease is transmitted through the feces of an infected individual, to the mouth of a susceptible person. This transmission can happen through: failures in sanitation frameworks along the sterilization chain (toilet, containment, conveyance, treatment, end-use and removal) and prompting introduction by means of different courses including food, water, hands, flies, objects or surfaces.

The infection of the gastrointestinal tract occurs by bountiful angiotensin-converting enzyme 2 (ACE2) receptors to which the virus attaches. In later phases of the contamination, intestinal manifestations have been appeared to happen prevalently. In any case, there are studies reporting a very less percentage of patients with uneasiness and loose bowel before fever and respiratory symptoms \unskip~\citep{971768:21118976}.

In a progression of infected children, some exhibited RNA in rectal swab as long as about fourteen days although their nasopharyngeal swabs were negative. Additionally, SARS-CoV2 has likewise been distinguished by reverse transcriptase-polymerase chain reaction (RT{\textendash}PCR) from a sample of stool in a youngster with no symptom and background of contact as long as 2 weeks after the negativity of pharyngeal swabs. According to the recent guidelines, a negative result of 2 sequential nasopharygeal swabs (at least 24 hr apart) by `RT{\textendash}PCR' are contemplated as proof of viral clearance. Notwithstanding, the recent finding of delayed shedding of the infection in stools raises the concern of pre-symptomatic or post-recovery spread of disease through feces. Discharge dependent on oral swab results might be hazardous and could lead to expected transmission in the community.

 Until this point in time, transmission by the fecal\ensuremath{-}oral route of the virus has not been reported. There is a report of detecting a live virus from a sample of the stool of a confirmed COVID-19 case from Province of Heilongjiang in China around 2 weeks after the beginning of symptoms \unskip~\citep{971768:21118977}.

Public education in regards to individual hygiene like hand washing (frequent), bleaching, avoiding defecation in open areas and confined utilization of public latrines should be supported proactively alongside avoiding social gatherings particularly in a nation like India where transmission of diseases by fecal-oral route is a significant general medical condition because of poor sanitation practices.

\textbf{Other Routes of Infection }

A few researchers have hypothesized that coronavirus can spread through the conjunctiva, although a study disapproved this possibility. This study stated that almost an imperceptible percentage of patients out of all with positive infection had conjunctivitis; the conjunctival sac secretions was negative for the viral nucleic acid test \unskip~\citep{971768:21118978}.
    
\section{Conclusion}
COVID-19 {\textendash} from discovering as a threat to a region has progressed to a pandemic. The infectivity rate of the disease (COVID-19) is very high. Mainly two modes of transmission exist{\textemdash}direct and indirect. Transmission via aerosols, anal (feco-oral), tears, saliva, semen and maternal-to-fetal are the direct mode. While the indirect route includes the spread of the infection via fomites. Ongoing researches on different other routes make it possible for more information about the disease and its spread. Therefore, underestimating other potential routes may hinder in preventing and containing it. It is necessary to maintain social distancing, hand hygiene, disinfection to prevent the spread and community transmission.

\textbf{Conflict of Interest}

The authors declare that they have no conflict of interest for this study.

\textbf{Funding Support}

The authors declare that they have no funding support for this study.
    

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