Respiratory tract infections caused by viruses have now emerged as a leading cause of death and morbidity worldwide [1], with COVID-19 taking a huge toll of human life and representing an enormous economic and disease burden. SARS-CoV-2 and the associated coronavirus disease 2019 (COVID-19) pandemic have been shown to be transmitted through respiratory droplets and direct contact with an infected person [2], but its transmissibility through airborne transmission or indirect contact via contaminated inanimate surfaces is still debated [3].

Scientific experts believe that SARS-CoV-2 is an airborne virus that can be spread by viral aerosols [4]. Infectious viral aerosols will hang around in the air for a long time. SARS-CoV-2 can live for up to 16 hours in the air, according to reports, which is why continuous successful air purification is recommended. Dining areas, common areas, waiting rooms, offices, restrooms, and elevators all pose a major risk of airborne transmission.

Droplets are larger (>5µm) entities that fall to the ground rapidly due to gravity, typically within 3 to 6 feet of the source entity. Aerosols are tiny particles less than 5µm that evaporate quickly and leave behind droplet nuclei, which are tiny and light enough to float in the air for hours. similar to pollen [5].

Various Google searches were done in the months of February and March 2021 for manuscripts on various COVID-19 and other Coronaviruses' modes of transmission like SARS and MERS. Various sites that were used for searching articles were WHO, PubMed, CDC and Google Scholar. The different types of keywords that were used in searches were COVID-19 transmission, routes, airborne, control measures and their combination.

More than a year ago, In Wuhan, China, a SARS-CoV-2 outbreak occurred, which slowly spread far and wide and erupted in the form of a pandemic. Based on the two different statements by CDC on its website, earlier mentioning the airborne transmission of COVID-19 and later withdrawing the same [8], a lot of controversies are there on the modes of transmission of this virus. Respiratory viruses can be transmitted through three different routes: direct or indirect touch, droplet transmission, and aerosol transmission [1]. Although being a Respiratory Virus, COVID-19 is suspected of having Faeco-oral [9] and Mother to child [10] transmission. Table 1 summarizes various studies indicating the different modes of transmission of COVID-19.

It's crucial to figure out if droplets or aerosols play a bigger role in SARS-CoV-2 transmission as type of precautions vary for the two modes [21]. Wearing a surgical mask, face shield, or standing at least 6 feet away from other people should be enough to prevent transmission if COVID-19 is primarily transmitted through respiratory droplets, contrary to which if the virus is spread by aerosols then it can remain in the environment for an extended period of time consequently wearing a surgical mask would be insufficient, face shields can only have limited defense and maintaining 6 feet distance would not protect you from the virus particles carried by air currents.

Respiratory droplets and direct contact have been a well-established mode of transmission of COVID-19 [22]. Cai et al. have laid the importance of COVID-19 transmission by fomites (Indirect Spread) and aerosolization of virus in confined spaces [11]. Their findings also suggest the transmission by Restrooms and Elevators (Fomites) and through asymptomatic persons. Doremalen et al. studied SARS-CoV-2 stability on different surfaces and calculated viral decay rates [13] after comparing the virus stability on artificially generated aerosols and surfaces like plastics, copper, cupboard, stainless steel, they inferred that COVID-19 spread by aerosol or fomite is possible since the virus can survive for hours in aerosols and days on surfaces. Contrary to this, in a study from Israel, surface and air samples accounted for 46% of the total viral RNA contamination. However, no viable SARS-CoV-2 was isolated from any environmental sample, but that may have happened as the CT value of samples >34. Another study has also reported the presence of non-viable viruses above CT value 34 [23].

Li et al. [12] looked into a restaurant outbreak involving three separate families in China’s Guangzhou and concluded that though near contact and fomite exposure are thought to be important factors in SARS-CoV-2 propagation, the virus can also be spread through short-range aerosol transmission in crowded and suffocating enclosures. After an outbreak following a weekly rehearsal in Skagit Valley, Washington, Miller et al. conducted a case study using questionaries [24] and suspected the aerosols released while singing rehearsal as a primary mode of spread in this case as all other direct and contact precautions were observed. Tang et al in their review article, have emphasized the credible evidences of airborne transmission of COVID-19 and their plausibility metrics scored 8 out of 9 for the same [15].

Mother-to-child transmission is another major concern in the spread of COVID-19.in a metanalysis done by Kotlyar et al. [10]. They found that among 936 infants from COVID-19 positive mothers, 27 (3.2%) had a positive result and concluded that vertical COVID-19 transmission is possible. and it tends to happen in a small percentage of cases mainly when the mother is infected in the last trimester. The incidence of infection is comparable to that of other pathogens that cause congenital infections. Anti-SARS-CoV-2 IgM and IgG antibodies were found in umbilical cord blood and milk samples, according to Fenizia et al. [16], emphasizing the virus's potential for vertical transmission. Dong et al. reported a case of elevated IgM antibodies in a newborn borne to a COVID-19 positive mother [25]. As IgM does not cross placenta and takes 3-7 days to rise after infection, the infection must have transferred in utero, yet again directing towards the vertical transmission of the disease.

Fig. (1). Preventive measures to break the chain of transmission.

Faeco-oral transmission by COVID-19 is yet another topic to explore as positive PCR results from stool specimen suggest that stool or sewage may also act as a mode of spread. The presence of virus in stool specimens was discovered by Tang et al. [18] even after negative respiratory tract specimens of the same individual and suggested stool should be considered for routine diagnostic screening in addition to respiratory tract specimens. SARS-CoV-2 RNA was positive in 8 out of 10 treated wastewater samples in an Iranian analysis [19]. We suggest that further studies are required to ascertain the faeco-oral transmission of COVID-19 as mere RT-PCR positivity may not be essential for virus's dissemination. To validate this mode of transmission, the virus must be viable when isolated from stool and sewage samples.

Droplet and direct contact are proven modes of transmission of SARS CoV-2. COVID-19 spread is thought to be caused by airborne transmission, with ventilation playing a key role. The evidence for SARS-CoV-2 transmission through aerosols, especially in well-ventilated environments, appears to be inconclusive. People should avoid closed and less ventilated spaces lest the pandemics come to a stop or should take precautions for airborne transmission as there are no perfect experimental data that can be used to prove or disprove SARS-CoV-2 transmission by aerosols. Mother Child transmission of COVID-19 through meagre but is possible. Proving faeco-oral transmission of this virus requires vast studies involving the culture of live virus from faecal specimens.