key: cord-0985121-e83xe4sp authors: Falahi, Shahab; Kenarkoohi, Azra title: Transmission routes for SARS-COV-2 infection: Review of Evidence date: 2020-10-06 journal: New Microbes New Infect DOI: 10.1016/j.nmni.2020.100778 sha: 28e7cd5004b12956e6b9157db46d1cdb03b126cc doc_id: 985121 cord_uid: e83xe4sp • Airborne transmission of SARS-COV-2 emphasizes the importance of using face masks in public, crowded and closed places as the most important preventive measure. • household contact tracing of index COVID-19 cases can be applied as important part of COVID-19 control programs. • Vertical transmission of SARS-COV-2 seems possible, but the risk is very low. • in theory, SARS-COV-2 may transmit through oral-anal intercourse, it is unlikely that this route will be a significant mean of transmission. • similarity of SARS-COV-2 with SARS, supports the hypothesis of non-transmission through blood. SARS-COV-2 was transmitted from animals to humans, and then between humans at high speed. Now, months after the start of the pandemic, the number of cases is increasing daily. The high transmission rate of SARS-COV-2 has raised many questions about the possible routes of infection transmission. In the early days of the epidemic, it was thought that transmission was occurring only from symptomatic individuals, but as the story progressed, it became clear that transmission from presymptomatic and asymptomatic individual is possible (1, 2). Due to uncertainty of the main transmission routes, the infection control policies faced with more challenges. The possible main route of transmission is thought to be the close contact and respiratory droplets secreted by the patient during coughing, sneezing, breathing and even normal speeching. Therefore, it is necessary to maintain physical distance and using the face mask. Another route of transmission is through contaminated objects or fomites. Touching the T-zone of the face, after contact with these objects, is also a means of transmission, and it emphasizes hand hygiene and hand washing. Subsequent studies have shown that the virus is also present in saliva; given the evidence of virus transmission from asymptomatic individuals(1) and the presence of the virus in saliva, it has been suggested that even secretory droplets during normal speeching may be a route to transmit SARS-COV-2 (2). Talking creates droplets that can carry the virus, and this hypothesis also supports the transmission of SARS-COV-2 by asymptomatic individuals. If we accept this possibility, it is necessary for everyone in the community to wear a mask. In addition to nosocomial transmission in hospitals, we must also pay attention to intra-family transmission. in one study, 16.3% of household transmission were reported by household contact tracing of index COVID-19 cases; indicating the high transmissibility of SARS-CoV-2 (6). household contact tracing of index COVID-19 cases can be applied as important part of COVID-19 control programs. Although contact tracing is necessary and effective for controlling COVID-19 infection, due to the presence of asymptomatic and presymptomatic transmission, it is not sufficient to interrupt transmission so physical /social distancing should be implemented. The SARS-CoV-2 RNA has been detected from the feces of infected patients and hospital toilet bowls. Although viral RNA detection is not equivalent to a viable virus, it is a trace of the virus, so fecal-oral transmission may be a possible route of transmission (5, 8) . ACE2 is present in the gastrointestinal tract, so SARS-CoV-2 may enter the gastrointestinal tract and replicate. Due to the digestive symptoms in some patients and the prolonged viral shedding in the feces, the virus appears to have active proliferation in the gastrointestinal tract (9) .Therefore, there is a possibility of infection transmission in toilets through creating aerosols by flushing toilets and surfaces contact. The potential for fecal-oral transmission also emphasize hand hygiene and washing. There is conflicting evidence about vertical transmission. Some studies have followed infants born to infected pregnant women and taken pharyngeal samples for RT-PCR testing, but the results were negative and showed that the infants were not infected with SARS-CoV-2 (10). Some studies did not rule out vertical transmission. In one study, PCR testing of neonatal pharyngeal sample was negative, but IgM and IgG antibodies against COVID-19 were positive on birthday. Mother's vaginal discharge was also negative (11) . In another study, 33 infants born to pregnant women with COVID-19 were screened, and 3 infants were tested positive for SARS-COV-2 PCR on the second day of life (12) . Also, one study identified SARS-COV-2 nucleic acid in the placenta ) 13 ( . On the other side of the coin, there are studies that show the impossibility of vertical transmission: in one study, all pregnancy products of two pregnant women were negative (10) . In general, due to the presence of ACE2 on placental villi and the uterus (14), vertical transmission can occur, although the risk seems very low. In a number of studies, the SARS-CoV-2 genome was identified in blood samples from a number of patients with COVID-19 (15, 16) , and subsequently raised the question: Is SARS-CoV-2 transmitted through transfusion? In a study, all blood samples of asymptomatic people with COVID-19 were negative for SARS-COV-2 PCR and RNAemia was detected in severe and symptomatic cases. Due to the fact that people with symptoms of infectious disease are not accepted in the blood transfusion organization, there is no chance of transmission through transfusion or risk is very low. In addition, the viral load is low in serum and plasma. In previous viruses of this family(SARS and MERS), no cases of transfusion transmission have been reported (16) , similarity of SARS-COV-2 with SARS also supports the hypothesis of non-transmission through blood. Sexual transmission: SARS-COV-2 is present in saliva and feces, and in theory it is possible to transmit through oral-anal intercourse (17) but this sexual habit is not common, so it is unlikely that this route will be a significant mean of transmission. In summary, considering high transmission capacity of SARS-COV-2, prolonged viral shedding, lack of vaccine and systematic medication at this time, the most important action to limit infection is to cut off the transmission chain. 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Transfusion medicine reviews SARS-CoV-2 asymptomatic and symptomatic patients and risk for transfusion transmission Sexual transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2): A new possible route of infection Airborne transmission of SARS-COV-2 emphasizes the importance of using face masks in public, crowded and closed places as the most important preventive measure. household contact tracing of index COVID-19 cases can be applied as important part of COVID-19 control programs Vertical transmission of SARS-COV-2 seems possible, but the risk is very low. in theory, SARS-COV-2 may transmit through oral-anal intercourse