Table 5.

Current evidence and outlook on the intestinal infection and potential faecal–oral transmission of SARS-CoV-2

Category	Current evidence	Further questions
Intestinal infection by SARS-CoV-2
Clinical evidence	Nearly half of patients with COVID-19 are positive for SARS-CoV-2 RNA detection in faecal samples15	How many patients with positive faecal tests have active viral replication as measured by viral subgenomic mRNA?
	Persistence of viral RNA in faecal compared with respiratory samples for as long as a month after discharge77,80,83	Can intestinal infection serve as a reservoir for re-infection in the lung? Does intestinal infection of SARS-CoV-2 enhance its mutation rate?
	Approximately 5–70% of patients with COVID-19 reported gastrointestinal symptoms11,24	How to unify the definition of gastrointestinal symptoms in COVID-19 among different studies? Clear distinctions should be established between gastrointestinal symptoms presented on admission and the gastrointestinal symptoms caused by medications A correlation is not yet established between gastrointestinal symptoms, the presence of faecal SARS-CoV-2 RNA and/or active viral replication
	Endoscopic and histological examination of patients with COVID-19 revealed virions and inflammatory cell infiltration in the duodenum and rectum59,60	More comprehensive autopsy or surgical specimens are needed to provide histological evidence of intestinal infection
	SARS-CoV-2 infection altered gut microbiota, correlated with elevated expression of inflammatory cytokines such as IL-2 and IL-18 (refs65,66,161) High levels of faecal calprotectin in patients with COVID-19 with diarrhoea, which were positively correlated with IL-6 levels in serum62	Does intestinal infection lead to increased expression of inflammatory cytokines in the intestines and/or serum? If so, do intestinal infection and elevated cytokine levels contribute to cytokine release syndrome or correlate with disease severity?
	IgA dominated in the early stage of SARS-CoV-2-specific humoral responses and was more potent in neutralization than IgG165	Does intestinal mucosa contribute to IgA production during SARS-CoV-2 infection? Will oral administration of SARS-CoV-2 vaccines achieve better efficacy?
In vitro evidence	SARS-CoV-2 infects intestinal cell lines and human intestinal organoids, thereby mediating the production of ISGs69,92,93,127,128	Can human intestinal organoids serve as a highly relevant infection model to characterize the complete SARS-CoV-2 life cycle and test viable candidate therapeutics?
	SARS-CoV-2 can establish an intestinal infection in hACE2 knock-in mice, hamsters, ferrets and non-human primates133–139	More careful virological and histological examination of intestinal infection in animal models can provide evidence not easily observed in humans
Potential faecal–oral transmission routes
Clinical/environmental evidence	Viral RNA detected in the sewage156–159 Infectious virions were isolated from faecal samples of patients with COVID-19 (refs89–92)	How long can SARS-CoV-2 survive in sewage or food surfaces? Can SARS-CoV-2 maintain sufficient concentration and infectivity in fomites for subsequent transmission?
In vivo evidence	Hamsters can be infected through SARS-CoV-2 fomites137 ACE2 knock-in mice can be infected by intragastric SARS-CoV-2 (ref.133) Naive ferrets can be infected by intragastric faecal supernatant from infected ferrets134	Exploration of the exact route and timelines for faecal–oral infection in animal models; systematic characterization of the host response for lung infection and intestinal infection in animal models
	Prolonged shedding of viral RNA in rectal swabs was observed from one infected Rhesus macaque even after nose and throat swabs returned negative135	More evidence in humans on whether SARS-CoV-2 can infect the next host via the faecal–oral route is needed

ACE2, angiotensin-converting enzyme 2; COVID-19, coronavirus disease 2019; hACE2, human ACE2; ISGs, interferon-stimulated genes; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.