How Long Is A SARS-CoV-2 Infected Person Contagious?

As the COVID-19 pandemic rages on, the global community has become accustomed to comprehensive, interventional strategies such as wearing a mask, social distancing, hand hygiene, and surface cleaning and disinfection. It is well known that transmission of SARS-CoV-2 occurs through direct, indirect, or close contact with infected people through infectious secretions, such as saliva and respiratory secretions or their respiratory droplets. In addition, the scientific community has confirmed that the airborne transmission of SARS-CoV-2 viruses in aerosols (smaller than 100 μm) can remain suspended in air for many seconds to hours and are highly concentrated near an infected person, thus infecting people most easily in close proximity (1). Furthermore, aerosols containing infectious virus can also travel more than 2 m and accumulate in poorly ventilated indoor air, leading to superspreading events. World Health Organization and Centers for Disease Control and Prevention have also acknowledged this under certain circumstances, such as enclosed spaces, prolonged exposure to respiratory particles (i.e., shouting, singing, and exercising), and inadequate ventilation or air (2, 3). Thus, effective control strategies and standardized guidance to the public are integral in mitigating COVID-19. In particular, understanding the duration of infectiousness in infected persons with SARS-CoV-2 is critical for developing evidence-based public health policies on isolation, contact tracing and returning to work. In general, the levels of viral RNA were determined by using quantitative reverse transcription-polymerase chain reaction. However, detection of viral RNA does not necessarily indicate that a person is infectious and able to transmit the virus to another person. Although it is critical to determine the levels of infectious virus particles in infected COVID-19 patients, requirement of Biosafety Level-3 laboratory for the virus titration has hindered this approach. In this perspective section, we will discuss currently available scientific data regarding the levels of infectious virus particles in asymptomatic individuals, in mild and severe COVID-19 patients and in children and young adults.

Asymptomatic and presymptomatic individuals represent a source of potentially transmissible virus (4). Asymptomatic infections have no specific incubation period due to no clinical signs. However, the viral loads detected in asymptomatic populations have been reported in several studies to be similar to those in symptomatic patients. In a nursing facility, quantitative SARS-CoV-2 viral loads detected in residents were similarly high in the four symptom groups (residents with typical symptoms, those with atypical symptoms, those who were presymptomatic, and those who remained asymptomatic). Notably, 17 of 24 specimens (71%) from presymptomatic persons had viable virus by culture 1 to 6 days before the development of symptoms. In a surveillance study, asymptomatic cases of samples were collected through swabbing of contacts or facility/family/household testing in the context of outbreak investigations. Despite the uncertainty of their date of exposure or start of infection, cultivable virus was isolated from samples collected from asymptomatic individuals (41% of tested samples).

Detection of infectious SARS-CoV-2 from upper respiratory tract of mild-to-moderate COVID-19 patients showed that infectious virus can persist for more than a week after symptom onset, declining over time (5). At 10 days after symptom onset, probability of culturing virus declines to 6%. This is in line with current WHO guidance on release from isolation. Similarly, shedding of the virus in mild COVID-19 patients was determined by measuring the levels of transcribed subgenomic mRNA and isolation of infectious viruses (6). Pharyngeal virus shedding was very high during the first week of symptoms, with a peak at 7.1 × 108 RNA copies per throat swab on day 4. In addition, infectious viruses were successfully isolated from these samples, confirming active virus replication in the upper respiratory tract. No viruses were isolated after day 7 onset. These findings suggest efficient transmission of SARS-CoV-2, through active pharyngeal viral shedding at a time at which symptoms are still mild and typical of infections of the upper respiratory tract. In a surveillance study in Manitoba, Canada, the presence of infectious viruses was determined by evaluating samples from day of symptom onset (day 0) up to 21 days after symptom onset (7). Within this range of samples, positive cultures were observed up to day 8 after symptom onset with the probability of obtaining peak titers on day 3. Similarly, in Hong Kong, virus was isolated from the samples of mild patients collected within the first 8 days of illness with median viral RNA load of 7.54 log10 genome copies/mL (8). From severe COVID patients, prolonged duration of cultivable virus was detected for up to 20 days after symptom onset, suggesting that prolonged excretion of infectious virus is associated to the severity of the disease (9).

Severity in most children is limited, and children do not seem to be major drivers of transmission. However, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infects children of all ages. In South Korea, large-scale testing, aggressive contact tracing and testing, and isolation/direct observation of asymptomatic or mildly symptomatic children have identified the presence of asymptomatic (20 of 91 [22%]), presymptomatic (18 of 91 [20%]), and symptomatic children (53 of 91 [58%]) (10). Presymptomatic children remained symptom free for a median (range) of 2.5 (1-25) days before exhibiting any symptoms. A minority of children (6 [7%]) were identified as infected; this highlights the concept that infected children may be more likely to go unnoticed either with or without symptoms and continue on with their usual activities, which may contribute to viral circulation in the community. In a separate study with 12 symptomatic children, infectious viruses were detected at a median of 2 days after symptom onset (11). Median viral RNA load at diagnosis was 3.0 × 106 copies/mL (mean 4.4 × 108 ranging from 6.9 × 103 to 4.4 × 108 copies/mL. A limitation of this study is the small number of children assessed. However, viral load at diagnosis is comparable to that of adults and symptomatic children of all ages shed infectious virus in early acute illness, a prerequisite for further transmission. Considering the relatively low frequency of infected children, even in severely affected areas, biological or other unknown factors could lead to the lower transmission in this population. Large serologic investigations and systematic surveillance for acute respiratory diseases and asymptomatic presentations are still needed to assess the role of children in this pandemic.

Although the spectrum of COVID-19 ranges from asymptomatic to severe infections, most patients experience mild disease (80%). Scientific data indicate that infectious virus in mild patients can persist for a week after symptom onset. Furthermore, infectious virus can be isolated from asymptomatic individuals. This clearly enforces the importance of wearing a mask, quarantine, and contact tracing to mitigate the transmission of SARS-CoV-2. Recent studies support that wearing masks can save lives not only by cutting down the chances of both transmitting and catching the coronavirus (12) but also by reducing the severity of infection in contracted individuals (13). It is well demonstrated that self-quarantine of close contacts exposed to COVID-19 prevents transmission to others. Contact tracing must be conducted for close contacts (any individual within 6 feet of an infected person for at least 15 minutes) of laboratory-confirmed or probable COVID-19 patients. However, relaxed social distancing and opposition of wearing masks are hindering the mitigation of SARS-CoV-2, resulting in continuous increase of COVID-19 cases. Even when the vaccine will be available, we would not immediately stop social distancing, wearing masks, and other interventional measures until reaching efficient levels of viral mitigation. The message is clear that a simple practice of wearing masks can protect ourselves and save other lives from circulating SARS-CoV-2.



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Effect of Suppressed Innate Immunity on Covid-19 Severity

One of the important determinants of severe Covid-19 appears to be an inappropriate response by the innate immune system. Specifically, this involves, on the one hand, insufficient or delayed expression and signaling by type 1 interferons (IFNs), which induce innate cell-mediated immunity. The interferon (IFN) response constitutes the major first line of defense against viruses. On the other hand, this involves an overly active inflammatory cytokine response, with its attendant tissue damage (this “cytokine storm” was explored in our previous Perspective at Let’s look at some of the lines of evidence for these contradictory events.

One way in which an aberrant immune response to SARS-CoV-2 is reflected at the cellular level is by a greatly increased ratio of neutrophils to lymphocytes(1). This is correlated with low expression of type I and III interferons and high expression of pro-inflammatory factors including IL-6 and a variety of chemokines(2) that act as attractants for neutrophils and monocyte/macrophages. These activities are the outcome of interactions between host factors that recognize pathogen associated molecular patterns (PAMPs), such as viral RNA in endosomes, and viral proteins that are antagonistic to these factors and their signaling pathways. Genetic differences in host factors can result in profound differences in host responses to pathogens (recent findings are described below). Different viruses also tend to have different or unique antagonists to host immune factors that can greatly influence the outcome of infection.

What are some of the molecular studies that point to defects in interferon activity in severe Covid-19? Recently, interest has been increasing in toll-like receptors (TLRs), especially TLR3 and TLR7, which recognize viral RNA and are important in interferon type I and inflammatory cytokine expression. TLRs play a key role in the recognition of PAMPs and trigger the activation of specific signaling pathways, thereby inducing the transcription of inflammatory and/or anti-inflammatory cytokine. One interesting report looked at two sets of two brothers(3) who, although young and otherwise healthy, had severe Covid-19 (one died). Whole exome sequencing revealed that both sets of brothers had mutations in TLR7, which serves as a sensor for viral RNA. One set had a missense mutation predicted to result in an inactive TLR7, while the other set had a frame shifting 4 nucleotide deletion, resulting in a nonsense protein. Stimulation of primary immune cells in vitro with the TLR agonist imiquimod resulted in defective expression of type I interferon-related genes normally regulated by TLR7. While the limited nature of the study does not permit a conclusion of causality, several factors make it likely that these loss of function mutations are significant. Exhibition of severe disease in young men is rare. Despite rare cases of loss of function mutations in TLR7, two different loss of function mutations in two young brother pairs with severe disease indicate its potential role in Covid-19 severity. It should be pointed out that TLR7 is located on the X chromosome, so a single mutant copy would cause loss of function. One of the mothers was heterozygous for wild type TLR7, making her a carrier. Thus, if problems with the TLR7 pathway exacerbate Covid-19, males might be likelier to have an insufficiency.

Another study analyzed the complete genomes or exomes of 659 patients with life threatening Covid-19 and compared them with those of 534 people with asymptomatic or benign infections(4). They characterized 13 genetic loci encoding factors in the TLR3-interferon regulatory factor 7 (IRF7) pathway, which also regulates type I interferon production and immunity to influenza virus. They found that 3.5% of the people with life threatening Covid-19 had loss of function variants at these loci. Moreover, when immune cells from patients with these variants were tested in vitro, they were found to be defective in type-I interferon immune activities, and further in vivo study confirmed impaired production of type I IFN during the course of SARS-CoV-2 infection. About half of these patients also had extremely low levels of serum interferon α, a type I interferon.

Yet, another study further implicates lack of appropriate interferon activity in severe Covid-19(5). In this study, 101 of 987 patients with life threatening Covid-19 had auto-antibodies against interferon α, interferon ω or both. These were not present in 663 patients with mild disease. The auto-antibodies were able to neutralize the antiviral effects of interferon in vitro (and likely in vivo). Interestingly, the auto-antibodies were about 5-fold more prevalent in men than in women.

None of these studies by themselves show a specific defect in interferon activity in a majority of cases. However, taken together, they certainly suggest that a great variety of different defects related to the antiviral activities of type 1 interferons may be surprisingly common. Probably, one of the factors explains why some people resist serious disease while, for others, it is life-threatening. Further, investigations in this area will be most interesting.

One other study identified a 3p21.31 gene cluster that conferred a risk of severe Covid-19. This region contains three chemokine receptor genes, all involved in innate immunity(6). It turns out to be a region derived from Neanderthals, and is present at variable incidence worldwide except in Africa. This provides yet another clue that the innate immune response to SARS-CoV-2 may be an important determinant of whether an infected individual will, or will not, develop critical Covid-19.

In this light, it has been proposed that some cross-protection could be afforded by administering live attenuated vaccines, such as measles-mumps-rubella, and oral polio vaccine(7) (non-specific effect of vaccination against SARS-CoV-2). The stimulation of innate immunity by these vaccines could provide temporary protection against Covid-19. If proven to be effective against Covid-19, emergency immunization with these vaccines could be used for protection against other unrelated emerging pathogens.



  1. Y. Liu et al., Neutrophil-to-lymphocyte ratio as an independent risk factor for mortality in hospitalized patients with COVID-19. J Infect 81, e6-e12 (2020).
  2. D. Blanco-Melo et al., Imbalanced Host Response to SARS-CoV-2 Drives Development of COVID-19. Cell 181, 1036-1045 e1039 (2020).
  3. C. I. van der Made et al., Presence of Genetic Variants Among Young Men With Severe COVID-19. JAMA 324, 663-673 (2020).
  4. Q. Zhang et al., Inborn errors of type I IFN immunity in patients with life-threatening COVID-19. Science, (2020).
  5. P. Bastard et al., Auto-antibodies against type I IFNs in patients with life-threatening COVID-19. Science, (2020).
  6. D. Ellinghaus et al., Genomewide Association Study of Severe Covid-19 with Respiratory Failure. N Engl J Med, (2020).
  7. K. Chumakov, C. S. Benn, P. Aaby, S. Kottilil, R. Gallo, Can existing live vaccines prevent COVID-19? Science 368, 1187-1188 (2020).

Transmission Dynamics of SARS-CoV-2: Superspreaders and Superspreading Events

The concept of Superspreaders and Superspreading events has recently attracted a lot of attention. In fact it is important to understand that most of SARS-CoV-2 infected persons are in fact not contagious! Thus, transmission is really dependent on a handful of individuals we call Superspreaders who nurture Superspreading events. Let’s distinguish between Superspreaders and Superspreading events. Superspreaders are individuals who infect a high number of persons; why?  This is not clear, and this is a most important issue to clarify in the future.  We know they yield high viral load and that they are generally, but not always, young people.  But, this cannot fully explain their massive contamination impact.  Moreover, they are frequently asymptomatic, thereby significantly increasing risk of dissemination. Superspreading events, which involve at least one Superspreader, are events which favor large scale transmission, such as close contacts in indoor situations. Secondary transmissions from infected people then result in a large number of further infections, and so on. Thus, by some estimations  only 20% of infected individuals cause 80% of infections(1).  Identification of Superspreading events depends upon contact tracing.  Furthermore, DNA sequencing of viral genomes adds a great deal to a clearer understanding of these phenomena, thus, confirming the substantial role of Superspreaders in the pandemic. Let’s look at some well-characterized superspreader events to try to better understand how the majority of SARS-CoV-2 infections occur, enabling us to gain an understanding of what might be done to prevent them.

One of the early recognized superspreading events occurred in mid-March in Skagit County, Washington(2) at a 2.5 hr-long choir practice in which 61 people sang in close proximity. Probable or confirmed infections occurred in 87% of the attendees. Given the low incidence of COVID-19 at the time of this event, it is likely that all the infections originated with a single individual, and that the act of singing vigorously launched many viral laden particles into the air. People were relatively closely positioned. Thus, it is likely that superspreading occurs in an unusually favorable environment. Looking at it another way, the virus got “lucky.”  Another study, unrelated to the choir event, analyzed the sequences of 453 viral genomes collected between February and March in Washington(3).  It is possible to infer the likelihood of how many people have been infected by a single person using viral genomic epidemiology, especially given the relative genetic stability of SARS-CoV-2. A phylogenetic analysis strongly suggested that 84% of the 453 viral genomes derived from a single introduction sometime in early February.  In the choir study, it is clear how virus was transmitted. In the genomic study, it is only clear that a single infected individual somehow infected a great number of people through primary, secondary, and other less direct routes.

Another well-studied superspreader event began with a single infected individual in a meat packing plant is Postville, Iowa. In this case, viral spread could be ascertained by both contact tracing and by genomic epidemiology. Fourteen independent viral introductions were identified in the region, but the only virus to spread widely was the one from the meat packing plant. The virus from this individual passed first to numerous other workers, then to family members, then to the community in Postville (87 cases), and finally to other locations in an area of 185 square miles in Iowa, Wisconsin and Minnesota. These conclusions were supported by genomic sequences from 27 different infected individuals. Again, it appears that prolonged close contact indoors facilitated transmission and suggests that this is a critical feature of superspreading events.

Perhaps, the best characterized superspreading event, originating from an international business meeting of Biogen in Boston(4) in which more than 90 individuals became infected.  This event recently received considerable attention. The large number of infections was indicative of a possible superspreading event. A recent study looked at this event and its consequences in detail, using genomic epidemiology(5). They were able to identify and track the virus in question by a single nucleotide polymorphism (SNP), C2416T. Among 80 separate introductions from four continents into the Boston area early in the pandemic, which they inferred from phylogenetic analyses, the C2416 SNP was unique to one virus. Comparing other viral genomes from various parts of the world with C2416T, the parental origin appears to be in Europe, perhaps France, with an estimated most recent common ancestor existing about two weeks prior to the conference aroundFebruary 26-27. Of all samples collected prior to March 10, the only instances of C2416T were from people who had attended the conference, indicating that a superspreading event had indeed occurred there.

Subsequent samples (744) from infected individuals in Boston and surrounding areas were collected over a period from February to June and genomes were sequenced. Remarkably, 35% of the samples had the C2416 SNP. Since no sample prior the March 10 had this SNP, it suggests that the superspreading event at the Biogen meeting February 26-27 resulted in virus from a single individual infecting more than a third of all infected people in the Boston area. Percentages of the C2416T SNP in regions around Boston ranged from 30-46% in four adjacent counties. In addition, a second SNP, G26233T, appears to have emerged during the event, enabling further tracking. Data from this SNP shows a likely export to other states and countries, with further community spread in Virginia, Michigan and Australia. Some caution is warranted, however, since genomic sampling is not generally done on a randomized basis.

The same report looked at infection clusters at homeless shelters, nursing facilities, and a hospital to gain a better understanding of transmission dynamics. They analyzed 193 viral genomes collected from the Boston Health Care for the Homeless Program and identified 4 clusters of 20 or more highly similar genomes, including two clusters containing the C2416T SNP. They also investigated a superspreading event at a skilled nursing facility, in which 82/97 (84%) residents and 36/97 (37%) of staff were infected. In fact, 75% of viral genomes from different individuals had highly similar genomes, suggesting that they arose from a single recent introduction. This took place even though strict interventional measures were in place. Interestingly, two other clusters of three closely related genomes were detected. This represents independent introductions, but these failed to massively spread. In the case of two clusters of infection at Massachusetts General Hospital, highly similar genomes were not found, suggesting a lack of significant in-hospital spread.

What is our conclusion? First, it is now clear that a majority of transmissions result from superspreading events, facilitated by conducive conditions. These include indoor location, close contact, lengthy contact, indoor activity such as singing or talking, poor air ventilation, and lack of mitigation procedures (i.e., wearing masks and physical distancing). Tracing Superspreading events in Hong Kong also confirmed that the largest cluster (106 cases) was traced to four bars followed by a wedding (22 cases) and attendance at a temple (19 cases) (1). This study suggests that disease control efforts should focus on avoiding gathering events and mitigating their impact.  The rapid tracing and quarantine of confirmed contacts, along with the implementation of physical distancing policies including either closures or reduced capacity measures targeting high-risk social settings such as bars, weddings, religious sites and restaurants, should be efficient to prevent the occurrence of superspreading events. Overall, the issue of Superspreaders and Superspreading events illustrates the impact of molecular epidemiology for deciphering the patterns of COVID-19 dissemination. What we still clearly lack, however, is the understanding of the very early phases of the pandemics in China. This would be very useful for the whole appraisal of transmission dynamics.

  1. D. C. Adam et al., Clustering and superspreading potential of SARS-CoV-2 infections in Hong Kong. Nat Med, (2020).
  2. L. Hamner et al., High SARS-CoV-2 Attack Rate Following Exposure at a Choir Practice – Skagit County, Washington, March 2020. MMWR Morb Mortal Wkly Rep 69, 606-610 (2020).
  3. T. Bedford et al., Cryptic transmission of SARS-CoV-2 in Washington state. Science, (2020).
  4. A. Schuchat, C. C.-R. Team, Public Health Response to the Initiation and Spread of Pandemic COVID-19 in the United States, February 24-April 21, 2020. MMWR Morb Mortal Wkly Rep 69, 551-556 (2020).
  5. J. Lemieux et al., Phylogenetic analysis of SARS-CoV-2 in the Boston area highlights the role of recurrent importation and superspreading events. medRxiv, (2020).