Potential sources, modes of transmission and effectiveness of prevention measures against SARS-CoV-2

SARS-CoV-1 has been frequently associated with droplet-based transmission [,]. Likewise, person-to-person transmission has been assumed for SARS-CoV-2 very early []. Importantly, a more efficient transmission of SARS-CoV-2 compared with SARS-CoV-1 has been suggested, due to active pharyngeal viral shedding while symptoms are still mild and typical of upper respiratory tract infection []. Table III summarizes the frequency and magnitude of SARS-CoV-2 viral RNA loads in respiratory tract samples obtained from COVID-19 patients.

The viral RNA load with SARS-CoV-2 can be as high as 11.1 log₁₀ cpm (Table III). It seems to be particularly high in the early and progressive stage of disease [] or two days before and one day after symptom onset []. However, in some cases RNA could still be found up to 51 days after the first positive test with negative results in between [,]. Influenza A virus RNA has even been released for up to 70 days with negative results in between although infectious virus was only detected for 5 days after symptom onset []. Age was also associated with high viral RNA load []. Most studies observed decreased viral RNA loads over time [,]. One study shows that SARS-CoV-2 was detected by culture in 19 of 25 clinical samples (nasopharyngeal swab) from COVID-19 patients []. The viral RNA load detected in the asymptomatic patient was similar to that in the symptomatic patients, which suggests the transmission potential of asymptomatic or minimally symptomatic patients []. It is important to differentiate between detection of RNA and the isolation of infectious virus in cell culture. PCR for RNA of SARS-CoV-2 does not distinguish between infectious virus and non-infectious nucleic acid. Thus, interpretation of duration of viral shedding and infection potential should be based on viable virus from cell culture and needs to be carefully evaluated when solely based on PCR results.

A strict distinction between droplet versus airborne transmission routes for infections is not possible []. Virus transmission via droplets and aerosols enables many viruses to spread efficiently between humans []. Airborne transmission is defined as the transmission of infection by expelled particles of comparatively small size and which can remain suspended in air for long periods of time []. The World Health Organization uses a particle diameter of 5 μm to delineate between airborne (≤5 μm) and droplet (>5 μm) transmission []. Transmission of infectious diseases by the airborne route is dependent on the interplay of several factors, including particle size (i.e. particle diameter) and the extent of desiccation []. Particle desiccation is a critical variable and depending on environmental factors as even large, moisture-laden droplet particles desiccate rapidly []. For example, Wells showed that particles begin desiccating immediately in a rapid fashion upon air expulsion: particles up to 50 μm can desiccate completely within approximately 0.5 s []. Rapid desiccation is a concern because the smaller and lighter the infectious particle, the longer it will potentially remain airborne. Hence, even when infectious agents are expelled from the respiratory tract in a matrix of mucus and other secretions, causing large, heavy particles, rapid desiccation can lengthen the time they remain airborne (the dried residuals of these large aerosols, termed droplet nuclei, are typically 0.5–12 μm in diameter) []. Of further concern, very large aerosol particles may initially fall out of the air only to become airborne again once they have desiccated []. One of the challenges facing practitioners, particularly in an enclosed building, is that even large-sized droplets can remain suspended in air for long periods. The reason is that droplets settle out of air on to a surface at a velocity dictated by their mass []. If the upward velocity of the air in which they circulate exceeds this velocity, they remain airborne. Hence, droplet aerosols up to 100 μm diameter have been shown to remain suspended in air for prolonged periods when the velocity of air moving throughout a room exceeds the terminal settling velocity of the particle [].

Respiratory virus shedding can occur via sneezing, coughing or talking. Sneezing distributes approximately 40,000 particles (droplets or airborne micro-organisms) per sneeze, coughing approximately 710 particles per cough, and talking approximately 36 particles per 100 words []. Using highly sensitive laser light scattering observations a recent study describes that loud speech can emit thousands of oral fluid droplets per second [], indicating that normal speaking may also contribute to virus transmission in stagnant air. Most of the 40,000 large-droplet particles caused by a single sneeze will desiccate immediately into small, infectious droplet nuclei, with 80% of the particles being smaller than 100 μm []. The transmission of infectious diseases via airborne or droplet routes may further also depend on the frequency of the initiating activity. A single sneeze may produce more total infectious particles, while overall coughing may potentially be a more effective route of airborne transmission (e.g. during infection with Coxsackievirus A) []. Coronavirus-infected humans coughed on average 17 times over 30 min during exhaled breath collection []. Given that dry cough is also a common symptom of COVID-19 patients [], it may therefore contribute to potential airborne transmission of this pathogen. In this context, airborne transmission has been considered to be possible in a cluster of infections in a restaurant with air conditioning [].

Few studies are available that evaluated the role of air for transmission of SARS-CoV-2, most of them obtained in hospitals with COVID-19 patients. From the data shown in Table IV, viral copies were only detected in large air volumes of 9000 L with a larger proportion in intensive care units (ICUs) (35% detection rate) compared with general wards (12.5% detection rate). In smaller volumes such as 90 L, 1200 L or 1.5 m³ no virus was detected. Even directly in front of a COVID-19 patient it was not possible to detect the SARS-CoV-2 RNA in the air []. The viral RNA loads of the first confirmed case were 3.3 × 10⁶ copies per mL in the pooled nasopharyngeal and throat swabs and 5.9 × 10⁶ copies per mL in saliva on the day of air sampling []. The air samples of 1000 L were collected at a distance of 10 cm at the level of patient's chin while the patient performed four different manoeuvres (i.e. normal breathing, deep breathing, speaking ‘1, 2, 3’ continuously, and coughing continuously) while putting on and taking off the surgical mask were all undetectable for SARS-CoV-2 RNA []. Nosocomial transmission of SARS-CoV-2 by an airborne route has been described to be very unlikely []. Nonetheless, SARS-CoV-2 can remain infectious in air for 3 h measured in a Goldberg drum with a decline of viral load from 3.5 log₁₀ to 2.7 log₁₀ per litre of air []. In a subset of four study participants with a symptomatic seasonal coronavirus infection but without any coughing during the 30 min exhaled breath collection, no coronavirus RNA was detected in respiratory droplets or aerosols [].

Other aspects influencing droplet or airborne transmission are temperature and humidity because they correlate with the spread of and deaths associated with COVID-19 [, , ]. In China, the number of confirmed cases increased with higher temperature and higher humidity in most of the provinces [,]. COVID-19 lethality significantly worsened (four times on average) with environmental temperatures between 4°C and 12°C and relative humidity between 60% and 80% []. Biktasheva et al., however, described that the COVID-19 mortality correlates with low air humidity, probably caused by a lower resistivity of dry or very dry mucous membranes []. Huang et al. described that 60% of all COVID-19 cases are found in places with an air temperature between 5°C and 15°C []. In Brazil a 1°C increase in temperature has been associated with a decrease in confirmed cases of 8% []. In Wuhan and Xiaogan, temperature was the only meteorological parameter constantly but inversely correlated with COVID-19 incidence []. At low temperature and low humidity, droplets tend to remain suspended in air []. High relative humidity will increase the droplet sizes due to the hygroscopic growth effect, which increases the deposition fractions on both humans and the ground []. Overall, a seasonal pattern of COVID-19 is very likely.

SARS-CoV-2 aerosolized from infected patients and deposited on surfaces could remain infectious outdoors for considerable time during the winter in many temperate-zone cities, with continued risk for re-aerosolization and human infection []. Conversely, SARS-CoV-2 should be inactivated in the environment relatively fast during summer in many populous cities of the world, indicating that sunlight should have a role in the occurrence, spread rate, and duration of coronavirus pandemics []. Simulated sunlight has been described to rapidly inactivate SARS-CoV-2 [,].

Indoor transmission of SARS-CoV-2 is much more likely compared with outdoor transmission []. In a closed seafood market, the risk of a customer acquiring SARS-CoV-2 infection via the aerosol route after 1 h exposure in the market with one infected shopkeeper was about 2.23 × 10⁻⁵. The risk rapidly decreased outside the market due to the dilution by ambient air and became below 10⁻⁶ at 5 m away from the exit []. Outdoor, these virus particles are very strongly diluted by the open air [].

Some patients displayed diarrhoea at the beginning or during the course of infection suggesting that SARS-CoV-2 may also affect the gastrointestinal tract. Viral RNA was detected in a proportion between 9.1% and 100% in COVID-19 patients with up to 8.1 log₁₀ viral copies per g (Table V). One study including 46 patients with 16 of them reporting gastrointestinal manifestations (35%) reported diarrhoea as the most common symptom (15%), followed by abdominal pain (11%), dyspepsia (11%), and nausea (2%) []. Analysing two groups of overall 12 patients, none of the stool samples resulted in successful virus isolation in cell culture, irrespective of viral RNA concentration [,]. In contrast, one study described the successful isolation of virus by cell culture from two of three patients []. Of note, another study showed higher viral RNA loads in faecal samples of mildly symptomatic or asymptomatic children compared with nasopharyngeal swabs []. These results indicate the possibility of faecal–oral transmission or faecal–respiratory transmission through aerosolized faeces. Furthermore, the presence of SARS-CoV-2 RNA in bile juice was reported from one patient and it was speculated that RNA in faecal specimens may partly originate from bile juice []. Finally, a recent study suggested that detectable SARS-CoV-2 RNA in the digestive tract could be a potential warning indicator of severe disease [], however further evidence will be needed.

Transmission of SARS-CoV-2 through the ocular surface was considered to be possible []. Conjunctivitis has been reported in a patient in the middle phase of COVID-19, the conjunctival swab specimens remained positive for SARS-CoV-2 on 14 and 17 days after onset and were negative on day 19 []. Another study showed among 30 COVID-19 patients that the virus was detected in tears and conjunctival secretions only in the one patient with conjunctivitis []. Furthermore, in another group of 38 COVID-19 patients two of them were identified with positive findings for SARS-CoV-2 in their conjunctival as well as nasopharyngeal specimens, a total of 12 patients had ocular manifestations consistent with conjunctivitis, including conjunctival hyperaemia, chemosis, epiphora, or increased secretions []. In addition, no virus was detected on the conjunctiva in five other COVID-19 patients []. One patient was described with persistent conjunctivitis with viral RNA detection until day 27 after symptom onset and confirmation of infectious virus in the first RNA-positive ocular sample []. Even though the virus can be detected rarely in the conjunctival sac at very low levels [,], there is no evidence that it can replicate locally []. That is why the conjunctiva were considered not to be the preferred gateway into the respiratory tract [].

A study analysed human post-mortem eyes for the expression of ACE2 (the receptor for SARS-CoV-2) and TMPRSS2. In all samples the expression of ACE2 and TMPRSS2 was detected in the conjunctiva, limbus, and cornea, with especially prominent staining in the superficial conjunctival and corneal epithelial surface []. In contrast, another study from Germany found no relevant conjunctival expression of the ACE2 receptor on mRNA and protein levels []. In summary, the detailed pathophysiology of ocular transmission of SARS-CoV-2 remains not completely understood [] and both the presence of viral particles in tears and conjunctiva, and the potential for conjunctival transmission remains controversial []. In conclusion, spread of COVID-19 from ocular secretions cannot be ruled out but seems to be very unlikely.

Indirect transmission of COVID-19 has been assumed to be possible via fomites although direct evidence is currently not available []. In hospitals some data were collected to describe the frequency of detection of SARS-CoV-2 RNA on inanimate surfaces in the immediate patient surroundings. The detection rate was variable on ICU surfaces (0–75%), in isolation rooms (1.4–100%) and on general wards (0–61%). The mean virus concentrations per swab were 4.4–5.2 log₁₀ on ICUs and 2.8–4.0 log₁₀ on general wards. A positive correlation between patient viral RNA load and positivity rate of surface samples was described []. However, on cleaned and disinfected surfaces viral RNA could mostly not be detected (Table VI). Detection of viral RNA on the floor is indicative for sedimentation of contaminated droplets.