SARS-CoV-2: Potential sources, modes of transmission and effective prevention measures

Studies provide information on the sources, transmission and prevention of SARS-CoV-2. In a review, researchers are systematically evaluating the current state of science – data published through June 26, 2020 were taken into account.

The scientific gain of knowledge does not follow a linear process – it develops from a diversity of data and perspectives as well as an expert discourse. This multi-directional path also applies to new research on the coronavirus. A review 1 by Guenter Kampf’s research team reveals a heterogeneous and partly contradictory picture – overall, this extensive data evaluation is a snapshot of a dynamically developing infection process, about which new data and findings are constantly being generated.

Viral infection dose and detection method

At the beginning of this publication, the scientists point out that the relationship between an identified viral load and the viral load necessary for an infection is not yet clear. In addition, the exact method for the detection of a possible infectivity (contagiousness) is also not yet conclusively defined. In this respect, analysis of the included publications is based on the assumption that the RNA load (ribonucleic acid) is a plausible surrogate to establish cautious clinical hypotheses. Kampf and his research team also state that it is not yet clear whether a high viral load also causes more severe symptoms. However, several studies suggest that this is the case.

Transmission dynamics

The first documented case of a multiple transmission chain outside of China allowed the classification of contacts into low or high risk classes. Other factors that influence transmission dynamics include the state of a person’s immune system, viral load, infectious virus carriers without any detectable symptoms (asymptomatic carriers) as well as in which health care facilities patients seek help, how often a transfer from one hospital to another takes place, and a large number or long duration of contacts. Temperature and humidity seem to have an influence on the transmission dynamics, but the study results are partially contradictory. In principle, a seasonality of the COVID-19 pandemic is considered likely – but the exact factors of influence are still unclear.

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SARS-CoV-2: Possible sources of infection

Asymptomatic virus carriers

According to the scientists, the proportion of virus carriers without symptoms at the time of a test fluctuates considerably. Among their examples:

  • Hospitalised patients showed a proportion between 5 percent and 27.8 percent.
  • One study showed a rate of 56.5 percent in a nursing home.
  • Within the family group, proportions between 25 percent and 57.1 percent were found.
  • A population study in Iceland with a total of 13,080 people tested showed a rate of 0.8 percent positive findings, 43 percent of whom were asymptomatic at the time of testing.

Asymptomatic carriers represent a very serious source of infection (up to 56 percent of SARS-CoV-2 infections in selected groups appear to be due to them) and are thus a significant factor in the rapid spread of the COVID-19 pandemic. However, according to the researchers, the majority of people identified as asymptomatic carriers at the time of the test developed moderate symptoms after a time lag, so they should be classified as pre-symptomatic.


The viral load in the respiratory tract is particularly high in the early stage of the infection, respectively two days before and up to one day after the onset of symptoms. In addition, the transmissibility of the virus in the early stage of the disease is facilitated by the fact that early symptoms are still mild, but the existing viral load is already high.

Surprisingly, a comparably high viral load was found in asymptomatic SARS-CoV-2 carriers and patients with COVID-19 disease, which further underlines the importance of asymptomatic carriers as a source of infection.

Transmission via droplets and aerosols

The scientists state that a strict separation between droplet and aerosol transfer is not possible – aerosols are smaller than droplets, with a diameter of no more than 5 µm, and can therefore float in the air longer. Further, even larger droplets can dry out within a very short time, so that virus particles contained in them then float in the air for a longer time. Air movement also plays a role here, as it causes even larger droplets to remain in the air for a certain length of time before sinking down. In addition, particles that have sunk down can be whirled up again by air movements in the room.

Sneezing, coughing and speaking lead to the distribution of viruses in the air, with each sneeze ejecting about 40,000 particles at once, each cough about 710 particles and speaking about 36 particles per 100 words.

So far, only a few studies have been able to investigate the role of pure airborne transmission (aerogenic infection), and virus particles have only been detected in large air volumes of 9000 L, but not in smaller ones. Even air samples taken ten centimeters from the chin of a COVID-19 patient – his nasal and throat swab and his saliva contained SARS-CoV-2 – did not show viral RNA. During the examination, the patient was asked to first breathe normally, then breathe in and out deeply, then speak and finally cough continuously – both without and with a surgical mask*. Pure aerosol transmission via the airway is therefore described as unlikely. However, the infectivity (contagiousness) of the SARS-CoV-2 in the air has been proven experimentally in a Goldberg drum for a period of three hours.

Close and long contact between asymptomatic or symptomatic patients and healthy hospital staff is most likely the greatest risk factor for infection. In addition, the main transmission route is most likely direct droplet transmission by coughing, sneezing or speaking.

Gastrointestinal tract/stool

Some patients suffer from diarrhea in the early stages of the disease, which suggests colonization of the gastrointestinal tract with SARS-CoV-2. Colonization rates vary between 9.1 percent and 100 percent in samples tested, with a high viral load in those tested positive. One study indicates that the viral load (RNA) in the stool of children is even higher than in their nasal swabs. A faecal-oral and even a faecal-respiratory transmission (inhalation of aerosolized stool particles) is therefore considered possible.


Previous findings on transmission via the ocular secretion are still controversial. However, the scientists of the present review assess the probability that this plays a significant role as a source of infection as low.

Inanimated surfaces

Tested surfaces in different hospital areas show different colonization rates with large variations: In intensive care units between 0 percent and 75 percent, in isolation rooms between 1.4 percent and 100 percent and in general wards between 0 percent and 61 percent. A positive correlation between the viral RNA load of patients and the colonization rate of their environment can be observed. At this point, the researchers note that the detection of RNA does not necessarily provide information about the infectivity of the viruses. In cell culture studies, SARS-CoV-2 remained infectious on steel and plastic for three to four days, on bank bills for two days and on wood for one day. Cleaned and disinfected surfaces no longer showed viral RNA.

Personal protective equipment

The personal protective equipment (PPE) of the health personnel is populated at a rate between 0 percent and 50 percent – here, most often shoes and gloves are populated.

Blood/urinary tract/seed fluid/breast milk/pets

Analysis of the data shows that transmission by blood and semen is very unlikely. Urine, breast milk and pets (in this case cats rather than dogs) may be a potential source of infection – however, there is still too little evidence to draw firm conclusions.

Effective prevention measures

Hand disinfection

The effectiveness of alcohol-based hand disinfectants* against SARS-CoV and SARS-CoV-2 is already well described, and their use in the healthcare system is the first choice. It could also be useful for COVID-19 patients to disinfect their hands before leaving a room, for example.

Face masks

Unprotected patient care with long and close patient contact is a significant risk factor for hospital staff to contract COVID-19. Masks* reduce the spread of viruses through the carrier into the environment. Wearing masks by COVID-19 patients during treatment or care activities increases the protection of healthy personnel from infection.

Conversely, a case study showed that nursing staff protected by surgical masks did not show positive PCR tests, despite close contact of more than 10 minutes with a COVID-19 patient and partially aerosol-generating activities.

What is important to know: Masks worn by COVID-19 patients are contaminated with the virus from the outside, and viruses can be detected on the outside of the masks for up to seven days. The re-use of already used masks in deficiency situations should be highly questioned in view of these results.

If mild symptoms occur in nursing staff who do not work on COVID-19 stations, the general early wearing of masks can be helpful in these cases, as a study showed that about 4 percent of these people tested positive for SARS-CoV-2.


Gloves can prevent the contamination of hands with germs and are therefore an effective personnel protection. It should be noted that wearing gloves 2 can negatively affect adequate hand hygiene among personnel. However, gloves should be worn to protect personnel when caring for COVID-19 patients.

Disinfection of surfaces with frequent hand contact

Examined alcohols – ethanol and isopropanol with a concentration between 30 percent and 80 percent – inactivate SARS-CoV-2 within 30 seconds. Chlorinated agents and 0.1 percent benzalkonium chloride act within 5 minutes. Routine disinfection* of surfaces with which COVID-19 patients come into contact is effective and necessary.

State: 10/2020

Further information


  2. (German)

*commercial communication

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