The fact that higher initial viral load leads to a higher isolation success is not surprising and well-known. However, the significance and predictability shown in our data set of at least 100 samples is astonishing and fits perfectly to actual data concerning transmission patterns of SARS-CoV-2 [1, 6]. We assume a Cq value between 27 and 30 in E-gene screening PCR as a breaking point (11 to 110 TCID70) for in vitro infection, where infectivity decreases significantly (p < 0.05) and transmission becomes less probable.
Our data revealed that 21% of our samples were infectious enough to transmit the virus in vitro, which perfectly fits to a recently published cluster analysis from Hong Kong, where Super-spreading events (SSE) were analysed and 20% of the infections led to the main component of transmissions . Therefore, the use of the dispersion parameter k contributes much better to the understanding of transmission than the reproduction number R0 and Public health policies can be adapted by discovering and preventing typical situations, where SSEs occur. An SSE or overdispersion emerges in conjunction with immune-suppressive patients, increased disease severity and therefore viral load, asymptomatic individuals and extensive social interactions . Additionally, future transmissions will depend on factors including the degree of seasonal variations in transmission, the duration of immunity, and the degree of cross-immunity between SARS-CoV-2 and other coronaviruses as well as the intensity and timing of control measures .
Isolation was also possible in the case of a recurrent patient, even though 13 days of incubation on cell culture were needed to obtain cytopathic effect and SARS-CoV-2 specific positive PCR for confirmation of successful isolation.
Besides the higher number of samples with a Cq of 35–39.9, the relative distribution of viral load is comparable between our sample (n = 100) and the whole amount of SARS-CoV-2 positive samples acquired during the first outbreak (n = 371). Therefore, in question of the initial viral load, our data reflect the distribution of an outbreak situation realistic.
Our quantitative comparison between an external standard using synthetic RNA and plaque-forming units received via virus titration showed the imperative necessity to distinguish between the both of it and carefully choose the type of quantitative standard. For example, Cq 32,56 shows 1 PFU/200 µl (1 TCID70) and simultaneously the difference of 40,000 copies/ml, suggesting an enormous number of RNA copies in a cell culture supernatant and probably also in an oropharyngeal sample, which is not viable, at least not possible to perform cytopathic effects and perhaps represent just fragmentary and ineffective RNA-copies.
We are aware of the limitations of our in vitro model, especially the circumstance, that we cannot suppose the same level of susceptibility between our cell line and the environment of the human oropharyngeal zone. Another limitation is caused by the use of only one cell line VeroB4, a line which is sufficiently susceptible to SARS-CoV-2, but seems to be inferior to the cell line VeroE6 due to the lower content of the cell surface protein TMPRSS2 (Transmembrane serine protease 2) . Further studies concerning serial isolation experiments on different cell lines simultaneously would be desired to determine the influence of the cell line on the experimental approach.
However, it is a fair approximation of viability. Our data reveal that 21% of our samples were infectious enough to transmit the virus in vitro, with the probability of a positive culture decreases steeply above a Cq value of 27, but transmission may still occur in patients with very close and long-lasting contact, especially household transmissions  and of course it must not be overseen that the viral load may vary over the duration of the infection and that the Cq value only represents a short-term current status of the infectivity.
Storage in the refrigerator for 1 week does not affect the initial viral load significantly, which confirms the high stability of an enveloped virus and facilitates the interpretations of patient’s material stored for some days before further handling. Actually, a significant loss of viral titer due to storage in the incubator under the same conditions apart from the temperature, was an interesting result. Outbreak situations are characterized by shortage of material and restrictions affecting transport and storage, so it is important to know that prolonged storage at room temperature has no major effect on PCR testing.
With regard to the distancing and hygiene rules, the viral shedding and therefore, the risk of transmission, should be minimal at a viral concentration of 11 TCID70 and lower.
If the assumption is correct and the meaning of SSEs in transmission of SARS-CoV-2 is as high as supposed, it will be the greatest challenge in the near future to find out which policies effectively tackle the dispersion, in ideal circumstances without the hard restrictions of a total lockdown.
In this respect, our data help to take well-directed precautions and Public health policies to prevent SSEs and simultaneously, relax measures that seem to have rather a psychological than an epidemiological effect. Applying quantitative PCR systems in diagnosis of SARS-CoV-2 can distinguish between patients providing a high risk of transmission and those, where the risk of transmission is probably limited to close and long-lasting contacts. Unlike medically indicated sick leave, quarantine represents a measure restricting liberty that must be well considered and should be imposed in the most moderate way. In further outbreaks, authorities will need a reliable diagnostic reason to impose restrictions that fit both the constitutional right of personal freedom and the needs of public health.