Importance of precise data on SARS-CoV-2 transmission dynamics control.

In December, 2019, COVID-19 was recognised as a novel respiratory disease in Wuhan, China, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Accurate and reliable data on SARS-CoV-2 incubation time, secondary attack rate, and transmission dynamics are key to successful containment. In late January, 2020, infection with SARS-CoV-2 was detected in Germany for the first time. By rapid response, the public health authorities identified a business meeting in a Bavarian company as the primary transmission site and a participating Chinese employee who had travelled from Shanghai to Munich as the index patient. Subsequently, the rigorous investigation of contacts led to detection of 16 people infected with SARS-CoV-2 and to successful containment of this outbreak. This well defined event with limited extent of transmission enabled Merle Böhmer and colleagues to provide a meticulous description of SARS-CoV-2 transmission dynamics in an Article published in The Lancet Infectious Diseases. The authors did standard and in-depth interviews with case patients and household members to determine the characteristics and the onset of symptoms. Data were used for calculation of SARS-CoV-2 secondary attack rates, defined as the probability that an infection occurs among susceptible people within the incubation period. In addition, whole genome sequencing of virus isolates was done in 15 of the 16 cases. As a result, Böhmer and colleagues report a detailed transmission network of the outbreak, which is accurately displayed in the main figure of the Article.

What are the main lessons to be learned from the analysis of this outbreak? First, the study allows some conclusions on the infectivity of the virus in relation to the intensity of contacts. While 11 out of 217 individuals (secondary attack rate of 5·1%, 95% CI 2·6–8·9) with high-risk non-household contact (defined as cumulative face-to-face contact to a laboratory-confirmed case for ≥15 min, direct contact with secretions or body fluids of a patient with confirmed COVID-19, or, in the case of health-care workers, had worked within 2 m of a patient with confirmed COVID-19 without personal protective equipment) got infected, none of the low-risk contacts tested positive for SARS-CoV-2. This observation underlines the value of current recommendations of physical distancing as a cornerstone of infection control in this pandemic. However, the intriguing case of a transmission event in two people sitting back to back in a canteen, who only had a very short face-to-face contact while exchanging a salt shaker, shows that the categorisation of high-risk and low-risk contacts has its limitations, too.

Second, SARS-CoV-2 could readily be isolated from throat swabs in all but one patient, who exhibited two negative tests initially. This is in line with the observation that viral replication occurs in the oropharynx in early phases of the disease, when patients still have no clinical signs of pneumonia. But it has also been described in other cases that pharyngeal swabs can convert to negative in later phases, while lung secretions yield positive results. Thus, for clinicians it is important to know when to use which diagnostic procedure, especially when initial results come back negative.

Third, SARS-CoV-2 can be transmitted very early in the course of the disease, when patients have only mild or even no symptoms. Böhmer and colleagues describe one presymptomatic transmission, four transmissions at the day of onset of symptoms, and up to two transmissions during the prodromal phase of the illness. This is in line with the results of others, who estimate the frequency of presymptomatic transmission to occur in up to a half of all infection events. This is one of the most serious obstacles to controlling the pandemic. While traditional tracing methods might be efficacious in controlling small events such as the Bavarian outbreak, they are clearly insufficient to control an epidemic at its peak. Therefore, novel technologies such as contact tracing applications are urgently needed to effectively control the pandemic. In the Bavarian cohort, only one infected individual was asymptomatic. However, it is likely that mild symptoms were reported only in the setting of such an investigation using standardised interviews. Under usual conditions, unspecific symptoms such as headache, fatigue, or a blocked nose might be not taken seriously enough by many people to isolate themselves.

In conclusion, Böhmer and colleagues' study elegantly shows that a thorough description and analysis of early outbreak events of COVID-19 can be very valuable to improve understanding of transmission dynamics and for applying appropriate infection control measures.