PandemicPreparedness

Emerging data on COVID-19 transmissibility:

28 October 2020: Viral load of contact is an important factor in risk of transmission following exposure- A study looked at 282 clusters in Catalonia, Spain. The findings showed that "time to onset of symptomatic disease decreased from a median of 7 days" to 6 days or less among contacts when the cases had a viral load of 107 or higher. The conclusion indicated that "The risk of symptomatic COVID-19 is strongly associated with viral load of the contact at baseline, which shortens the incubation time in a dose-dependent manner."
Source: medRxiv

16 April 2020 - Infectiousness profile of COVID-19 - A study was carried out to discover the “infectiousness profile” of SARS-CoV-2, the virus responsible for COVID-19, by looking at clinical data on virus shedding. Infectiousness relates to several key parameters including the incubation period, which is the time between infection and symptom onset, and the serial interval, which is the time between onsets of symptoms of successive cases.

The authors found that the average serial interval was approximately 5.8 days and the average incubation period was 5.2 days. They found that infectiousness started 2.3 days before the onset of symptoms, peaked at 0.7 days before symptom onset, and declined within seven days.

In this study, throat swabs were collected from symptom onset up to 32 days after, and it was found that viral load was high soon after symptom onset but gradually decreased towards day 21. There was substantial transmission potential before the onset of symptoms. Analysis from this study suggests that viral shedding occurs 2 to 3 days before symptoms appear, and viral load decreases after symptom onset. Findings also suggest that the infectiousness profile more closely resembles influenza than SARS.

The authors note that there may have been “recall bias” since symptom onset depends on a patient’s ability to identify the first sign of symptoms, and people tend to not spot the earliest signs of an infection when symptoms are relatively mild. Also, viral shedding dynamics were based on data from patients who had received treatment of various forms, including antivirals, corticosteroids and Chinese medicine preparations, all of which could have affected the dynamics of viral shedding.

Despite the limitations, this study concluded that viral shedding of COVID-19 patients peaked on or a little before the onset of symptoms, and a large proportion of transmission occurred before the first symptoms appeared. For effective control of the outbreak, more inclusive criteria for contact tracing should be considered to capture potential transmission 2 to 3 days before the onset of symptoms.