Question: For the Delta variant, how long is it between exposure and symptom onset and/or positive test? 

Answer: We are still learning many things about Delta, including its average incubation period.  A preprint posted last week, Viral infection and transmission in a large well-traced outbreak caused by the Delta SARS-CoV-2 variant, begins to shed some light.  I’ve shared a bit more about the study herein.  To synthesize– researchers found that average time from exposure to positive test was 3-4 days, much shorter than the 5-6 day average associated with an earlier 2020 outbreak (Figure 1). Researchers also found that viral load was much higher at positive test, indicating that individuals infected with Delta variant are also more highly contagious during the pre-symptomatic period. Delta is more dangerous.    

The researchers tracked a local outbreak in China caused by the Delta variant, which ultimately spread from the index case to 167 others.  All individuals who were close contacts of the index case were quarantined and tested daily.  If an individual’s test came back positive, then all close contacts of that newly positive individual were also traced, centrally isolated, and tested daily.  Viral loads were assessed on the day of each first positive test.  Researchers found that not only were viral loads much higher with Delta (~1000x higher than earlier strain), but time from exposure to symptom onset (e.g. incubation period) was shorter. “Our results showed the time interval from the exposure to first PCR positive in the quarantined population (n=29) was 6.00 (IQR 5.00-8.00) days in the 2020 epidemic (peak at 5.61 days) and was 4.00 (IQR 3.00-5.00) days in the 2021 (n=34) epidemic (peak at 3.71 days).”      

Note: In case you’re wondering, based on data collected pre-Delta, the median time to symptom onset is 4-5 days.     

Figure 1. Time from Exposure to Positive Test, 2020 and 2021 (from Virological)