Evidence for pre‐symptomatic transmission of coronavirus disease 2019 (COVID‐19) in China

Abstract

Background Between mid‐January and early February, provinces of mainland China outside the epicentre in Hubei province were on high alert for importations and transmission of COVID‐19. Many properties of COVID‐19 infection and transmission were still not yet established. Methods We collated and analysed data on 449 of the earliest COVID‐19 cases detected outside Hubei province to make inferences about transmission dynamics and severity of infection. We analysed 64 clusters to make inferences on serial interval and potential role of pre‐symptomatic transmission. Results We estimated an epidemic doubling time of 5.3 days (95% confidence interval (CI): 4.3, 6.7) and a median incubation period of 4.6 days (95% CI: 4.0, 5.2). We estimated a serial interval distribution with mean 5.7 days (95% CI: 4.7, 6.8) and standard deviation 3.5 days, and effective reproductive number was 1.98 (95% CI: 1.68, 2.35). We estimated that 32/80 (40%) of transmission events were likely to have occurred prior to symptoms onset in primary cases. Secondary cases in clusters had less severe illness on average than cluster primary cases. Conclusions The majority of transmissions are occurring around illness onset in an infected person, and pre‐symptomatic transmission does play a role. Detection of milder infections among the secondary cases may be more reflective of true disease severity.

Interventions targeting nonsymptomatic cases can be important to prevent local outbreaks: SARS-CoV-2 as a case-study

“…For example, we set the relative rates at which presymptomatic and asymptomatic individuals generate new infections compared to symptomatic individuals so that 48·9% of transmissions arise from presymptomatic infectors, and 10·6% arise from asymptomatic infectors. 11 While this is in line with other reported estimates, 28,29 there is substantial variation between studies. We therefore also conducted sensitivity analyses in which we explored a range of different values of model parameters (Supplementary Material).…”

Transmission of SARS-CoV-2 before and after symptom onset: impact of nonpharmaceutical interventions in China

“…For the post-lockdown period, estimates ranged from 68.1% to 80.6%, with 95% CIs extending from 64.5% to 83.5%. We note that the estimates for the pre-lockdown period are lower than many published estimates of the frequency of presymptomatic transmission [27,29,[37][38][39][40], which raises the possibility that many estimates might be inflated by the effects of nonpharmaceutical interventions. Points and lines show posterior means and 95% credible intervals, respectively.…”

Routine saliva testing for the identification of silent coronavirus disease 2019 (COVID-19) in healthcare workers

“…Our results show that routine NP testing every two weeks or every week, as recommended by some jurisdictions for frontline healthcare workers 10,11 , would lead to a significant percentage of undetected silent COVID-19 cases, indicating that institutional outbreaks could occur even in the presence of symptom-based screening 9 . Recent studies suggest that a significant portion of disease transmission occurs prior to symptom onset 14,24,25 , highlighting the importance of early detection. Given the practical considerations with NP testing, non-invasive saliva testing presents an attractive alternative for improving case detection with increased testing frequency 26,27 .…”