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Evidence for pre-symptomatic transmission of coronavirus disease 2019 (COVID-19) in China
Xiang Ren1, Yu Li1, Xiaokun Yang1
1The Chinese Center for Disease Control and Prevention, Beijing, China.
Insights
Early COVID-19 transmission outside Hubei province showed a 5.3-day epidemic doubling time. Pre-symptomatic transmission occurred in 40% of cases, indicating its significant role in the early spread of the novel coronavirus.
Area of Science:
- Epidemiology
- Infectious Disease Dynamics
Background:
- Provinces outside Hubei were on high alert for COVID-19 importations and transmission in early 2020.
- Key properties of COVID-19 infection and transmission were not yet established during this period.
Purpose of the Study:
- To analyze early COVID-19 cases outside Hubei province.
- To infer transmission dynamics, incubation period, and severity of the infection.
Main Methods:
- Collated and analyzed data from 449 early COVID-19 cases outside Hubei.
- Analyzed 64 transmission clusters to estimate serial interval and pre-symptomatic transmission.
Main Results:
- Estimated epidemic doubling time at 5.3 days (95% CI: 4.3, 6.7).
- Median incubation period estimated at 4.6 days (95% CI: 4.0, 5.2).
- Estimated 40% of transmissions occurred before symptom onset; secondary cases showed milder illness.
Conclusions:
- Majority of transmissions occur around symptom onset, with a significant role for pre-symptomatic spread.
- Milder infections in secondary cases may better reflect true disease severity.
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.
