Transmissibility of asymptomatic COVID-19: Data from Japanese clusters

Nakajo, Ko; Nishiura, Hiroshi

doi:10.1016/j.ijid.2021.02.065

Cited by 23 publications

(25 citation statements)

References 7 publications

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“…Our point estimate of the relative reproduction number for asymptomatic cases (0.19) was consistent with this finding. Recently, we reported that the relative transmissibility of asymptomatic cases was 0.27 by analyzing transmission networks within an early cluster in Tokyo and Kanagawa [16]; this result was also broadly consistent with the findings of the current study. Two other contact tracing studies assessed the transmissibility of asymptomatic infections using epidemiological measurements other than the reproduction number.…”

Section: Discussionsupporting

confidence: 92%

“…He et al [15] analyzed transmission data in Ningbo from January 21 to March 6, 2020 [17] and estimated the reproduction numbers of asymptomatic and symptomatic cases as 0.20 and 0.78, respectively. This finding indicated that the relative transmissibility of asymptomatic cases was below 1, and was further supported by the work of Nakajo and Nishiura in analyzing transmission trees among older adults [16].…”

supporting

confidence: 62%

“…The transmissibility of asymptomatic infections remains unclear [7][8][9]. The results of several studies have been contradictory [8,[10][11][12][13][14][15][16]. In a retrospective study of 303 symptomatic and asymptomatic patients in a community treatment center in Korea, the viral loads of asymptomatic patients were similar to those of symptomatic patients [10].…”

mentioning

confidence: 99%

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Exploring secondary SARS-CoV-2 transmission from asymptomatic cases using contact tracing data

Nakajo

Nishiura

2021

Theor Biol Med Model

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Background Individuals with asymptomatic severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection can propagate the virus unknowingly and thus have been a focus of public health attentions since the early stages of the pandemic. Understanding viral transmissibility among asymptomatic individuals is critical for successful control of coronavirus disease 2019 (COVID-19). The present study aimed to understand SARS-CoV-2 transmissibility among young asymptomatic individuals and to assess whether symptomatology was associated with transmission of symptomatic vs. asymptomatic infections. Methods We analyzed one of the first-identified clusters of SARS-CoV-2 infections with multiple chains of transmission that occurred among university students in March 2020 in Kyoto prefecture, Japan, using discrete and two-type branching process models. Assuming that the number of secondary cases resulting from either primary symptomatic or asymptomatic cases independently followed negative binomial distributions, we estimated the relative reproduction numbers of an asymptomatic case compared with a symptomatic case. To explore the potential association between symptomatology and transmission of symptomatic vs. asymptomatic incident infections, we also estimated the proportion of secondary symptomatic cases produced by primary symptomatic and asymptomatic cases. Results The reproduction number for a symptomatic primary case was estimated at 1.14 (95% confidence interval [CI]: 0.61–2.09). The relative reproduction number for asymptomatic cases was estimated at 0.19 (95% CI: 0.03–0.66), indicating that asymptomatic primary cases did not result in sufficient numbers of secondary infections to maintain chains of transmission. There was no apparent tendency for symptomatic primary cases to preferentially produce symptomatic secondary cases. Conclusions Using data from a transmission network during the early epidemic in Japan, we successfully estimated the relative transmissibility of asymptomatic cases of SARS-CoV-2 infection at 0.22. These results suggest that contract tracing focusing on symptomatic index cases may be justified given limited testing capacity.

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Section: Discussionsupporting

confidence: 92%

supporting

confidence: 62%

mentioning

confidence: 99%

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Exploring secondary SARS-CoV-2 transmission from asymptomatic cases using contact tracing data

Nakajo

Nishiura

2021

Theor Biol Med Model

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“…In generation 1, the generation-based reproduction number was estimated to be 6, falling below 1 by generation 3 (Supplementary Figures 4–5; Supplementary Table 2). Inspection of probabilistic transmission networks (Supplementary Figure 6) confirmed a high value of the case reproduction number, R , for B1.1, but also supported sequential transmission of the virus within households resulting in a greater estimated value of

compared to earlier studies ( Bi et al, 2020 , Endo et al, 2020 , Nakajo and Nishiura, 2021 ). A previous study by Ng et al ( Ng et al, 2020 ) involved an analysis of data from Taiwan from 2020, and found an estimated value of

that was substantially larger, in part due to the small sample size in their analysis (the posterior mean of

was 19.20).…”

Section: Resultssupporting

confidence: 59%

A hospital-related outbreak of SARS-CoV-2 associated with variant Epsilon (B.1.429) in Taiwan: transmission potential and outbreak containment under intensified contact tracing, January–February 2021

Akhmetzhanov¹,

Jung²,

Cheng³

et al. 2021

International Journal of Infectious Diseases

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Objectives A hospital-related cluster of 22 COVID-19 cases occurred in Taiwan in January–February 2021. Rigorous control measures were introduced and could only be relaxed once the outbreak was declared over. Each day after the apparent outbreak end, we estimated the risk of future cases occurring, to inform decision-making. Methods Probabilistic transmission networks were reconstructed and transmission parameters (the reproduction number R and overdispersion parameter k ) were estimated. We estimated the reporting delay during the outbreak (Scenario 1). We also considered a counterfactual scenario with less effective interventions characterized by a longer reporting delay (Scenario 2). Each day, we estimated the risk of future cases under both scenarios. Results The values of R and k were estimated to be 1.30 (95% credible interval: 0.57,3.80) and 0.38 (0.12,1.20), respectively. The mean reporting delays considered were 2.5 days (Scenario 1) and 7.8 days (Scenario 2). The inferred probability of future cases occurring declined more quickly in Scenario 1 than Scenario 2. Conclusions Following outbreak containment, rigorous control measures allowed the outbreak to be declared over quickly. This highlights the need for effective interventions, not only to reduce cases during outbreaks but also to allow outbreaks to be declared over with confidence.

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“…In a meta-analysis, Madewell et al compared the secondary attack rate around symptomatic (18%) and asymptomatic COVID-19 cases (0.7%, p < 0.001) [22]. Nowadays, questions are being raised about the real role of asymptomatic infected people in the spread of the disease [23][24][25][26][27]. Among SARS-CoV-2 positive individuals, there is a confusion between those who are asymptomatic (= no symptoms at all), which is the case for 20% of COVID-19 patients, CI 95%, 17-25 according to the review of Buitrago-Garcia et al [28], and those who are presymptomatic (= no symptoms at the time of sample collection for RT-PCR) [3,29].…”

Section: Discussionmentioning

confidence: 99%

Lessons learned from the investigation of a COVID-19 cluster in Creil, France: effectiveness of targeting symptomatic cases and conducting contact tracing around them

et al. 2021

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Background This study presents the methods and results of the investigation into a SARS-CoV-2 outbreak in a professional community. Due to the limited testing capacity available in France at the time, we elaborated a testing strategy according to pre-test probability. Methods The investigation design combined active case finding and contact tracing around each confirmed case with testing of at-risk contact persons who had any evocative symptoms (n = 88). One month later, we performed serology testing to test and screen symptomatic and asymptomatic cases again (n = 79). Results Twenty-four patients were confirmed (14 with RT-PCR and 10 with serology). The attack rate was 29% (24/83). Median age was 40 (24 to 59), and the sex ratio was 15/12. Only three cases were asymptomatic (= no symptoms at all, 13%, 95% CI, 3–32). Nineteen symptomatic cases (79%, 95% CI, 63–95) presented a respiratory infection, two of which were severe. All the RT-PCR confirmed cases acquired protective antibodies. Median incubation was 4 days (from 1 to 13 days), and the median serial interval was 3 days (0 to 15). We identified pre-symptomatic transmission in 40% of this cluster, but no transmission from asymptomatic to symptomatic cases. Conclusion We report the effective use of targeted testing according to pre-test probability, specifically prioritizing symptomatic COVID-19 diagnosis and contact tracing. The asymptomatic rate raises questions about the real role of asymptomatic infected people in transmission. Conversely, pre-symptomatic contamination occurred frequently in this cluster, highlighting the need to identify, test, and quarantine asymptomatic at-risk contact persons (= contact tracing). The local lockdown imposed helped reduce transmission during the investigation period.

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Transmissibility of asymptomatic COVID-19: Data from Japanese clusters

Cited by 23 publications

References 7 publications

Exploring secondary SARS-CoV-2 transmission from asymptomatic cases using contact tracing data

Exploring secondary SARS-CoV-2 transmission from asymptomatic cases using contact tracing data

A hospital-related outbreak of SARS-CoV-2 associated with variant Epsilon (B.1.429) in Taiwan: transmission potential and outbreak containment under intensified contact tracing, January–February 2021

Lessons learned from the investigation of a COVID-19 cluster in Creil, France: effectiveness of targeting symptomatic cases and conducting contact tracing around them

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