Author | Location | Number of relevant study subjects | Measure of relative infectiousness |
PCR studies | |||
Letizia et al13 | South Carolina, USA | 51 | The viral load at diagnosis was approximately four times as high (4.39, S.gene; 4.3, N.gene; 5.22, ORF 1ab) in the five symptomatic participants compared with the 46 asymptomatic participants. However, some asymptomatic participants had higher viral loads estimated on the basis of the cycle threshold (Ct). |
Uhm et al11 | South Korea | 396 | There was no significant difference in the first follow-up Ct value for the E, RdRP and N genes between symptomatic and asymptomatic individuals. |
Lavezzo et al1 | Vo’, Italy | 58 | There was no significant difference between asymptomatic and symptomatic participants based on viral PCR recovered from the nasopharyngeal swabs. |
Danis et al12 | Les Contamines-Montjoie, France | †† | The viral load was described as being similar among the six study subjects (one asymptomatic and five symptomatic cases). |
Long et al9 | Wanzhou, China | 74 | The initial Ct values were similar among the asymptomatic and mild symptomatic individuals, whereas the duration of shedding among asymptomatics was longer than for mild symptomatic individuals. |
Modelling studies, estimate (95% CI) | |||
Nabi2 | Dhaka, Bangladesh | NA* | 0.57† |
Li et al10 | China | NA* | 0.55 (0.46 to 0.62)‡,** 0.50 (0.37 to 0.69)§,** 0.43 (0.31 to 0.61)¶,** |
Contact tracing studies | |||
Park et al14 | Seoul, South Korea | 225 | Secondary attack rate:
Relative risk (RR)=0, but Fisher’s exact test (null hypothesis of no difference), p=1.0. Therefore, the point estimate for RR (equivalent to relative infectiousness) is equal to 0, but we do not have sufficient evidence to conclude that it is significantly different to 1. |
Cheng et al15 | Taiwan | 2735 | Secondary attack rate:
RR=0. Fisher’s exact test, p=1.0. Therefore, the point estimate for RR (equivalent to relative infectiousness) is equal to 0, but we do not have sufficient evidence to conclude that it is significantly different to 1. |
Han16 | South Korea | 192 (all contacts) and 36 (close contacts only) | Secondary attack rate among all contacts:
Secondary attack rate among close contacts only:
RR=0. Fisher’s exact test, p=0.19 (all contacts), p=<0.001 (close contacts). Conclusion:
|
Plucinski et al3 | Diamond Princess cruise ship, Japan | 75 | 0.78 (0.58 to 1.04) |
Luo et al17‡ | Guangzhou, China | 2610 | Adjusted OR for transmission risk, based on the severity of the primary cases‡:
|
The number of relevant study subjects refers to the number of individuals with available PCR results (PCR studies) or the number of close contacts (contact tracing studies).
*Population studied at national level.
†The average of the following set of values for relative infectiousness: {0.5249, 0.5855, 0.5995, 0.59, 0.5498}.
‡After controlling for age, sex, exposure setting and symptoms in the index (primary) cases.
§Model for the period 10–23 January 2020.
¶Model for the period 24 January–3 February 2020.
**Model for the period 24 January–8 February 2020.
††Asymptomatics were defined as ‘undocumented cases’.