Table 1.
Parameter definitions and assumptions for the baseline model
| Assumed value | Details and references | |
|---|---|---|
| Individual-level dynamics | ||
| R in absence of control measures | 2·6 | SARs were chosen to be consistent with empirical estimates (table 2) and produce an R consistent with a meta-analysis of early studies;15 sensitivity analysis shown in the appendix (p 4) |
| Duration of infectiousness | 5 days (for cases that will become symptomatic, first day is pre-symptomatic) | Given incubation period of about 5 days, this assumption implies serial interval of about 6·5 days;16 |
| Relative infectiousness of asymptomatic cases | 50% | One published point estimate was 65%,17 but secondary cases from asymptomatic individuals were more likely to, in turn, be asymptomatic, suggesting lower contribution to transmission; sensitivity analysis shown in the appendix (p 3) |
| Proportion of cases who are eventually symptomatic | 30% of children, 70% of adults | Based on evidence synthesis of age-stratified COVID-19 data;18 sensitivity analysis shown in the appendix (p 3) |
| Probability that symptomatic individual will eventually self-isolate and be tested | 90% | We assumed that the virus is only detectable by PCR during the infectious period; 90% of UK survey respondents said they would likely comply with app request to self-isolate if rapid test available19 |
| Effective duration of infectiousness if an individual self-isolates when symptomatic | Mean delay from onset to isolation of 2·6 days; distribution shown in the appendix (p 2) | We assumed that individuals are most likely to self-isolate 0–4 days after onset (ie, 1–5 days after becoming infectious); for 269 cases with known date of onset and confirmation in Singapore, of those who were confirmed within 5 days, 2% were confirmed on date of onset, 26% on day 2, 27% on day 3, 14% on day 4, and 31% on day 5;20 we assumed that isolation could occur 1 day before confirmation; sensitivity analysis shown in the appendix (p 5) |
| SAR among contacts in home | 20% | Details in the SAR section of the Methods |
| SAR among other contacts | 6% | Details in the SAR section of the Methods |
| Contact tracing | ||
| Proportion of contacts who are acquaintances (ie, have been met before) | 100% in household, 90% at school, 79% at work, 52% in other settings | Data from BBC Pandemic dataset;13 for each contact reported, participants were asked “have you met this person before?” |
| Proportion of potentially traceable household contacts who are successfully traced | 100% | Assumed |
| Proportion of potentially traceable workplace, school, or other scenario contacts who are successfully traced | 95% | Assumed, with sensitivity analysis shown in figure 2 |
| Probability that traced contacts adhere to quarantine | 90% | Proportion of traced contacts who are successfully removed from the potentially infectious group; we assumed virus is only detectable by PCR during the infectious period; 90% of UK survey respondents said they would likely comply with app request to self-isolate if rapid test available;19 we assumed contacts traced by app would be quarantined immediately and manually traced contacts would take 2 days to quarantine after isolation of the index case9, 16 |
| App-based tracing | ||
| Proportion of population that would have the app installed | 53% | We assumed that 71% of the population were smartphone users (details in appendix, p 1); 75% of UK survey respondents said they would probably or definitely download the app;19 therefore, we assume that 71% × 75% = 53% of the population would have the app installed. |
| Mass testing | ||
| Proportion of the population that are tested per week | 5% (ie, 460 000 tests per day for UK) | 0·7% of the population tested per day, which is equal to the highest number of daily per person tests done anywhere in world as of mid-April, 2020 (details in appendix, p 1) |
R=reproduction number. SAR=secondary attack rate.