Table 2.
Input parameters used for modelling the potential spread of COVID-19 infections with the stochastic version of CovidSIM (v1.1) with New Zealand as a case study.
| Parameter | Value/s used | Further details for parameter inputs into the modelling |
|---|---|---|
| Latency period | 5 days | We used the best estimate from CDC of a mean of 6 days to symptoms (i.e., the latency period plus the prodromal period)18. We used a standard deviation (SD) of 25% (1 day) (calculated using 16 stages; Erlang distribution) |
| Prodromal period | 1 day | There is still uncertainty about the length of the prodromal period for COVID-19, so we used an assumed value for influenza (SD = 25%; 0.25 days, Erlang distribution) |
| Symptomatic period | 10 days (split into 2 periods of 5 days each) | The WHO-China Joint Mission report stated that “the median time from onset to clinical recovery for mild cases is approximately 2 weeks and is 3–6 weeks for patients with severe or critical disease”19. But given that mild cases may have been missed in this particular assessment, we used a slightly shorter total time period of 10 days (SD = 25%; 2.5 days, Erlang distribution) |
| Infections that lead to sickness (symptomatic illness) | 60% | We used the best estimate from CDC of 60% symptomatic and 40% asymptomatic18* |
| Contagiousness | ||
| Risk of in-flight transmission | 0.00214 per hour of flying | This risk was estimated for transmission from an infectious case on a flight in which there was mandated masking (i.e., masks are mandated for all international flights arriving in NZ at the time of writing in March 2020). It is the risk that an index case infects one of the fellow passengers, not the individual risk of each fellow passengers to acquire infection. See the Supplementary Information for our estimates derived from our review of the literature* |
| Flight duration | 3 h | For the Australia to NZ flights (e.g., Sydney to Auckland). Times for flights from Japan and the US in scenario analyses are shown in Table 5* |
| Effective reproduction number (Re) in the NZ post-pandemic setting | 2.5 | We used the best estimate from CDC of R0 = 2.518. We assumed for NZ that the social behaviour with elimination status was fairly similar to the pre-COVID-19 situation (i.e. negligible additional physical distancing, normal occurrence of indoor events in public settings and no routine mask use by the great majority of the population). We also assumed a population with no specific immunity to SARS-CoV-2 (acquired or via vaccination) |
| Relative contagious-ness in the prodromal period | 100% | We assumed this was high given that the CDC estimate that 50% of transmission occurs prior to symptom onset18 |
| Contagiousness after the prodromal period | 100% and 50% | In the first five days of symptoms, cases were considered to be fully contagious. In the second five-day period, this was assumed to be at 50%. The latter figure is still uncertain, but is broadly consistent with one study on changing viral load20 |
*See Supplementary Information for consideration of uncertainty and probabilistic sensitivity analysis.