Contact patterns under national interventions
We assume the population is stratified into predefined age groups with age-specific mixing pattern represented by a contact matrix M with an element of mij representing the contacts between someone of age group i∈G with someone of age group j∈G. The baseline contact matrix (with no interventions in place) is taken from the POLYMOD survey conducted in the UK.22 The contact pattern may also be influenced by a range of interventions (social distancing was encouraged on 15 March 2020, schools were closed and lockdown occurred on 23 March 2020). We implement these interventions by assuming that the percentage of 0–18 year-olds attending school after 23 March 2020 was 5% (reducing all contacts between school-age individuals by 95%) and that social distancing reduced all contacts by 0%–50%. We take the element-wise minimum for each age group’s contact with another age group from all active interventions (distancing, schools/lockdown). A study on postlockdown contact patterns (CoMix11) is used to inform contacts after lockdown (first survey 24 March 2020, with an average of 73% reduction in daily contacts observed per person compared with POLYMOD).
Moving between contact matrices of multiple interventions was implemented by assuming a phased, linear decrease. After lockdown, we vary a parameter (endphase) to capture the time taken to fully adjust (across the population, on average) to the new measures (allowed to vary from 1 to 31 days). This assumption represents the time taken for individuals to fully adapt to new measures (and household transmission), and is in line with data on the delay in the control of COVID-19 (reductions in hospital admissions and deaths after lockdown).23 The parameter endphase can be interpreted as accounting for the time taken to adjust to all interventions (and not just lockdown).