Table 1 Scenario definitions.

Baseline

During the lockdown, the social contacts related to B2B and in the community reduced to 15% of observed behaviour prior the lockdown. Schools are closed and household mixing did not change. For the deconfinement phase, we consider four social mixing assumptions based on B2B and community-related activities. The social contacts related to B2B increase to 25% or 50% of the pre-pandemic mixing patterns from May 4th. Community mixing is assumed to remain 15% or increase up to 30% from May 25th onward. All runs contain an age-specific partial school reopening from May 18th according the Belgian regulations at that time (see Supplementary Table 7) and assuming a 50% reduction of transmission at school due to precautionary measures.

Baseline w/o B2B.

Baseline scenario without increase in B2B mixing (=fixed to 25%).

Baseline w/o community

Baseline scenario without increase in community mixing (=fixed to 15%).

Baseline w/o school*

Baseline scenario without partial school reopening.

Baseline w/o PM at school*

Baseline scenario without precautionary measures at schools.

School 0–5 years

Baseline scenario with reopening of pre-schools with precautionary measures.

School 0–11 years

Baseline scenario with reopening of pre- and primary schools with precautionary measures.

School 0–17 years

Baseline scenario with reopening of pre-, primary and secondary schools with precautionary. measures

School 0–11 years w/o PM

Baseline scenario with reopening of pre- and primary schools without precautionary measures.

Household bubbles

Community mixing is partially replaced by social contacts within household bubbles consisting of two households of which the oldest two household members are part of the same weekend community and their ages can differ by up to 3 years. The mixing intensity equals the equivalent of being fully connected 4 days per week. Interaction within the household bubbles is possible from May 11th onward.

Household bubbles: 7/7 days

Household bubble scenario in which all members are in contact every day (connected 7 days per week), hence almost no community contacts remain.

Household bubbles: 2/7 days

Household bubble scenario in which the members are less connected, the equivalent of a visit 2 days per week, hence more community contacts remain.

Household bubbles: size 3

Household bubble scenario with three households per bubble. The equivalent of being fully connected 4/7 days a week equals the assumed number of community contacts in our baseline scenario. As such, most leisure contacts are within the household bubble.

Household bubbles: size 4

Household bubble scenario with four households per bubble and all leisure contacts are within the household bubble. The size of the household bubble surpasses the number of leisure contacts in the baseline scenario, hence the absolute number of contacts increases in this scenario.

Household bubbles: age gap 20 years

Household bubble scenario in which the ages of the two oldest household members can differ by up to 20 years.

Household bubbles: age gap 60 years

Household bubble scenario in which the ages of the two oldest household members can differ by up to 60 years.

Baseline with CTS

Baseline scenario with contact tracing strategy starting on May 11th in which 70% of the symptomatic cases are included, 90% of the household contacts and 70% of the non-household contacts are successfully traced, tested and isolated if infected. The false-negative predictive value is 10%.

Household bubbles and CTS

Household bubble scenario with contact tracing as specified above.

(child) or (c)

Scenarios including age-specific susceptibility in which children (0–17 years) are only half as susceptible compared to adults (+18 years). During the lockdown, the social contacts related to B2B and in the community reduced to 24% and 14%, respectively, of observed behaviour prior the lockdown.