Mapping social distancing measures to the reproduction number for COVID-19

Abstract

In the absence of a vaccine, severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2) transmission has been controlled by preventing person-to-person interactions via social distancing measures. In order to re-open parts of society, policy-makers need to consider how combinations of measures will affect transmission and understand the trade-offs between them. We use age-specific social contact data, together with epidemiological data, to quantify the components of the COVID-19 reproduction number. We estimate the impact of social distancing policies on the reproduction number by turning contacts on and off based on context and age. We focus on the impact of re-opening schools against a background of wider social distancing measures. We demonstrate that pre-collected social contact data can be used to provide a time-varying estimate of the reproduction number (R). We find that following lockdown (when R= 0.7, 95% CI 0.6, 0.8), opening primary schools has a modest impact on transmission (R = 0.89, 95% CI 0.82-0.97) as long as other social interactions are not increased. Opening secondary and primary schools is predicted to have a larger impact (R = 1.22, 95% CI 1.02-1.53). Contact tracing and COVID security can be used to mitigate the impact of increased social mixing to some extent; however, social distancing measures are still required to control transmission. Our approach has been widely used by policy-makers to project the impact of social distancing measures and assess the trade-offs between them. Effective social distancing, contact tracing and COVID security are required if all age groups are to return to school while controlling transmission. This article is part of the theme issue 'Modelling that shaped the early COVID-19 pandemic response in the UK'.

Keywords: COVID-19 reproduction number; school closures; social contact data; social distancing measures.

Figure 1.

The time-varying R number in the UK, estimated using incidence death data prior to lockdown, SCS data and Google Community Mobility Reports. (Online version in colour.)

Figure 2.

The impact of tracing and isolating contacts of symptomatic cases when all children are back at school. The empty bars are under the assumption that children under 11 years of age are as infectous as adults; the filled bars are where children under 11 years of age are half as infectious as adults. Blue bars (left most bars in each grouping) are with 100% of contacts traced; red bars (right most bars) are with no contact tracing. (Online version in colour.)

Figure 3.

The COVID-19 R number as a function of the percentage of active work and leisure contacts under different contact tracing, COVID security and school closure scenarios. The shaded regions indicate the values with schools closed (grey, bottom ribbons), 50% of primary school pupils attending school (cyan), all primary school pupils at school (yellow) and primary and secondary schools open (red, top ribbons). The panels illustrate the values of the R number with (a) no contact tracing or COVID security and with increasing measures (b–i). The number 1 marks the UK position in March 2020, number 2 marks the UK position in April/May 2020. The width of the ribbons indicates 95% confidence intervals. These figures are generated with the assumption that children under 11 years of age are as infectious as adults. (Online version in colour.)

Figure 4.

An early version of the “ready reckoners” showing the relationship between the reproduction number and social distancing outside the home. (Online version in colour.)