Determining the optimal strategy for reopening schools, the impact of test and trace interventions, and the risk of occurrence of a second COVID-19 epidemic wave in the UK: a modelling study

Abstract

Background: As lockdown measures to slow the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection begin to ease in the UK, it is important to assess the impact of any changes in policy, including school reopening and broader relaxation of physical distancing measures. We aimed to use an individual-based model to predict the impact of two possible strategies for reopening schools to all students in the UK from September, 2020, in combination with different assumptions about relaxation of physical distancing measures and the scale-up of testing.

Methods: In this modelling study, we used Covasim, a stochastic individual-based model for transmission of SARS-CoV-2, calibrated to the UK epidemic. The model describes individuals' contact networks stratified into household, school, workplace, and community layers, and uses demographic and epidemiological data from the UK. We simulated six different scenarios, representing the combination of two school reopening strategies (full time and a part-time rota system with 50% of students attending school on alternate weeks) and three testing scenarios (68% contact tracing with no scale-up in testing, 68% contact tracing with sufficient testing to avoid a second COVID-19 wave, and 40% contact tracing with sufficient testing to avoid a second COVID-19 wave). We estimated the number of new infections, cases, and deaths, as well as the effective reproduction number (R) under different strategies. In a sensitivity analysis to account for uncertainties within the stochastic simulation, we also simulated infectiousness of children and young adults aged younger than 20 years at 50% relative to older ages (20 years and older).

Findings: With increased levels of testing (between 59% and 87% of symptomatic people tested at some point during an active SARS-CoV-2 infection, depending on the scenario), and effective contact tracing and isolation, an epidemic rebound might be prevented. Assuming 68% of contacts could be traced, we estimate that 75% of individuals with symptomatic infection would need to be tested and positive cases isolated if schools return full-time in September, or 65% if a part-time rota system were used. If only 40% of contacts could be traced, these figures would increase to 87% and 75%, respectively. However, without these levels of testing and contact tracing, reopening of schools together with gradual relaxing of the lockdown measures are likely to induce a second wave that would peak in December, 2020, if schools open full-time in September, and in February, 2021, if a part-time rota system were adopted. In either case, the second wave would result in R rising above 1 and a resulting second wave of infections 2·0-2·3 times the size of the original COVID-19 wave. When infectiousness of children and young adults was varied from 100% to 50% of that of older ages, we still found that a comprehensive and effective test-trace-isolate strategy would be required to avoid a second COVID-19 wave.

Interpretation: To prevent a second COVID-19 wave, relaxation of physical distancing, including reopening of schools, in the UK must be accompanied by large-scale, population-wide testing of symptomatic individuals and effective tracing of their contacts, followed by isolation of diagnosed individuals.

Funding: None.

Figure 1

Modelled disease states Blue shading indicates that an individual is infectious and can transmit the disease to other susceptible individuals. States in a darker shade of blue are considered to be symptomatic for the purpose of testing eligibility. This schematic is reproduced from existing work from members of this group.

Figure 2

Model estimates of daily new SARS-CoV-2 infections from Jan 21, 2020, to Dec 31, 2021 (A) New infections with 68% tracing and 18% testing in the full-time school reopening scenario. (B) New infections with 68% tracing and 18% testing in the part-time rota school reopening scenario. (C) New infections with 68% tracing and 75% testing in the full-time school reopening scenario. (D) New infections with 68% tracing and 65% testing in the part-time rota school reopening scenario. (E) New infections with 40% tracing and 87% testing in the full-time school reopening scenario. (F) New infections with 40% tracing and 75% testing in the part-time rota school reopening scenario. Medians across ten simulations are indicated by solid lines and 10% and 90% quantiles by shading. The results do not change if we run a larger number of simulations, and we tested 1, 3, 6, 8, 10, and 20 simulations. The difference is that the noise in the simulations increases with increased size of simulations; therefore, we chose ten simulations for these figures. SARS-CoV-2=severe acute respiratory syndrome coronavirus 2.

Figure 3

Model estimates of cumulative COVID-19 deaths from Jan 21, 2020, to Dec 31, 2021 (A) Deaths with 68% tracing and 18% testing in the full-time school reopening scenario. (B) Deaths with 68% tracing and 18% testing in the part-time rota school reopening scenario. (C) Deaths with 68% tracing and 75% testing in the full-time school reopening scenario. (D) Deaths with 68% tracing and 65% testing in the part-time rota school reopening scenario. (E) Deaths with 40% tracing and 87% testing in the full-time school reopening scenario. (F) Deaths with 40% tracing and 75% testing in the part-time rota school reopening scenario. Medians across ten simulations are indicated by solid lines and the 10% and 90% quantiles by shading.

Figure 4

Model estimates of effective reproduction number R from Jan 21, 2020, to Dec 31, 2021 (A) Reproductive number R with 68% tracing and 18% testing in the full-time school reopening scenario. (B) Reproductive number R with 68% tracing and 18% testing in the part-time rota school reopening scenario. (C) Reproductive number R with 68% tracing and 75% testing in the full-time school reopening scenario. (D) Reproductive number R with 68% tracing and 65% testing in the part-time rota school reopening scenario. (E) Reproductive number R with 40% tracing and 87% testing in the full-time school reopening scenario. (F) Reproductive number R with 40% tracing and 75% testing in the part-time rota school reopening scenario. Medians across ten simulations are indicated by solid lines and the 10% and 90% quantiles by shading. An R value of less than 1 is necessary for virus suppression.