Releasing the lockdown in the UK Covid-19 epidemic: a stochastic model

BackgroundIn a classic epidemic, the infected population has an early exponential phase, before slowing and fading to its peak. Mitigating interventions may change the exponent during the rising phase and a plateau can replace a peak. With interventions comes the risk that relaxation causes a second-wave. In the UK Covid-19 epidemic, infections cannot be counted, but their influence is seen in the curve of the mortality data. This work simulated social distancing and the lockdown in the UK Covid-19 epidemic to explore strategies for relaxation. MethodsCumulative mortality data was transposed 20 days earlier to identify three doubling periods separated by the 17th March--social distancing, and 23rd March--lockdown. A set of stochastic processes simulated viral transmission between interacting individuals using Covid-19 incubation and illness durations. Social distancing and restrictions on interactions were imposed and later relaxed. Principal FindingsDaily mortality data, consistent with that seen in the UK Covid-19 epidemic to 24th April 2020 was simulated. This output predicts that under a lockdown maintained till early July 2020, UK deaths will exceed 31,000, but leave a large susceptible population and a requirement for vaccination or quarantine. An earlier staged relaxation carries a risk of a second-wave. The model allows exploration of strategies for lifting the lockdown. InterpretationSocial distancing and the lockdown have had an impressive impact on the UK Covid-19 epidemic and saved lives, caution is now needed in planning its relaxation. FundingUnfunded research. Research in contextO_ST_ABSEvidence before this studyC_ST_ABSThe classical Susceptible, Infected, Recovered, (SIR) epidemiological model with additional compartments and sophistications have been widely used to make forecasts in the Covid-19 pandemic but are not easily accessible. Added value of this studyThis study adds reassurance that the interventions of social distancing introduced on the 17th March and the lockdown of the 23rd March 2020 have reduced mortality. The risks of a second-wave on their relaxation are real and illustrated graphically. Implications of all the available evidenceTogether with other models, credence is given to the risks of a second-wave in the UK Covid-19 epidemic on the relaxation of restrictions.