Question to the Department of Health and Social Care:

To ask Her Majesty's Government what estimate of the proportion of COVID-19 cases that are asymtomatic they applied to the forecasting models that were used to inform the decision to place England under national restrictions in March to address the COVID-19 pandemic; and what was the evidence base used for this estimate.

The Scientific Advisory Group for Emergency (SAGE) is responsible for ensuring that timely and coordinated scientific advice is made available to support decisions by the Government. The SAGE subgroup, Scientific Pandemic Influenza Group on Modelling, Operational use their own estimates of metrics such as asymptomatic case proportions, infection hospitalisation rates, or infection fatality rates. These are based on a wide range of available data sources, including testing data, hospital admission, intensive care unit admissions, and deaths. Their models are regularly updated to fit to the observed transmission of the disease.

In the reasonable worst-case planning scenario from late March, SAGE’s best estimate of the infection fatality ratio was approximately 1%, however this was highly age-dependent. Precise estimates of the case fatality ratio – the proportion of people with clinical symptoms who die – are much harder, as the proportion of cases who are asymptomatic is difficult to estimate. Due to the difficulty with ascertaining the proportion of infections that are truly asymptomatic, modelling is based on estimates of the total number of infections in a population. At the time, the best estimate of the proportion of cases that were asymptomatic was 33%.

Estimates of mortality rates for those hospitalised were around 12%. However, again this was highly age-dependent, with 50% mortality in those hospitalised who require invasive ventilation.

SAGE’s estimate of the proportion of infections that required hospitalisation was 5% overall, but that this was also highly dependent on age. This reasonable worse-case planning scenario used an estimate for the number of patients requiring ventilation, mechanical or otherwise, of 30%. A copy of the SAGE paper Reasonable Worst-Case Planning Scenario – 29/03/2020 is attached.