COVID-19: Current understanding

Grant Hill-Cawthorne

On 20th March, the Scientific Advisory Group for Emergencies (SAGE) released the evidence behind the government response to Coronavirus disease (COVID-19). This series of short articles summarises these 32 documents. You can view all our reporting on this topic under COVID-19.
This article goes over the key epidemiological terms used in the COVID-19 response.

SPI-M-O: Consensus statement on 2019 novel coronavirus (2 March 2020)

This document is from a number of weeks ago. It set out some initial figures on the behaviour of the virus to use in modelling.

Reproduction number (R0)

This is the average number of people infected by each single infected person. If R0 is greater than 1 then an infection will move through the population. R0 figures in Wuhan early on were between 2 and 3 i.e. each infection led to two or three further infections. In the UK this would lead to 80% of the population being infected.

Doubling time

The time it takes for the number of new infections to double in size. In Wuhan this was 4–6 days.

Transmission and control

The aim of the response will be to reduce the frequency with which people are coming into contact with each other. The effectiveness of this strategy will depend upon:

How well people follow any guidance.
By how much people reduce their contact rates.
Whether people can transmit the virus without showing symptoms (asymptomatic transmission).
Whether children can transmit the infection.

Reducing contact rates may not reduce the overall number of people infected but will flatten the peak of the epidemic. The ultimate aim will be to reduce R0 to below 1 as this will slow or stop the epidemic. However, once restrictions are lifted there may be a rebound of cases.

Role of school closures

Modelling suggests that school closures will impact transmission, but less than with influenza.

For influenza the figures given are:

R0 around 1.9 to 2.3 and school closures for 6 to 12 weeks.
Will reduce the peak of incidence by 7.5% to 25%.
The final size of the epidemic will not change.
The peak of the epidemic may be delayed by 3 weeks.

Epidemic timescales

The estimates on 2nd March were:

Highly likely that Wuhan outbreak has peaked.
Given the Wuhan model, cases in the UK would peak 3 to 5 months after start of widespread transmission.
Peak timings in different parts of the UK may vary by 4 to 6 weeks if unmitigated.

Fatality ratios

Infection Fatality Ratio (IFR): the proportion of infected people that die.

Case Fatality Ratio (CFR): the proportion of people with symptoms that die.

Hospitalised Case Fatality Ratio (HFR): the proportion of people hospitalised that die.

IFR estimated to be 0.5% to 1%.
- 0.01% in under 20 year-olds to 8% in over 80s.
CFR estimated to be 0.25% to 4%.
HFR estimated at 12%.
- 4% in under 50 year-olds, 20% for over 80 year-olds, 50% for those needing invasive ventilation.
Hospitalisation rate estimated to be 2% for under 50 year-olds to 44% of over 80 year-olds (8% of those infected overall).

Other key figures

Serial interval: the time between symptoms occurring in one person to symptoms appearing in the person they infect. Average 3 to 8 days.

Incubation period: the time between being infected and showing symptoms. On average symptoms take 5 days to appear, but can appear as soon as 1 day or as late as 11 days after infection. A duration outside of the maximum of 14 days is used to inform the extent of isolation. 

Operational considerations

Forecasting to allow for accurate estimates will depend on defining the figures above for the UK. This will be achieved weeks after a widespread outbreak. The accuracy of models will improve as more data become available. Key among these figures are the hospitalisation rate and CFR.

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