Coronavirus models: preventing a second wave - Riskaware

Coronavirus models: preventing a second wave

By Siân Jenkins

The UK governments’ current response to the Coronavirus pandemic aims to curb the spread of the virus. The measures being taken seek to reduce the number of seriously ill people at the peak of the outbreak, allowing the NHS to cope with an increased demand for high dependency beds and minimising the need to prioritise care amongst sufferers.

The core advice to the public and businesses has been as follows: 

  • Only go outside for food, health reasons or work (but only if you cannot work from home).
  • If you go out, stay two metres (six feet) away from other people at all times.
  • Wash your hands as soon as you get home. 

In addition, those experiencing possible symptoms of the virus have been asked to self isolate at home for seven days, and all members of their household to do similarly for 14 days from the onset of the sufferers symptoms. 

These steps represent a multi-pronged attack on the spread of the virus; combining quarantine with social distancing to reduce the communication of the virus. Hand washing with soap reduces the chance that an individual who has come into contact with the virus, will catch it or spread it further, by killing it.

How modelling can be used to understand the impact of such measures on the spread of the virus, was explored in our previous article. Here we consider how modelling allows authorities to determine when to end lockdown.

What would happen if we ended lockdown too soon?

Most households have been implementing the governments’ recommendations for roughly five weeks now. The government has said that it expects this to continue for at least another week on top of this, with the next review due to be held on May 11th. 

Current data is showing either a decline or a leveling off of newly recorded cases and the number of new fatalities each day is decreasing. Whilst this is a cause for optimism, we cannot afford to become complacent and end the lockdown too early.

Ending lockdown in the UK too soon would result in a resurgence of the virus amongst the population. Current efforts mean that relatively few people have had the virus. As a result, the majority of the population is still thought to be susceptible to it. Allowing the virus to freely spread amongst these individuals would result in another significant spike in the number of cases, which could potentially overwhelm the NHS.

The figure below shows the results from an SEIR (Susceptible-Exposed-Infected-Recovered) model applied to a population of 10000 people. This represents the effects of a virus on a small population and is the type of model whose results are used to inform government decisions. More information on this type of model can be found in our previous article

Here we see an initial uncontrolled outbreak for the first 40 days as the virus begins to spread in a purely susceptible population. During this period, the virus has a basic reproductive number of three, meaning that every infectious individual is on average infecting three people over the duration of their infection. This value is typically denoted by R0 and has been described as the R value during Downing Street press conferences. 

The lockdown has resulted in a reduction in transmission of the virus, reducing R0 to less than one. In this scenario, the spread of the virus decreases and it should eventually die out. The figure below shows this scenario between day 40 and day 70, where the number of exposed and infected people decreases and the susceptible population levels off. The exposed and infected populations appear to be almost zero by day 70.

On day 70, the lockdown restrictions are completely removed and there is a sudden resurgence of the virus. This resurgence is much larger than the initial outbreak, represented by the much larger peak in infected near day 110, and would likely overwhelm the NHS. If this scenario occurred, another period of lockdown would need to be implemented to protect the NHS and save lives. 

As the incubation and symptomatic periods of the virus remain unchanged, this would essentially result in the current period of lockdown being at least doubled in length.

What scenarios could lead to a resurgence in the virus?

As already discussed, ending lockdown too early is one such scenario that would lead to a resurgence of the virus. However any scenario that allows contact between infected individuals and a largely susceptible population will result in a rapid spread of the virus. In the case of ending lockdown too early, there would be an inadequate reduction in cases across all communities in the country, resulting in infected individuals being returned to the general population, causing a growth in cases. 

Another risk of re-infection comes from virus hotspots. We have seen in the data presented at daily Downing Street press conferences, that the spread of the virus in London was a few weeks ahead of the spread in the rest of the country. Even if lockdown is ended at the correct point for most regions, there is still potential for those that had higher rates of infection to re-infect neighbouring regions.

Testing is required to identify virus hot spots – especially those in the local community. These can  arise as a result of individuals in these communities failing to adequately follow social distancing guidelines during the current period of lockdown, allowing the virus to continue to spread. If the majority of the country is sufficiently free of the virus, the government may choose to re-open the majority of the country whilst maintaining a more stringent lockdown on specific areas of virus activity, in order to balance management of the virus with getting the economy up and running again.

Another source for re-introduction of the virus is from people entering the country and bringing it with them. For example, all recent new cases in China have arisen from the virus being imported into the country. Australia is addressing this scenario by currently quarantining all international visitors upon arrival to the country. This hopes to prevent the introduction of new virus sources whilst the country works to prevent further spread within its own borders. 

How long will lockdown continue?

Currently, plans are in place to continue lockdown measures in the UK until mid-May. Scientists won’t know if the current period of social distancing has done enough until much closer to this date. Data needs to be continually collected and analysed throughout the period leading up to this date, so an informed decision can be made. 

Current testing is focused on identifying if symptomatic key workers have the virus so that they can return to work as soon as possible. The government has announced that a new study will begin, run by the Office of National Statistics and the University of Oxford. The plan is to regularly monitor individuals with regard to the virus, through questionnaires and swabs, providing a general picture of the virus’ presence within each area of the population.

The data collected can be fed into SEIR type models to provide an estimate for the number of infected and immune individuals in each community. The more information available, the more accurate these estimates will be. 

Only by closely monitoring virus levels and immunity within the population can models receive enough data to create robust forecasts for the progression of the virus, allowing a date to be set for the end of lockdown and eventually social distancing. This is why testing is central to current efforts, along with the development of a vaccine.

The new NHS contact tracing app will help implement the test, track and trace initiative, aiding in rapid identification of new individuals with the virus and their contacts. Contacts can then be informed and potentially placed in quarantine to prevent further spread of the virus and another wide-scale outbreak. By authorities immediately becoming aware of the people an infected individual has come into contact with, effective disease modelling can be conducted to identify the impact of the individual and identify if the wider community also needs to be quarantined.

It is likely that a phased reopening of activities will be implemented when lockdown ends – an approach being taken by many other countries in Europe. A phased reopening allows governments to test the impact of each activity on the spread of the virus, identify those that are shown to lead to an increase in the spread of the virus and swiftly suspend them to reduce infection. Disease models will be essential for helping to identify high and low risk activities, as they can be used to model the different social interactions that each activity entails and the resultant spread of infection.

It is also likely that social distancing and the requirement for anyone with symptoms, and their household, to self-isolate, will continue for the foreseeable future. In this scenario, certain areas of normal life where large numbers gather, such as in pubs and restaurants, will be some of the last activities to return to normal. 

Without a vaccine or a sufficient number having gained natural immunity (assuming people remain immune), we won’t be able to achieve the herd immunity required to provide protection to the people most at risk in the population. Disease modelling can additionally be used to determine the level of immunity in a population required to provide herd immunity. Consequently, we will have to accept these changes to our lives until required levels can be attained.

Regardless of whichever strategy is ultimately applied to return the country back to some sense of normality, informed disease modelling utilising robust data, is a vital element for providing situational awareness to support informed decision making as the lockdown is gradually relaxed.

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