Understanding the trade-off between factors affecting rate changes in the coronavirus data

February feels like an important month for Boris Johnson and his government. On 22nd of this month, England’s exit strategy from its third national lockdown will be presented to the public. This exit has only been made possible by the tremendous public efforts to bring coronavirus infections, hospitalisations and deaths down from their seemingly insurmountable levels over the Christmas period and early January. Cases (by specimen date) are now 70% below the peak; hospital admissions have fallen by 43%; and deaths (by date of death) by 39%. These numbers will continue to fall under current conditions, but when the conditions change – which they inevitably will – so will the % change in the coronavirus data. The current decline may accelerate further or decelerate, for example. We can make reasonable assumptions about what we think will happen, but we will not know for sure. Thus, while we are making significant strides towards the point in which we feel like we can return to enjoying a greater quality of life, there are still plenty of pitfalls to negotiate over the next few months.

The best way to view the upcoming challenge of managing the virus is through the prism of “trade-offs”where one thing increases, another must decrease. This concept is important for understanding the factors that affect COVID-19’s ability to grow or decline in the community, such as seasonality, susceptibility and utilisation of control measures (restrictions). Below is a simplified example of such factors and their effect:

Note: This is a simplified version. There are more factors affecting the process and each factor does not have an equal impact.

As the diagram shows, certain factors – e.g. control measures (restrictions), increasing number of people vaccinated, and warmer weather (leading more people to socialise outdoors) – can contribute to the decline in prevalence of the virus, whereas others – fewer control measures (restrictions), prevalence of certain variants (more transmissible or lethal), and colder weather (leading more people to socialise indoors) – have the opposite effect, contributing to the virus’s increase.

However, what is important to remember is that each factor does not have an equal impact on the virus’s ability to transmit. The conditions are constantly changing. We can see which side is having a greater impact on transmission by looking at the rate of change in the coronavirus data. If numbers are going down, it tells us that the conditions pushing down on the virus are greater than those pushing up, and the reverse is true if the numbers are increasing. These two dynamics are simplified in the diagram below:

Note: This is a simplified version. There are more factors affecting the process and each factor does not have an equal impact.

This is where the concept of “trade-off” becomes increasingly important. Currently, the conditions pushing down on the virus are greater than those pushing up, which is why numbers have consistently fallen for the past month. The reasons for this fall appear largely due to increased restrictions. Lockdown upholds severe control measures that restrict people’s movements and social activity, offering less opportunity for the virus to transmit. In all three lockdowns, it has pushed cases to fall by 20-25%. We can see this by plotting the percentage change in cases:

Note: Covers period of 1st June – 7th February. Source: UK Government’s COVID-19 Dashboard.

Yet, we all know that lockdown is unsustainable because of the severe restrictions it places on people’s lives. Thus, over the next few months, we will rightfully be easing some restrictions, but this will inevitably change the dynamics. It will take some strength away from the conditions pushing down on the virus and give it to the conditions pushing up on the virus. How much so depends on what restrictions are eased, but to prevent the conditions pushing up on the virus becoming greater than those pushing down, further reinforcements are needed.

The good news is that we have a factor which we did not have before: vaccinations. Our vaccine roll-out has been successful; we have already vaccinated over 15 million people (first dose only) and numbers will continue to grow over the next few months. As increasing numbers of people vaccinated provides further protection – particularly from serious illness and death – it will increasingly strengthen the conditions pushing down on the virus. Though it is important to highlight that the extent to which it will help with transmission is still an unknown. We expect it to halt some transmission, but no one can say with any certainty by how much.

Warmer weather will also provide an added boost because more people should be socialising outdoors than indoors, which is a harder environment for the virus to transmit. This factor certainly helped us somewhat over the previous summer when we were easing restrictions from the first lockdown. Thus, there are plenty of reasons to be cautiously optimistic.

However, there is one difference for the factors pushing up on the virus that we must be careful of: the more transmissible Kent variant. It gives the virus an advantage, expanding its opportunities to transmit, even among an increasingly vaccinated population. It is probably the starkest argument against an open-everything-at-once exit strategy because it is now the dominant variant in the community. If schools are to be the first setting to reopen, they will provide real-time data on how much this variant can push up cases in a non-vaccinated environment. Early reports have indicated that Johnson believes it might be enough to push R above 1. Thus, waiting a few weeks to see whether this comes to fruition in the data seems like a sensible decision.

It highlights how timing is an undervalued element of any lockdown easing debate. It is not solely about what you do, but when you do it. Waiting a few extra weeks could mean the difference between ~15 million people vaccinated and ~24 million, or ~24 million and ~33 million, for example. This is important because a larger number of people vaccinated can provide a greater force pushing down on the virus. The same can be said for the number of cases, hospitalisations and deaths. The lower they go, the better chance we have of managing this virus. It is something worth considering as talk turns to when and how England will emerge from its third lockdown.

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