Why we are where we are with our high Covid-19 rates…

Dr Claire Donnelly is a consultant physician with expertise in infectious diseases and an MSc in epidemiology

I want to try to explain from an epidemiological perspective how NI is approaching a cliff-face with Covid 19. Professor Devi Sridhar has used the analogy of boiling milk to explain this very well. Initially, it starts to boil slowly and it seems like nothing much is happening. As you get closer to the boiling point, if you take your eye off the cooker it can boil over in seconds. That is where we are now in NI with Covid. We have reduced restrictions but case numbers are still high. Increases in cases could and likely will happen quickly. On 17^th December, the NI Covid dashboard reported 656 new cases, with 3531 cases diagnosed in the previous 7 days.

The exponential rise in case numbers can be explained in different ways. Many people are familiar with Ro. However, I find doubling time a more useful figure to understand. This is the number of days it takes for case numbers to double. The fast exponential rise is illustrated by the NI Covid figures in September and early October before the introduction of a series of restrictions. On 30^th September, the NI Covid dashboard showed 424 new cases, and on 7^th October that number increased to 828 new cases in a day. On the 7^th October, there were 4417 cases diagnosed in the previous 7 days, compared to 2049 in the 7 days preceding that. The number of cases more than doubled over a 7 day period. If that had continued without restrictions or a change in people’s behaviour, the number would have continued to increase exponentially. To put this into figures, without a change in peoples contacts, 4417 could have increased to over 8000 and then to 16000 etc. It is likely that many people would have stopped going out and the exponential growth would have slowed down, but those figures are what would happen without any change.

No healthcare system in the world can cope with those numbers, and even countries like Germany with over 5 times the intensive care capacity as NI, has had to introduce restrictions leading up to Christmas.

Mathematical models have been used to monitor, predict and direct public health policies throughout the pandemic. It is important that people understand the limitations of models. They are trying to predict what will happen based on different factors, which include people’s behaviour and the number of interactions they have with other people. Assumptions will be made about how people will interact in the run-up to Christmas. However, there is no data that will accurately tell you how people in NI will behave in the lead up to Christmas during a pandemic. That data will be available after Christmas. Collateral information such as mobility data will help, but to rely on assumptions like this overestimates the accuracy of models as prediction tools. If our contacts are even similar to late September, then our numbers will surge from a high number, and we could easily exceed the record numbers we saw in October over a short period of time. Literally, the milk could boil over before you get to turn off the cooker.

Sometimes it seems as if few people in NI are complying with Covid guidance because only the non-compliant people are highlighted in the media. Ro was approximately 3 at the start of the pandemic. Over the last several months, it has been often around 1 and always considerably under 2. This shows many people are complying with guidance.

My view is waiting to introduce restrictions after Christmas is a very risky strategy. It is very possible that the numbers will surge before that to a high level that could overwhelm healthcare in NI. We will not know for sure until January, but that is the problem with trying to predict the future with a mathematical model based on human behaviour.

The higher the numbers rise over Christmas then the longer and harder restrictions will be in the New Year to bring the figures down. To people who say this is just the flu. It is not. I have worked in infectious diseases for over 20 years and have been a doctor for over 25 years, and Covid is not remotely like the flu.

My interpretation of the guidelines is that Christmas is a time to support our families and friends. For many people, the most effective way of doing that is not to meet them in person and meet them virtually. However, for many people who are struggling with the restrictions and loneliness, the pandemic has brought, the guidelines allow them to meet a small number of contacts.

My Christmas dinner will be with my household family members only. I will say an outdoor, socially distanced Happy Christmas to my parents, and will not meet them inside until we have all received the vaccine.

Photo by Queven is licensed under CC BY-NC-SA