COVID-19 projections are not about ‘getting it right’

Estimations about the progression of the pandemic are projections but never accurate predictions. The future remains unpredictable. However, projections provide us with valuable information about potential risks and enable us to make informed decisions.

There is much discussion about the value of modelling studies and other approaches to estimate how the COVID-19 pandemic is going to develop. Many people judge scientists on whether their predictions were ‘right’ or ‘wrong’ in the past. However, this view misses the point of such estimations, which are actually projections and no predictions. Their purpose is not to predict accurately what is going to happen, not least because this is impossible. Instead, their purpose is to provide a perspective on the potential risks and opportunities to inform our actions.

Many projections are self-defeating prophecies. If actions are taken, the situation will automatically change. If projections indicate that a COVID-19 wave will overwhelm the healthcare system and cause major devastation, measures will reduce the damage and prevent the projected scenario.

Realism is also needed to accept that even under ideal circumstances, it is impossible to predict exactly the future development of the pandemic. This is because many things happen by chance and are, therefore, unpredictable per se. It is only possible to estimate the probabilities of different scenarios. This is why projections come with huge error margins.

Even if probabilities of certain scenarios are estimated correctly, sometimes the unlikely happens. This is why we often see very different levels of virus spread in very similar settings. Some areas are simply be more or less lucky.

Another example of the limitations of predictions is the formation of viral escape variants that are not covered by vaccine-mediated immune protection. Since the formation of such escape variants depends on random mutations that may or may not occur, you can only be sure once one has formed.

However, we know that some scenarios are more likely to bring about such escape variants. Two factors promote the formation of escape variants. The first one is the presence of a pressure that drives the selection of escape variants, once they emerge. The second one is a high level of virus spread, because higher levels of virus replication mean more mutations and, hence, more chances that the required mutations occur.

In the UK, we currently see high levels of virus replication in a population with a high level of immunity. Thus, we are in a situation of high risk for the formation of escape variants. This is why many experts argue that vaccination programmes would be much safer if rolled-out in a low COVID-19 incidence setting. However, we can never be 100% sure. If we are lucky, we may not see escape variants despite taking a considerable risk. If we are unlucky, escape variants may emerge even in a very cautious scenario.

This is similar to trying to predict sports results. You can never know the result before a competition is over. We just had the Euros with many surprising results. Who would, for example, have predicted that Switzerland would beat France? Or at the Olympic Games that an Italian wins the hundred metres? It is not a lack of knowledge, if experts are surprised by such results. In the same way, virologists and epidemiologists can only estimate the likelihood of outcomes in the pandemic but not reliably predict the future.

Hence, projections about the progress of the pandemic are not and can never be exact predictions of what is going to happen. They can only highlight likely scenarios and risks and inform our response. Based on these estimations we can decide in a more informed way, which risks we are prepared to accept and/ or actions we want to take (or not).