It's not The Peak, it's This Peak

Many people are misinterpreting the COVID-19 forecast model from the University of Washington’s Institute for Health Metrics and Evaluation.

One key misunderstanding

There is a widespread misunderstanding at the heart of almost all of the misinformation related to the IHME model: Many people think that the IHME model is predicting 'when the epidemic will peak' and then subside before it's all over. But that’s not what the model is about.

The forecast charts from the model seem easy to understand: They show one big hump. So that means that the models predicts that the infection will be one wave and then over, right? No, it just means that forecasting future waves of COVID-19 outbreaks is beyond the scope of the model.

THIS peak

The IHME model is not projecting that the epidemic will peak and subside and then it will all be over. It’s projecting when the first wave will peak, assuming that social distancing stays in place.[1] It’s a planning tool for health planners and governments to project how long before social distancing could be relaxed after one wave of the epidemic. It’s not a crystal ball that’s predicting what will happen with the overall pandemic.

Projecting that first wave is incredibly difficult because exponential growth curves can lead to wildly different outcomes from tiny tweaks in the input parameters. Trying to model future outbreaks would be even more difficult and unreliable at this point, and so the IHME researchers are not even trying to do that:

It is critical to note that we restrict our projections to the first wave of the pandemic under a scenario of continued implementation of social distancing mandates and do not yet incorporate the possibility of a resurgence or subsequent waves.

The problem is that the widely-cited dashboard for the model’s forecasts is not clear about this crucial background information about the model. Giving people an easily-accessible dashboard that they can easily misunderstand could potentially backfire. In the worst case, people will misinterpret the IHME model's reports as saying that the threat of COVID-19 is over, and that it’s okay to stop the social distancing.

Conclusions

The IHME model is not forecasting when the COVID-19 epidemic will be over. It isn’t forecasting that there will only be one peak.

It’s only trying to estimate the timing of the current wave of the epidemic. It assumes that we have taken measures to slow the spread of the virus. And it estimates how long it might take to reduce the number of new infections to a level where social distancing could end.[2] Temporarily. Future outbreaks would require a repeat of the whole mitigation cycle.

References

1. 1. Forecasting the impact of the first wave of the COVID-19 pandemic on hospital demand and deaths for the USA and European Economic Area countries. IHME COVID-19 health service utilization forecasting team; 21-April-2020 link (Accessed 2020-04-27 16:00)
2. 2. IHME COVID-19 model FAQs. Institute for Health Metrics and Evaluation; link