Because Covid-19 is new, no one has any immunity, they said. “The length of the pandemic will likely be 18 to 24 months, as herd immunity gradually develops in the human population,” they wrote.

Their predictions are different from models presented by groups such as the Institute for Health Metrics and Evaluation (IHME) at the University of Washington or the models produced by Imperial College London, whose report predicting millions of deaths in the US and UK helped galvanize responses by both governments.

The CIDRAP-led team used those reports, historical data on past pandemics, and published reports about the medical details of Covid-19 to put together their forecast.

“I have said for a long time that when you are trying to understand how infectious disease is going to unfold, you should rely on history as well as models,” Lipsitch told CNN. For instance, pandemic infections don’t tend to die down in the summer, like seasonal flu does., he said.