by Aylin Woodward and Ruobing Su

This pandemic won’t end until a majority of the human population becomes immune to the virus — that is almost a certainty — but how it will spread and the way case numbers might rise and fall over time are less clear.

In a report published last week, researchers at the Center for Infectious Disease Research and Policy (CIDRAP) laid out three scenarios for what the next 18 to 24 months might look like.

The projections apply to the temperate Northern Hemisphere, which includes the US, though the study authors said “similar patterns could occur in the in global south.”

The worst-case scenario of the three involves a second, larger wave of infections this fall and winter. The researchers suggested this is the most likely outcome and said states should prepare for it.

“This thing’s not going to stop until it infects 60% to 70% of people,” Michael Osterholm, a report author and the director of CIDRAP, told CNN. “The idea that this is going to be done soon defies microbiology.”

People arrive at Hankou Railway Station in Wuhan, China, to take one of the first trains leaving the city on April 8, 2020. Noel Celis/AFP via Getty Image

Here are three charts that show what future waves of COVID-19 cases could look like.

Scenario 1: Repeating, smaller waves of infections 

In this scenario, the first COVID-19 wave, which is happening right now, would be followed by a series of smaller waves throughout the summer and beyond. Those waves would bring a lower number of infections than the US and other countries are currently experiencing. However, they would persist over the next 18 to 24 months (before gradually diminishing). 

The authors noted that the geographic areas in which these subsequent waves would grow depends on the measures various places put in place to encourage social distancing, and how those restrictions eventually get rolled back. 

Scenario 2: A second, larger wave of infections this fall

The worst — and most likely — of the three scenarios is one in which the first wave is followed by a larger wave in the fall or early winter. After that would come one or more smaller subsequent waves in 2021.

This mirrors what happened during the 1918 Spanish influenza pandemic and the 2009 H1N1 flu.

A second wave of infections with an even higher peak would require US states (and other countries in the Northern Hemisphere) to reinstitute mitigation measures like lockdowns, the authors wrote. 

“States, territories, and tribal health authorities should plan for the worst-case scenario,” they added.

Scenario 3: A ‘slow burn’ of ongoing transmission

The final scenario suggests that this first wave of coronavirus infections is the biggest we’d see. In the coming months, the COVID-19 pandemic would shift into a “slow burn” of ongoing transmission and new cases.

“While this third pattern was not seen with past influenza pandemics, it remains a possibility for COVID-19,” the experts reported.

This possibility would mean that states likely wouldn’t need to lock down again, although we’d still see many more cases and deaths.

‘There is no crystal ball’

Osterholm and his colleagues’ conclusions are based on an examination of multiple models that predict future coronavirus impacts, as well as research about how COVID-19 spreads and data from past pandemics.

The coronavirus outbreak shares important similarities with an influenza pandemic, like the 1918 Spanish flu (which infected 500 million people worldwide). Both viruses spread via droplets people emit when coughing or sneezing and can be transmitted even when infected people show no symptoms. That makes this type of flu a solid model for comparison, but experts still aren’t sure what to expect from the coronavirus.

That’s because this new virus spreads more easily than the flu does. An average person with the coronavirus infects between 2 and 2.5 new people, a metric known as the R0 value. Seasonal influenza’s Ro value, by contrast, is about 1.3.

“There is no crystal ball to tell us what the future holds and what the ‘end game’ for controlling this pandemic will be,” the report authors wrote.

Of course, the report’s three scenarios would change if a vaccine is developed more quickly than expected. But the authors noted that a vaccine probably won’t become available for a long time — the earliest it’s expected is in 2021. 

“We don’t know what kinds of challenges could arise during vaccine development that could delay the timeline,” they added.