The new coronavirus is likely to keep spreading for at least another 18 months to two years—until 60% to 70% of the population has been infected, a team of longstanding pandemic experts predicted in a report released Thursday.

They recommended that the US prepare for a worst-case scenario that includes a second big wave of coronavirus infections in the fall and winter. Even in a best-case scenario, people will continue to die from the virus, they predicted.

“This thing’s not going to stop until it infects 60 to 70 percent of people,” Mike Osterholm, who directs the Center for Infectious Disease Research and Policy (CIDRAP) at the University of Minnesota, told CNN.

“The idea that this is going to be done soon defies microbiology.”

Osterholm has been writing about the risk of pandemics for 20 years and has advised several presidents. He wrote the report along with Harvard School of Public Health epidemiologist Marc Lipsitch, who is also a top expert on pandemics; Dr. Kristine Moore, a former Centers for Disease Control and Prevention epidemiologist who is now medical director for CIDRAP; and historian John Barry, who wrote the 2004 book “The Great Influenza” about the 1918 flu pandemic.

Waiting for herd immunity

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.

Covid-19 is most comparable to a pandemic strain of influenza, they said.

“Because of a longer incubation period, more asymptomatic spread, and a higher R0, COVID-19 appears to spread more easily than flu,” they wrote in the report. R0 is the average number of other people infected by each patient.

“A higher R0 means more people will need to get infected and become immune before the pandemic can end,” they add. “Based on the most recent flu pandemics, this outbreak will likely last 18 to 24 months.”

Preparing for the worst

They said government officials should stop telling people the pandemic could be ending and instead prepare citizens for a long haul.

Three scenarios are possible, they said:

Scenario 1: The first wave of Covid-19 in spring 2020 is followed by a series of repetitive smaller waves that occur through the summer and then consistently over a one- to two-year period, gradually diminishing sometime in 2021

Scenario 2: The first wave of Covid-19 is followed by a larger wave in the fall or winter and one or more smaller waves in 2021. “This pattern will require the reinstitution of mitigation measures in the fall in an attempt to drive down spread of infection and prevent healthcare systems from being overwhelmed,” they wrote. “This pattern is similar to what was seen with the 1918-19 pandemic.”

Scenario 3: A “slow burn” of ongoing transmission. “This third scenario likely would not require the reinstitution of mitigation measures, although cases and deaths will continue to occur.”

States and territories should plan for scenario 2, the worst-case scenario, they recommended.

“Government officials should develop concrete plans, including triggers for reinstituting mitigation measures, for dealing with disease peaks when they occur,” they advised.

Lipsitch and Osterholm both said they are surprised by the decisions many states are making to lift restrictions aimed at controlling the spread of the virus.

“I think it’s an experiment. It’s an experiment that likely will cost lives, especially in places that do it without careful controls to try to figure out when to try to slow things down again,” Lipsitch said.

Plus, he said, some states are choosing to lift restrictions when they have more new infections than they had when they decided to impose the restrictions.

“It is hard to even understand the rationale,” Lipsitch said.

A vaccine could help, the report said, but not quickly. “The course of the pandemic also could be influenced by a vaccine; however, a vaccine will likely not be available until at least sometime in 2021,” they wrote.

“And we don’t know what kinds of challenges could arise during vaccine development that could delay the timeline.”

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