Coronavirus may last 2 years, study warns. And its second wave could be worse.

(USA TODAY) If COVID-19 follows a pattern set by the 1918 Spanish flu, the pandemic is likely to last up to two years and return...

(USA TODAY) If COVID-19 follows a pattern set by the 1918 Spanish flu, the pandemic is likely to last up to two years and return with a vengeance this fall and winter – a second wave worse than the first, according to a study issued from the University of Minnesota.

"States, territories and tribal health authorities should plan for the worst-case scenario," warns the report out of the university's Center for Infectious Disease Research and Policy, "including no vaccine availability or herd immunity."

"Risk communication messaging from government officials should incorporate the concept that this pandemic will not be over soon and that people need to be prepared for possible periodic resurgences of disease," the authors suggest. 

The study team, headed by Dr. Kristine A. Moore, medical director at the University of Minnesota center, included pandemic experts from Harvard and Tulane universities. 

Antibody tests were supposed to help guide reopening plans. They've brought more confusion than clarity amid coronavirus.

In a preface to the report, researchers said they are striving to produce critical and timely information "with straight talk and clarity."

The worst-case scenario – with a major resurgence by year's end – is one of three laid out in the paper.

A second possibility suggests the outbreak this year could be followed by a series of smaller waves into 2021.

And a third scenario, not seen in previous pandemics, would feature a "slow burn" of viral transmission with no clear pattern.

"The virus caught the global community off guard, and its future course is still highly unpredictable," says the report. "There is no crystal ball to tell us what the future holds and what the 'end game' for controlling this pandemic will be."

However, it stresses, a COVID-19 vaccine is not likely to be available until 2021. And, because up to a quarter of those infected may have no symptoms – and others spread the disease for days before feeling ill – historic influenza pandemics provide the best model.

Second wave: When will it hit, and what will it look like?

Because humans don't have natural immunity and the virus is so easily transmitted, up to 70% of the population may have to develop immunity before COVID-19's spread diminishes naturally. That means the pandemic length "will likely be 18 to 24 months," and the virus will remain endemic afterward.

The worst-case scenario – a more lethal resurgence this fall and winter – is based on the Spanish flu outbreak a century ago, when a small wave hit in early 1918, followed by a huge spike that fall and a third major wave in early 1919.

Studies suggest social distancing measures had worked against the 1918 epidemic until they were hastily lifted by some cities, like Denver, in early celebrations. Instead of continuing to “flatten the curve,” these cities experienced a second spike in cases.

“A lot of the confusion, in general, is premised on the misunderstanding that if you control the epidemic once, then you’re done,” Harvard epidemiologist Marc Lipsitch told the USA TODAY Editorial Board last month. “There’s no reason to think that.”

The researchers urged government agencies and officials to gird for resurgences and develop triggers for re-instituting mitigation measures so health care systems won't once again be overwhelmed.

Contributing: Adrianna Rodriguez, USA TODAY.