May 1 (UPI) -- Leaders across the United States should plan for a worst-case scenario, second wave of the COVID-19 pandemic, including no vaccines or herd immunity, experts at the University of Minnesota said in an analysis published Friday.
If the pandemic follows the pattern of the 1918-1919 Spanish flu, it likely to last up to two years and return as a more serious outbreak this fall and winter, researchers at the university's Center for Infectious Disease Research and Policy said.
Humans lack natural immunity to the virus, and it is highly contagious -- and up to 70 percent of the U.S. population may need to develop immunity before spread slows naturally. That could take up to 18 to 24 months in the absence of a vaccine, the authors indicated.
Governors in several states expressed the same concern this week, saying that plans to reopen businesses closed due to the COVID-19 pandemic need to factor in a possible second wave of infections.
The findings were posted on the university's website Friday, hours after Delaware Gov. John Carney, following officials in several other states, declared that a robust testing system needs to be in place to prevent a second wave of cases.
"That's what we need to have in order to start reopening in a phased way," he noted.
The report outlines three possible scenarios for the pandemic for the rest of 2020 and into 2021. The first and worst-case scenario is a major resurgence before the end of the year, while the second suggests that the current outbreak could be followed by a series of smaller ones well into 2021.
The worst-case scenario would follow the same pattern as the Spanish flu outbreak more than a century ago, when a small wave hit in early 1918, followed by a huge spike in infections later that year and a third round in early 1919.
A third possibility, unprecedented across the history of global pandemics, would feature a "slow burn" of disease spread with no clear pattern.
Regardless of which scenario comes to pass, the report urges governments to prepare for at least another 18 to 24 months of significant COVID-19 activity in several still as-yet-unknown "hot spots."
Even as the pandemic draws to a close, the researchers predict the virus will continue to circulate in the population, perhaps morphing into a seasonal virus much like other pandemic influenza viruses.
Even the development of widespread human immunity -- or "herd" immunity -- to the virus could be "complicated by the fact that we don't yet know the duration of immunity" to the new coronavirus, the authors noted.
Blood testing has revealed that humans can develop antibodies to the virus, but it is not known whether these antibodies are sufficient to protect against future infection -- and for how long.
Based on other seasonal coronaviruses, however, the authors suggest that even if immunity is not long-lasting, it may still protect against more severe disease and reduce "contagiousness." However, whether is the case for SARS-CoV-2 remains unknown.
"There is no crystal ball to tell us what the future holds and what the 'end game' for controlling this pandemic will be," the authors wrote.