Nearly half of US states haven't contained their coronavirus outbreaks, a new study finds

• Twenty-four US states haven't contained their coronavirus outbreaks, according to a new study from Imperial College London.
• The researchers modeled the basic reproduction number — a number that helps determine how quickly the virus is spreading — for each state.
• They concluded that most states in the Midwest and the South are still seeing relatively high rates of transmission.

All 50 states have begun easing their lockdown restrictions, but a new report suggests that 24 states still haven't contained their outbreaks.

Researchers from Imperial College London modeled the basic reproduction number (R0) — the average number of people a single patient is expected to infect — in every US state. The figure is a moving target: As more people resume normal activities, the rate of transmission among a population could rise.

"The R0 depends on how fast people are interacting with each other," Dr. Elizabeth Halloran, a biostatistician at Fred Hutchinson Cancer Research Center and the University of Washington, previously told Business Insider. "You have to interact with a certain number of people to be able to infect them."

So the Imperial College researchers used cell phone data to estimate how often people were interacting at pharmacies, grocery stores, parks, restaurants, offices, shopping centers, transit stations, and more as of May 17. From there, they determined whether states have successfully brought down their R0.

When an outbreak has run its course, the R0 will drop below 1, meaning every person will infect fewer than one other person on average. The researchers (whose work has not been peer reviewed) determined that this was not the case in 24 states, though most states have decreased their R0 since the beginning of their outbreaks.

"While we are confident that some states have controlled transmission, we are similarly confident that many states have not," the researchers wrote. "Most states in the Midwest and the South have rates of transmission that suggest the epidemic is not yet under control."

States with lots of deaths had lower reproduction numbers

Of the ten states with the highest basic reproduction numbers, five were located the Great Lakes region: Illinois, Ohio, Minnesota, Indiana, and Wisconsin. Texas had the highest R0 of all 50 states, according to the study. Arizona, Colorado, Iowa, and Alabama rounded out the top ten.

That means the virus is spreading more rapidly in these states compared to the rest of the country.

The researchers found lower reproduction numbers in states that already witnessed lots of coronavirus deaths. That could be due to the fact that these states have had to implement strict lockdowns to contain their outbreaks.

In New York, for instance, around 17% of the population has been infected so far. But the researchers estimated that New York's R0 is currently below 1 — a dramatic decline from the start of the outbreak, when one person in New York infected five others, on average. The researchers concluded that New York's stay-at-home order was successful at containing its outbreak. But they also theorized that the state may have reduced transmission by ramping up testing, mask-wearing, and hand-washing.

In two states that reported early coronavirus outbreaks — California and Florida — the R0 is still above 1, even though less than 2% of the population has been infected in these states. In Florida, the researchers noted, an increase in mobility "appears to have driven transmission up recently." Still, they said there was "considerable uncertainty" surrounding these estimates.

Overall, the researchers estimated that 4% of the US population was infected as of May 17 and 1.3 million individuals remain infectious across the country. Over the next two months, they added, the US could see more than double the amount of deaths that have already been recorded nationwide.

"These estimates suggest that caution must be taken in loosening current restrictions if effective additional measures are not put in place," the researchers wrote. They added that interventions like mass testing, contact tracing, or increased mask-wearing could substantially reduce the rate of transmission.