June 8, 2020: EPI's COVID-19 forecast model
New EPI mathematical modeling results for projecting COVID-19 in Florida.
Ira Longini, Ph.D., Department of Biostatistics, College of Public Health and Health Professions, College of Medicine, Emerging Pathogens Institute, University of Florida, Gainesville, USA
Where we are now
While daily reported cases have declined on average nationally between April and May, Florida’s daily reported cases continue to rise. The top chart shows the total case counts reported daily for the nation. The bottom chart graphs new daily cases in Florida. Rt is used by researchers to describe how fast a virus is spreading between people. It signals how many people will become infected, on average, by someone who is infectious.
Current trend of COVID-19 deaths in Florida
Current reported deaths attributed to COVID-19 have declined since late April.
Current trend of COVID-19 infections in Florida
Despite the downward trend in reported deaths, our daily reported incidence of infection has continued to steadily rise.
Estimates of close contacts
This graph shows an estimate of Floridian’s typical daily close contacts. These contacts began decreasing shortly after a state of emergency was declared on March 9, 2020 and continued to decrease to about half of normal levels just after shelter-in-place orders were issued on April 4, 2020. Daily close contacts have been rising slowly but steadily since mid-April, but are not yet back to pre-pandemic typical levels.
Mobility in Florida
This graph shows an estimate of distance traveled by typical Floridians. Similar to the graph above, mobility began decreasing shortly after a state of emergency was declared on March 9, 2020. Mobility continued to decrease to below half of normal levels by the time shelter-in-place orders were issued on April 4, 2020. Daily mobility has risen slowly since mid-April, but it is not yet back to pre-pandemic typical levels.
Rate of spread
This chart shows an estimate for Rt. (This is a measure of how fast a virus is spreading between people. It describes how many people will become infected, on average, by someone who is infectious.) At the beginning of the pandemic, Rt in Florida was close to three but is now hovering slightly above one. This means that in March every infected person infected, on average, three more people; today, each infected person only spreads the virus to slightly more than one person. To halt the virus from spreading, Rt needs to fall below one.
Where is Rt less than one? Not even close to one?
Green states have a high probability of Rt being less than one, which is a good thing. (The darker the green, the higher the probability.) Pink states have a low probability of Rt being less than one, which means their viral spread could be better controlled. (The darker the pink, the less likely it is that Rt falls below one; it's more likely equal to, or greater than, one in these states.)
What’s the forecast for fall 2020?
The best outcome to control Florida’s epidemic of COVID-19 is to do so much testing that at least 50% of symptomatic cases are caught. After these are identified, at least 20% of the contacts of each symptomatic person would need to be quarantined to reduce potential transmission.
What happened with swine flu?
Swine flu, known to the medical world as Influenza A subtype H1N1, swept through the U.S. between late summer 2009 and spring 2010. This is what the flu peaks looked like, from mid-2006 to early 2010. Note the much higher than normal peak in flu-like illnesses reported from mid-2009 to early 2010.
But what about successive waves of outbreak?
Nearly 100 years before the swine flu, another much deadlier Influenza A subtype H1N1 swept through the U.S. The 1918 Spanish flu killed millions of people across the globe in several waves of infection. The second and thirds waves were far deadlier than the first.
What about a vaccine?
Safe and effective vaccines take time. We will hopefully have at least one viable vaccine candidate by this fall.
What happens till then?
We need to rely on strict public health measures until a safe and effective vaccine is available for mass distribution.
Editor's note: an earlier version of this webpage mistakenly stated, "The best outcome to control Florida’s epidemic of COVID-19 is to do so much testing that at least 50% of asymptomatic cases are caught. After these are identified, at least 20% of the contacts of each asymptomatic person would need to be quarantined to reduce potential transmission." This was corrected on June 25th to read "50% of symptomatic cases" and "20% of the contacts of each symptomatic person..."