Have We Passed the Peak?


By: Kyle Freese, PhD, MPH

Weekly Summary 4/17/2020

For this update, I will be focusing on COVID-19 in Arizona. There has been a lot of chatter that the U.S. has reached the peak of the pandemic (i.e. the top of the epidemic curve), but we need to examine the data by location since each locale will have different epidemic curves. As a preface, the data presented are from the New York Times’ efforts to collect all confirmed cases and deaths from COVID-19. It is possible that there are missing cases and deaths due to lack of testing or incomplete ascertainment of data; therefore, we should interpret these results with caution.

To recap the progression of the COVID-19 outbreak in Arizona as of April 15, we have confirmed 4,234 cases, 150 deaths, and have performed 47,398 tests. Approximately 25% of cases have been in individuals 65 years of age and older; however over 70% of the deaths (n=107) in Arizona have occurred among those in this age group. Among deaths, 61% have been male, 40% have been non-Hispanic White, 20% have been Native American, and 10% Hispanic or Latino.

Figure 1. Cumulative cases and deaths from COVID-19 in Arizona


                  While mapping the cumulative cases and deaths is informative for understanding the overall toll of an epidemic, we should dig deeper. From the information presented in Figure 1, we cannot determine if public health interventions aimed at social distancing are having an effect. From this graph alone, it does not appear that the outbreak is slowing to a significant degree. Though not covered in detail here, this is an inherent challenge of interpreting exponential curves associated with epidemics on the linear scale. Log scales are better at helping to understand if the rate of growth is slowing. Instead of presenting the cumulative cases and deaths for a region, we can construct a traditional epidemic curve, whereby the number of new cases of COVID-19 is plotted over time. Figure 2 below show these data alongside newly recorded deaths. It appears that Arizona might have reached the peak of this wave of the outbreak. If these data are correct and the trend continues, this figure indicates that aggressive public health measures have likely worked to “flatten the curve”. This is crucial to remember. Infectious diseases epidemics for which the pathogen is community spread do not exist alone (i.e. as a single epidemic peak). See the Spanish Flu of 1918 and the Swine Flu of 2009. In addition to the fact that Arizona is still experiencing new cases every day, there will likely be additional waves of outbreaks in the coming months. As such, these data should not support the notion to abandon the ongoing public health distancing measures. But, we should be encouraged that the sacrifices we have made are having an impact.

Figure 2. New, daily, confirmed cases and deaths from COVID-19 in Arizona

Finally, because Arizona is a diverse state, Figure 3 presents the epidemic curve (smoothed estimates shown) by counties with populations >200,000. These estimates are crude counts and not standardized by population. Still, there is a similar trend in Maricopa County as the state at large. One might argue that Pima, Yavapai, and Pinal Counties might also be experiencing a slowing the number of new, daily cases. The slight uptick in cases in Mohave County should be monitored closely.

Figure 3: New, daily, confirmed cases of COVID-19 by Arizona county with >200,000 inhabitants.

In conclusion, we should feel a sense of hope, but not fall into complacency. We are still at an important juncture in slowing the spread of SARS-CoV-2. If we remain vigilant and follow public health guidelines, it is possible that Arizona can avoid the worst burden of this pandemic seen by far too many communities around the world.

For additional references, resources, or questions, please email Dr. Kyle Freese at