The University of Tennessee, Knoxville.
Making Comparisons: Swine Flu vs. COVID-19

A Policy Brief by the Howard H. Baker Jr. Center for Public Policy

in Partnership with the Coronavirus-19 Outbreak Response Experts (CORE-19)

April 24, 2020
Tennessee State Capitol and Flag
Using publicly available data from emerging research on COVID-19, this brief was written and reviewed by the Coronavirus-19 Outbreak Response Experts (CORE-19) at the University of Tennessee, Knoxville.  
In the absence of a vaccination or cure, mitigation policy and behaviors rely on the extent to which the public is informed about the outbreak. Comparing past outbreaks, such as the 2009 H1N1 Swine Flu pandemic, is a common framework for communicating to the public the nature or severity of a novel disease. This brief provides insight into what was learned during the 2009 Swine Flu pandemic and the differences between H1N1 and COVID-19.

A Brief History

Before COVID-19 emerged in December 2019, there was H1N1 (commonly known as Swine Flu). In April 2009, the first cases of a new flu virus were detected in Mexico. This was the second H1N1 flu pandemic to emerge and spread globally, the first one being the flu pandemic of 1918 that resulted in perhaps 50-100 million deaths worldwide (between 3-5% of the world’s population at that time). The new strain of H1N1 appears to have originated in Veracruz, Mexico, at a pig farm located in the southeastern part of the state and, shortly after, two children in southern California tested positive for the H1N1 virus
In the United States, Patient A was a 10-year old boy living in San Diego, California, and Patient B was a 9-year old girl from Imperial County, which borders San Diego to the east. On July 1, 2009, the World Health Organization declared the outbreak a pandemic as the number of H1N1 cases surged from two dozen in early April 2009 to over 10,000 within 30 days and nearly half a million within 6 months. Of course, confirmed case counts likely understate the extent of the infection significantly. Despite fewer than 200,000 confirmed cases of H1N1 in all of North America as of October 2009, the CDC estimates between 14-34 million people in the U.S. alone were infected with the novel H1N1 virus. 
Likewise, COVID-19 emerged in Wuhan, China in December 2019 and spread to the United States by January 2020. The first US case was diagnosed on January 20, 2020, after a 35-year old man in Snohomish County, Washington, visited an urgent care clinic with a persistent cough and fever. The man had recently returned from Wuhan, China where he was visiting family. The virus subsequently spread quickly across the United States. Globally, the virus proliferated rapidly, reaching 100,000 cases worldwide by March 7, 2020. On that same day, the United States was reporting only 338 confirmed cases of novel virus but, within three weeks, the U.S. would also cross the 100,000 cases threshold.
The novel H1N1 virus went from 0 to over 1,000 confirmed cases in about 10 days. It took COVID-19 approximately 25 days to reach the same mark. However, by Day 34, COVID-19 surpassed total H1N1 global cases and while H1N1 daily increases were beginning to slow, COVID-19 cases continued to surge. On April 11 (Day 103), total global COVID-19 cases surpassed 1.5 million (as displayed in Figure 1). The H1N1 curve began to flatten around day 20, but it has taken over twice as long to see a flattening of the curve with COVID-19. Of course, with both H1N1 and COVID-19, there is likely extensive undercounting of cases, which impacts dynamic trends in overall cases. Undercounting is less problematic with regard to mortality and hospitalization trends, as these measures are independent of testing capacity.   
Figure 1: Total Global Cases of H1N1 and COVID-19 over first 75 days of Outbreak

Figure 1: Total Global Cases of H1N1 and COVID-19 over first 75 days of Outbreak
Although the number of deaths increased more quickly for the H1N1 virus, its mortality rate remained significantly lower than initial estimates of the mortality rate for COVID-19 (as displayed in Figure 3). The World Health Organization (WHO) estimated the mortality rate of H1N1 from April 2009 to April 2010 to be around 0.02% while the current rate for COVID-19 appears closer to 6% globally (300 times more deadly) and over 3% in the United States.
Initial mortality rate estimates show COVID-19 is significantly more lethal than H1N1.
Additionally, the two diseases have also affected somewhat different populations. Attack rates, morbidity, and mortality for H1N1 were much higher among the young and middle-aged than the elderly. As displayed in Figure 2, the CDC estimates that, for H1N1, “approximately 90% of estimated hospitalizations and 87% of estimated deaths… occurred in people younger than 65 years old.”
Figure 2: Hospitalizations and Deaths by Age for the Seasonal Flu and H1N1
Figure 2: Hospitalizations and Deaths by Age for the Seasonal Flu and H1N1
Based on preliminary data, the opposite appears to be the case for novel COVID-19.
In the United States, for example, there were nearly six times more hospitalizations from H1N1 among those between the ages of 18 and 65 than those over the age of 65, which differs from how typical flu virus affects the population in a normal year. Some research suggests an existing level of immunity to the H1N1 flu virus in older populations may account for the disparities in morbidity across age groups.    
Figure 3: Deaths from H1N1 and COVID-19 Viruses
Figure 3: Deaths from H1N1 and COVID-19 Viruses
While the trajectory of novel COVID-19 cases and its economic impact remains uncertain, we can look back at H1N1 for guidance.
On Day 103 of the outbreak (corresponding to April 14 for COVID-19), the total number of confirmed H1N1 infections reached 162,000 with nearly 1,200 deaths. Over the next 3 months, H1N1 infections increased by nearly 100% and deaths by nearly 400% (as shown in Figure 4), indicating the novel COVID-19 is more virulent and more contagious than the H1N1 virus.
The rate of transmission for H1N1 (also known as the Basic Reproductive Number or R0) was estimated to be around 1.5—which means that, on average, one and a half people will catch the virus from 1 infected person. But the transmission rate, or R0, is much higher for COVID-19. The Imperial College COVID-19 Response Team estimates the R0 for COVID-19 is between 3.01 and 4.66, and a recent estimate by the CDC found a median R0 value of 5.7 for COVID-19. These differences between the two diseases can have huge downstream effects, vastly increasing the total number of infected people at any given time as well as cumulatively. 
Figure 4: Trajectory of H1N1 after the 106th day (April 14th for COVID-19)
Figure 4: Trajectory of H1N1 after the 106th day (April 14th for COVID-19)
Another factor to consider is the potential for a re-emergence of infections later in the year. There were a second wave of infections during both the 1918 (Spanish Flu) and 2009 (Swine Flu) H1N1 outbreaks, meaning a second wave of COVID-19 infections is plausible, if not likely. Notably, the second wave of infections during the 1918 H1N1 outbreak accounted for “most of the deaths attributed to the pandemic,” proving to be even more fatal than the initial outbreak. While infections also re-emerged during the 2009 H1N1 outbreak, much of the impact was mitigated by a vaccination that had become available in the months prior. In other words, the trajectory and impact of the 2009 H1N1 outbreak was altered significantly by the rapid development of a vaccine but, unlike in 2009, most experts agree a COVID-19 vaccine will not be widely available for at least 12 to 18 months.
Broadly speaking, when compared to the 2009 H1N1 virus, COVID-19 is both more deadly and more contagious. Lessons taken from the previous pandemics can be fruitful, but it is important to keep in mind that the scope and nature of H1N1 and COVID-19 are different. Despite some positive signs indicating that new COVID-19 infections might be decreasing, it is clear there is a long road ahead.
Send additional questions to the CORE-19 research team. 
core19@utk.edu | 865-321-1299

Coronavirus-19 Outbreak Response Experts (CORE-19) 

Dr. Brandon Prins

Dr. Brandon Prins, PhD

Brandon Prins is Professor of Political Science at the University of Tennessee-Knoxville and a Global Security Fellow with the Howard Baker Center for Public Policy. Currently, he serves as co-lead editor of International Studies Quarterly. Dr. Prins’ research and teaching interests center on the conditions associated with political violence, including interstate and civil war, terrorism, and maritime piracy.  He recently completed a book-length manuscript forthcoming at Oxford University Press on the political geography of modern maritime piracy.
Hancen Sale

Hancen A. Sale 

Sale is an undergraduate research assistant at the Center. He is a senior majoring in Economics with minors in Public Policy Analytics and Political Science. His experience includes work on an NSF-funded project regarding civil wars and rebel groups, as well as supporting The White House's American Workforce Policy Advisory Board. 
Dr. Kathleen Brown

Dr. Kathleen C. Brown, PhD, MPH

Brown is an Associate Professor of Practice in the Department of Public Health and the Program Director for the Master's in Public Health (MPH) degree. Her research focuses on the health and well-being of individuals and communities. She has experience in local public health in epidemiology, risk reduction and health promotion.
Dr. Katie Cahill

Dr. Katie A. Cahill, PhD

Cahill is the Associate Director of the Howard H. Baker Jr. Center for Public Policy. She also is the Director of the Center's Leadership & Governance program and holds a courtesy faculty position in the Department of Political Science. Her area of expertise is public health policy. She leads the Healthy Appalachia project. 
Dr. Kristina Kintziger

Dr. Kristina W. Kintziger, PhD, MPH

Kintziger is an Assistant Professor in the Department of Public Health and the co-Director of the Doctoral Program. She has worked in academia and public health practice. Prior to coming to Tennessee, she served as an epidemiologist and biostatistician at the Florida Department of Health. She is an environmental and infectious disease epidemiologist.
Dr. Matthew Murray

Dr. Matthew N. Murray, PhD

Murray is the Director of the Howard H. Baker Jr. Center for Public Policy. He also is the Associate Director of the Boyd Center for Business and Economic Research and is a professor in the Department of Economics in the Haslam College of Business. He has led the team producing Tennessee's annual economic report to the governor since 1995. 
Dr. Agricola Odoi

Dr. Agricola Odoi, BVM, MSc, PhD

Odoi is a professor of epidemiology at the University of Tennessee College of Veterinary Medicine. He teaches quantitative and geographical epidemiology and his research interests are in population health and impact of place on health and access to health services. He was a public health epidemiologist before joining academia.
Dr. Marcy Souza

Dr. Marcy J. Souza, DVM, MPH

Souza is an associate professor and Director of Veterinary Public Health in the UT College of Veterinary Medicine.  Her teaching and research focuses on zoonotic diseases and food safety issues. 
Disclaimer: the information in this policy brief was produced by researchers, not medical or public health professionals, and is based on their best assessment of the existing knowledge and data available on the topic. It does not constitute medical advice and is subject to change as additional information becomes available. The information contained in this brief is for informational purposes only. No material in this brief is intended to be a substitute for professional medical advice, diagnosis or treatment, and the University of Tennessee makes no warranties, expressed or implied, regarding errors or omissions and assumes no legal liability or responsibility whatsoever for loss or damage resulting from the use of information provided.
Howard H. Baker Jr Center for Public Policy
1640 Cumberland Avenue
Knoxville, TN 37996
Phone: 865-974-0931
Email: bakercenter@utk.edu
Online: bakercenter.utk.edu
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