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Influenza vs Covid-19



How COVID-19 Infection Differs from Seasonal Influenza in Transmission, Hospitalizations and Mortality


By Stuart M. Caplen, M.D.


When this pandemic started, many public figures thought it would be just a little more severe than seasonal influenza. After seeing scenes from hospitals in Wuhan, Italy, and New York City, it was clear that COVID-19 was much worse. This article will explore exactly how much worse COVID-19 is than seasonal influenza from the perspective of infection, mortality, and hospitalization rates.


Infection Rate


The reproduction number of a disease, or R0 (pronounced R naught), is the average number of new individuals that will be infected by each person with the disease. An R0 equal to 1 means one person is on average infecting one new person, and the level of disease in the population is stable. R0 values greater than 1 indicate that each person is on average infecting more than one person and the infection will continue to grow. This is what typically occurs in a pandemic. A value of R0 less than 1 indicates that the level of infection is declining in the population, as each person on average is infecting less than one other person. The term generation of infection is used to represent the total number of new people infected by the previous group of infected individuals based on the R0 value.


For seasonal influenza the R0 value is estimated to be 1.28 in the community setting, meaning each person infects an average of 1.28 other people. For the severe influenza pandemic of 1918, the R0 value was estimated to be 1.8. [1]


The actual R0 value in this COVID-19 pandemic is an estimation. Twelve studies from China were analyzed and the median R0 value was 2.79, which is much higher than influenza’s R0 value.[2] Another study from Wuhan published in a major journal found a R0 of 2.20.[3] Typical estimates for the COVID-19 R0 value are somewhere between 2.00 and 2.50.[4]


In the chart below, starting with 10 people infected with either seasonal influenza or COVID-19, the effect of R0 values on disease spread can be seen. This assumes no mitigation procedures such as social distancing are in place. Using an R0 of 1.28 for seasonal influenza, 386 people would be infected by the 10th generation of infection. Using an R0 of 2.79 for COVID-19, 159,652 people would be infected by the 10th generation of infection. At a COVID-19 R0 of 2.20 there would be 22,125 estimated total infections. This is a clear reminder of why social distancing and wearing masks is so important to decrease the transmission rate of COVID-19 as it is so much more infectious than seasonal influenza.







The doubling time, or the time it takes for the number of infected people to double, is another measure of the disease transmission rate. Doubling time does not really distinguish between the COVID-19 and influenza, as they overlap. The doubling time for influenza is variable depending on the virulence of the strain, and location of the outbreak. For the 2009-10 H1N1 influenza virus, the doubling time was estimated to be anywhere from two days to over 6 days. [5,6] The doubling time of COVID-19 infections in New York City on March 23rd was 3 days. As of May 14th, due to societal mitigation actions, the doubling time was 134 days.[7]


Mortality Rate


In this section, I will be referring to mortality rates for the general population. When mortality rates are broken down by age, the elderly have significantly higher mortality rates than younger people.

According to the CDC, yearly seasonal influenza infections from 2010 to 2019 ranged from 9.3 million to 45 million.[8] During the same time period, the estimated average yearly mortality rate for influenza was about 37,400 deaths per year (range 12,000 to 61,000).[8] For seasonal influenza the mortality rate is considered to be about 0.1% of infections. [9]


Using the diagnosed 1,645,094 COVID-19 infections in the United States on May 22nd, and 97,647 deaths, yields a mortality rate of 5.94% or about 60 times the influenza rate.[10] New York City random antibody testing suggests that the actual COVID-19 prevalence rate, including asymptomatic and undiagnosed individuals who had the infection, is approximately ten times the current known rate.[11,12] This would make the corrected COVID-19 mortality rate in the United States about 0.6%, or 6 times the influenza mortality rate.


The mortality rate in New York City as of May 1st, using the number of people with confirmed and probable infections was an estimated 1.08%.[10] The CDC recently reported there was an excess number of deaths in New York City over the usual rates. These deaths were not explained by reported COVID-19 deaths, and the CDC suggested that the actual COVID-19 related mortality rate may be higher than the reported rate.[13] One website calculated the mortality rate, including all the excess deaths found from that time, which resulted in an estimated mortality rate of 1.4%. This is probably an overestimation, as not all the excess deaths may have been COVID-19 related.[10]


We are only in the first few months of this COVID-19 pandemic and the number of deaths is already higher than any recent full influenza season, and over twice the average number of influenza deaths in a typical year.[8]


Part of the difference in mortality rates between the two viruses may be due to the fact that the influenza vaccine can lower the infection and death rates. Also, there are some antiviral drugs that can attenuate influenza symptoms. The inherent lethality of the viruses may play a role. Finally, COVID-19 is a novel virus that no one has ever been exposed to, as opposed to influenza where a percentage of the population may have some residual immunity.


We are in the beginning phases of this pandemic and doctors are just starting to learn which medications and therapeutic treatments will work to treat COVID-19 patients. As more research is done, the COVID-19 fatality rate may very well decrease.


Another aspect to pandemic planning is preventing hospitals from being overwhelmed. The number of infections differ by region. Some hospitals can have open bed space and others may be totally overwhelmed. A rapid increase in seriously ill patients is much more problematic for hospitals than a slower steady flow of patients. There is data that when intensive care unit patients stay in emergency departments for more than 6 to 12 hours, the mortality rate is higher than for those patients who go to the ICU more rapidly.[14,15] An influx of larger numbers of patients than normal can lead to increased emergency department holding times, which may have an adverse effect on the mortality rate. Nurses having more patients than normal to take care of, as can occur in a seriously overwhelmed hospital, has also been shown to increase mortality rates.[16]


Hospitalization Rates


The hospitalization rate for this past 2019-2020 influenza season was estimated by the CDC to be 69.4 hospitalizations per 100,000 population,[17] or 228,777 total hospitalizations. The hospitalization figure is based on an estimated 47,500,000 cases of influenza last season (range 39 million to 56 million) and a US population of 329,650,000.[18,19]


On May 23rd according to CDC estimates, COVID hospitalizations are averaging 67.9 per 100,000 population,[20] or 223,832 total hospitalizations so far. That number will continue to rise as more people are infected. That 67.9 hospitalizations per 100,000 population estimate is based on 1,645,094 known COVID-19 infections.[10]


If we assume COVID-19 infected 47,500,000 people, the same number of people as influenza infected last season, there would be an estimated 1,961 COVID-19 hospitalizations per 100,000 population. This would result in an estimated 6,462,875 hospitalizations or about 28.2 times the seasonal influenza hospitalization rate. If we assume there are actually ten times more occult COVID-19 infections than officially diagnosed ones, the number of estimated hospitalizations would drop to 646,287, or 2.82 times the seasonal influenza rate.


It is quite possible, given how much more infectious COVID is than seasonal influenza, that more than 47,500,000 people might become infected, resulting in an even higher number of hospitalizations.


Conclusions


  • As can be seen by their respective R0 values and number of new infections over time, COVID-19 is much more contagious than seasonal influenza.

  • Mortality rates from COVID-19 appear to be at a minimum 6 times higher than for seasonal flu in the United States and may have been higher in New York City during the first wave of infection when hospitals were overwhelmed.

  • COVID-19 hospitalization rates are likely to be at least 2.82 times higher than seasonal influenza hospitalization rates.

  • The high infectivity and resultant hospitalization rate of COVID-19 can lead to hospital systems being overwhelmed, possibly leading to increased mortality rates in the affected areas.

  • Finally, social distancing, mask-wearing, and other societal and personal interventions can reduce infectivity, hospitalization and mortality rates, until a vaccine or effective therapy is developed.

Author Note:

Many of the statistics given in this article are from estimates with wide ranges. Calculations of R0 from China have wide variations in value. There is a major difference in estimated mortality and hospitalization rates depending on whether you use the known numbers of COVID-19 infections, or a ten times higher estimate which includes undiagnosed and asymptomatic infections. I have tried to sort out the data and use best information to allow a comparison between COVID-19 and influenza, but like any statistical analysis, it may or may not accurately reflect real-world numbers.


There are three kinds of lies: lies, damned lies and statistics."

Mark Twain attribution to Benjamin Disraeli

References

[1]Biggerstaff, M., Cauchemez, S., Reed, C. et al. Estimates of the reproduction number for seasonal, pandemic, and zoonotic influenza: a systematic review of the literature. BMC Infect Dis 14, 480 (2014). Retrieved from: https://doi.org/10.1186/1471-2334-14-480


[2]Ying Liu, Albert A Gayle, Annelies Wilder-Smith, Joacim Rocklöv, The reproductive number of COVID-19 is higher compared to SARS coronavirus, Journal of Travel Medicine, Volume 27, Issue 2, March 2020, taaa021. Retrieved from: https://doi.org/10.1093/jtm/taaa021


[3]Li Q, et al., Early Transmission Dynamics in Wuhan, China, of Novel Coronavirus–Infected Pneumonia, N Engl J Med 2020; 382:1199-1207, March 26, 2020, On line publication January 29, 2020. Retrieved from https://www.nejm.org/doi/full/10.1056/NEJMoa2001316


[4]Fisher, M, R0, the Messy Metric That May Soon Shape Our Lives, Explained, New York Times, April 23, 2020. Retrieved from https://www.nytimes.com/2020/04/23/world/europe/coronavirus-R0-explainer.html


[5]2009 H1N1 Early Outbreak and Disease Characteristics, Centers for Disease Control and Prevention, October 27, 2009. Retrieved from: https://www.cdc.gov/h1n1flu/surveillanceqa.htm


[6]Storms AD, Van Kerkhove MD, Azziz-Baumgartner E, et al. Worldwide transmission and seasonal variation of pandemic influenza A(H1N1)2009 virus activity during the 2009-2010 pandemic. Influenza Other Respir Viruses. 2013;7(6):1328‐1335. Retrieved from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4634296/


[7] Tokyo & N.Y Doubling Time, Japan Macro Advisors, May 12, 2020. Retrieved from: https://www.japanmacroadvisors.com/page/category/economic-indicators/covid19-related/covid19-days-for-deaths-to-double/


[8]Past Seasons Estimated Influenza Disease Burden, Centers for Disease Control and Prevention, last reviewed: January 9, 2020. Retrieved from: https://www.cdc.gov/flu/about/burden/past-seasons.html


[9]Higgins-Dunn N, Lovelace Jr. B, Top US health official says the coronavirus is 10 times ‘more lethal’ than the seasonal flu, CNBC, Mar 11 2020. Retrieved from: https://www.cnbc.com/2020/03/11/top-federal-health-official-says-coronavirus-outbreak-is-going-to-get-worse-in-the-us.html


[10]Worldometer, Reported Cases and Deaths by Country, Territory, or Conveyance. Retrieved from: https://www.worldometers.info/coronavirus/


[11]Dupras M, Billmyer S, Where is coronavirus in NY? See map, charts of COVID-19 cases, deaths, hospitalizations (Thursday, April 23), syracuse.com, Retrieved from: https://www.syracuse.com/coronavirus/2020/04/where-is-coronavirus-in-ny-see-map-charts-of-covid-19-cases-deaths-hospitalizations-thursday-april-23.html


[12]LaVito A, et al. New York Finds Virus Marker in 13.9%, Suggesting Wide Spread, Bloomberg, April 23, 2020. Retrieved from: https://www.bloomberg.com/news/articles/2020-04-23/new-york-finds-virus-marker-in-13-9-suggesting-wide-spread


[13] Preliminary Estimate of Excess Mortality During the COVID-19 Outbreak — New York City, March 11–May 2, 2020, Centers for Disease Control and Prevention, first posted online May 11, 2020, officially dated May 15, 2020 / 69(19);603–605. Retrieved from: https://www.cdc.gov/mmwr/volumes/69/wr/mm6919e5.htm?s_cid=mm6919e5_X


[14]Chalfin D, et al. Impact of delayed transfer of critically ill patients from the emergency department to the intensive care unit, Crit Care Med. 2007 Jun;35(6):1477-83. Retrieved from: https://www.ncbi.nlm.nih.gov/pubmed/17440421


[15]Singer A, et al. The Association Between Length of Emergency Department Boarding and Mortality, Academic Emergency Medicine 2011; 18:1324–1329. Retrieved from: https://onlinelibrary.wiley.com/doi/full/10.1111/j.1553-2712.2011.01236.x


[16]Griffiths P, et al. Nurse staffing, nursing assistants and hospital mortality: retrospective longitudinal cohort study, BMJ Qual Saf 2019;28:609–617. Retrieved from: https://qualitysafety.bmj.com/content/qhc/28/8/609.full.pdf


[17] Weekly U.S. Influenza Surveillance Report, Centers for Disease Control and Prevention, last reviewed: May 15, 2020. Retrieved from: https://www.cdc.gov/flu/weekly/index.htm


[18]2019-2020 U.S. Flu Season: Preliminary Burden Estimates, Centers for Disease Control and Prevention, last reviewed: April 17, 2020. Retrieved from https://www.cdc.gov/flu/about/burden/preliminary-in-season-estimates.htm


[19]U.S. and World Population Clock, United States Census Bureau, May 15, 2020. Retrieved from: https://www.census.gov/popclock/


[20]Coronavirus Disease 2019 (COVID-19)-Cases, Data, & Surveillance, Centers for Disease Control and Prevention, Updated May 8, 2020. Retrieved from: https://www.cdc.gov/coronavirus/2019-ncov/covid-data/covidview/index.html

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ABOUT THE AUTHOR

Stuart M. Caplen, MD, FACEP, MSM

Dr. Caplen is a former emergency physician and emergency department medical director, now retired from clinical practice. His current interests include how quality is produced and maintained in health care, and he recently achieved greenbelt certification in lean/six sigma.

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