Recommendations and Reports TABLE 9. Prepandemic influenza planning scenarios to guide implementation of nonpharmaceutical interventions, by severity of pandemic and the Pandemic Severity Assessment Framework quadrant Possible no. of hospitalizations and † Implications of clinical deaths if unmitigated, by age group Severity of pandemic severity and transmissibility Age groups No. of No. of and PSAF quadrant in this scenario* (yrs) hospitalizations deaths Historical experience Low to moderate • Clinical severity and All ages 340,000 17,000 2009 pandemic severity (mild to transmissibility similar to the 0–18 50,000 1,000 • First detected in North America, the 2009 H1N1 moderate pandemic) range seen during annual 18–64 135,000 6,000 pandemic quickly spread to all continents. In the PSAF quadrant: A influenza seasons. ≥65 155,000 10,000 United States, persons at high risk for severe • Estimated overall attack and complications included pregnant women and those case-fatality rates: 18% and with neuromuscular disease, lung disease, morbid 0.03%, respectively. Rates of obesity, and other chronic conditions. severe outcomes are greater • An estimated 43–89 million people in the United among younger persons than States became ill with H1N1 from April 2009 through § during influenza seasons. April 2010, and approximately 12,000 people died. A total of 87% of deaths were among persons aged ≤65 yrs, with a mean age of 43 yrs.¶ During typical influenza seasons, 80%–90% of deaths are among persons aged ≥65 yrs, and the mean age of influenza- related deaths is approximately 76 yrs.** Moderate to high • Clinical severity similar to the range All ages 550,000 35,000 1968 pandemic severity (moderate to seen during annual influenza 0–18 80,000 2,500 • First detected in Hong Kong in July 1968, a new severe pandemic) seasons. Transmissibility greater 18–64 220,000 12,000 influenza virus (H3N2) spread worldwide. PSAF quadrant: B than during influenza seasons. ≥65 250,000 20,000 • The first cases in the United States were detected in • Estimated overall attack and September 1968. The 1968 influenza pandemic case–fatality rates: 22% and 0.05%, resulted in approximately 30,000 deaths in the respectively. Rates of severe United States, with approximately half among those outcomes are greater than during aged ≥65 yrs.††,§§ influenza seasons, especially among younger persons. High severity (severe • Clinical severity similar to the range All ages 1,100,000 86,000 1957 pandemic pandemic) seen during annual influenza 0–18 150,000 6,000 • A new influenza virus, H2N2 (the Asian strain), PSAF quadrant: B seasons. Transmissibility greater 18–64 450,000 30,000 emerged in China in February 1957 and spread to than during influenza seasons. ≥65 500,000 50,000 approximately 20 countries, including the United • Estimated overall attack and States, by June 1957. case-fatality rates: 28% and 0.1%, • An estimated 25% of the U.S. population became ill respectively. Rates of severe with the new pandemic virus strain. U.S. infection outcomes are greater than during rates were highest among school-aged children and influenza seasons. adults aged ≤40 yrs, with most (64%) of the approximately 70,000 deaths occurring among older adults.††,§§,¶¶ Very high severity • Both clinical severity and All ages 7,500,000 1,400,000 1918 pandemic (very severe to transmissibility are greater than 0–18 1,000,000 100,000 • The 1918 pandemic resulted in death for 2%–3% of extreme pandemic) during annual influenza seasons. 18–64 3,000,000 500,000 those infected, a case-fatality rate that was much PSAF quadrant: D • Estimated overall attack and ≥65 3,400,000 800,000 greater than the rate during an average influenza case-fatality rates: 30% and 1.5%, season. The pandemic virus was easily transmitted. respectively. Rates of severe • Approximately one fourth of the U.S. population outcomes are greater than during became ill, and approximately 500,000 died; 99% of influenza seasons, especially deaths occurred in persons aged ≤65 yrs.††,*** among young adults. Abbreviation: PSAF = Pandemic Severity Assessment Framework. * Based on PSAF (Source: Reed C, Biggerstaff M, Finelli L, et al. Novel framework for assessing epidemiologic effects of influenza epidemics and pandemics. Emerg Infect Dis 2013;19:85–91). † Point estimates for hospitalizations and deaths, by age group, are based on the estimated overall attack and case-fatality rates provided in the second column (clinical severity and transmissibility). Age-specific point estimates of hospitalizations and deaths are based on U.S. Census 2010 population data. § Source: Shrestha SS, Swerdlow DL, Borse RH, et al. Estimating the burden of 2009 pandemic influenza A (H1N1) in the United States (April 2009–April 2010). Clin Infect Dis 2011;52(Suppl 1):S75–S82. ¶ Source: Fowlkes AL, Arguin P, Biggerstaff MS, et al. Epidemiology of 2009 pandemic influenza A (H1N1) deaths in the United States, April–July 2009. Clin Infect Dis 2011;52(Suppl 1):S60–S68. ** Source: Viboud C, Miller M, Olson DR, Osterholm M, Simonsen L. Preliminary estimates of mortality and years of life lost associated with the 2009 A/H1N1 pandemic in the U.S. and comparison with past influenza seasons. PLoS Currents 2010;2:RRN1153. †† Source: Simonsen L, Clarke MJ, Schonberger LB, Arden NH, Cox NJ, Fukuda K. Pandemic versus epidemic influenza mortality: a pattern of changing age distribution. J Infect Dis 1998;178:53–60. §§ Source: Cox NJ, Subbarao K. Global epidemiology of influenza: past and present. Annu Rev Med 2000;51:407–21. ¶¶ Source: Henderson DA, Courtney B, Inglesby TV, Toner E, Nuzzo JB. Public health and medical responses to the 1957–58 influenza pandemic. Biosecur Bioterror 2009;7:265–73. *** Source: Collins SD. Age and sex incidence of influenza and pneumonia morbidity and mortality in the epidemic of 1928–29 with comparative data for the epidemic of 1918–19. Public Health Rep 1931;46:1909–37. US Department of Health and Human Services/Centers for Disease Control and Prevention MMWR / April 21, 2017 / Vol. 66 / No. 1 31