Pandemics are explosive global events in which most, if not all, populations worldwide are at risk for infection and illness. In historical pandemics of influenza, the disease spread worldwide within months, and with modern travel patterns and increased population density, a pandemic of influenza is expected to spread much faster in present day. A pandemic has the potential to be a catastrophic disaster of unprecedented scale.

Influenza pandemics are believed to have occurred for at least 300 years at unpredictable intervals. It is difficult to predict when the next influenza pandemic will occur or how severe it will be - wherever and whenever a pandemic starts, everyone around the world is at risk.

Defining the potential magnitude of a pandemic is difficult because of the large differences in severity for the three 20th Century pandemics. The 1918 pandemic resulted in an estimated (at least) 500,000 deaths in the United States, the 1958 pandemic killed approximately 70,000 Americans, and the 1968 pandemic caused an estimated 34,000 U.S. deaths. This difference largely relates to the severity of infections and the virulence of the influenza viruses that caused the pandemics. The 20th Century pandemics also have shared similar characteristics. In each pandemic, about 30 percent of the U.S. population developed illness, with about half of those Americans seeking medical care. Children under the age of 18 have tended to have the highest rates of illness, though not of severe disease and death. Geographical spread in each pandemic was rapid and virtually all communities experienced outbreaks.

An especially severe pandemic of influenza could lead to high levels of illness, death, social disruption, and economic loss. Everyday life will be disrupted because so many people in so many places become seriously ill at the same time. Impacts can range from school and business closings and the cancellation of social/mass gatherings (such as concerts and movies) to the interruption of basic services such as public transportation and food delivery.

While an annual influenza season in the U.S., on average, results in approximately 36,000 deaths, 226,000 hospitalizations, and between $1 billion and $3 billion in direct costs for medical care, it is predicted that the number of illnesses, deaths, hospitalizations, and the direct costs will be significantly higher during a pandemic. A substantial percentage of the world's population will require some form of medical care. 5 Health care facilities will likely overwhelmed, creating a shortage of hospital staff, beds, ventilators, and other supplies. Surge capacity at non-traditional sites (such as schools) may need to be created to cope with demand.

In addition, in this scenario, the need for vaccine is likely to outstrip supply and the supply of antiviral drugs is also likely to be inadequate early in a pandemic. Difficult decisions will need to be made regarding who gets antiviral drugs and vaccines.

As described in the Interim Pre-pandemic Planning Guidance:Community Strategy for Pandemic Influenza Mitigation in the United States - Early, Targeted, Layered Use of Nonpharmaceutical Interventions, without implementing mitigating interventions, even a less severe pandemic would likely result in dramatic increases in the number of hospitalizations and deaths. In addition, an unmitigated severe pandemic would likely overwhelm our nation's critical healthcare services and impose significant stress on our nation's critical infrastructure. Therefore, communities, individuals and families, employers, schools, and other organizations must plan for the use of non-pharmaceutical interventions (NPI) to help limit the spread of a pandemic, prevent disease and death, lessen the impact on the economy, and keep society functioning. For additional information on these NPIs, refer to Section D of this appendix.

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A. Influenza Viruses

Influenza viruses are negative-stranded ribonucleic acid (RNA) viruses that have been divided into types: A, B, and C viruses. Type C is not known to cause disease in humans, and only type A viruses are known to cause pandemics. Type A viruses are further divided into subtypes based on the specific hemagglutinin (H) and neuraminidase (N) proteins on the virus surface.

Two subtypes of A viruses are currently circulating among humans - H3N2 and H1N1. When these subtypes in emerged in the 20th Century, humans had not been exposed to them previously and resulted in the pandemics of 1918 pandemic (H1N1) and 1968 (H3N2). In addition, the 1957 pandemic was associated with the emergence and spread of the H2N2 virus; however, this virus subtype stopped circulating in 1968.

Pandemics of influenza are unusual events and their timing cannot be predicted. For example, only three pandemics occurred in the 20th Century (1918, 1957, and 1968). The infrequency and unpredictable timing of these events is explained by the fact that influenza pandemics occur only when a new (or novel) influenza virus emerges and spreads globally. By definition, most people have never been exposed to these viruses; and therefore, are susceptible to infection by them. In contrast, seasonal influenza virus strain variants are modified versions of influenza viruses that are already in widespread circulation. Therefore, there is usually some level of pre-existing immunity to strain variants. Antigenic drift and shift account for the differences described here between seasonal and pandemic strains of influenza.

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1. Drift and Shift

Influenza viruses replicate their RNA sloppily, and as a result mutate fairly easily. Mutations of these viruses can result in either modification (drift) or replacement (shift) of the hemagglutinin and neuraminidase on the viral surface. Because these proteins are the main targets for the immune system, changes in these proteins can have minor to profound effects on the influenza virus' ability to spread throughout human and animal populations.

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a) Drift

When influenza viruses undergo antigenic drift, mutations to the virus genome produce changes in the viral hemagglutinin and neuraminidase. As noted earlier, since RNA replication in influenza virus is sloppy, drift is a continuous ongoing process, and results in the emergence of new strain variants. The amount of change can be subtle or dramatic, but eventually one of the new variant strains becomes dominant, usually for a few years, until a new variant emerges and replaces it. In essence, drift affects the influenza viruses that are already in worldwide circulation. This process allows influenza viruses to change and re-infect people repeatedly through their lifetime.

Since influenza virus strains shift dominance on such a constant basis, each year, many laboratories, including in the United States, collect influenza viruses circulating in different parts of the world. Some of these virus samples are sent to one of four WHO reference laboratories (one of which is at the CDC in Atlanta) for detailed testing. The testing includes determining how well antibodies in the current seasonal influenza vaccine react to the circulating virus and new influenza viruses (how well the current vaccine protects against from getting sick). This information, along with information about global influenza activity, is used by the WHO and an advisory committee of the FDA to select three viruses (two subtypes of influenza A viruses and one influenza B virus) to be the components of the annual influenza vaccines produced for the following fall and winter. Usually, one or two of the three virus strains in the vaccine are changed each year. (This is why it is so important to get the seasonal vaccine every year.)

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b) Shift

In contrast to drift, in antigenic shifts, the surface existing viral hemagglutinin and neuraminidase proteins are not modified, but are replaced by significantly different hemagglutinin and neuraminidase proteins. As a result, the influenza A viruses bear entirely new (or novel) H or H/N combinations which are perceived by our immune systems as something majority of people do not have pre-existing antibody protection against. As a result, there is widespread illness and mortality among populations as the disease spreads around the world in a pandemic.

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B. Animal Reservoirs

Wild birds are considered the reservoir for influenza viruses because more influenza A subtypes (15) circulate among wild birds than humans or other animal species. Normally, animal influenza viruses do not infect humans. However, avian influenza viruses can sometimes cross this barrier and directly infect humans.

Pandemic viruses can also arise when some of the genes from animal influenza viruses mix (or re-assort) with some of the genes from human influenza viruses to create a new hybrid influenza virus. Re-assorted viruses have been frequently identified and are thought to have been responsible for the 1957 and 1968 pandemic viruses.

There are three requirements for influenza (or any infectious disease) to become pandemic in humans:

  • New viral subtype of influenza needs to emerge
  • The virus strain that emerges needs have the ability to infect humans and cause serious infection
  • Virus must spread human-to-human in sustained transmission.

As described below, the first two requirements have been met.

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a) H5N1 Strain of Influenza and Current Situation

Although it is unpredictable when the next pandemic will occur and what strain may cause it, the continued and expanded spread of a highly pathogenic - and now endemic - avian H5N1 virus across much of Asia, Russia, and Europe, and parts of Africa, represents a significant pandemic threat. Human H5N1 influenza infection was first recognized in 1997 when it infected 18 people (and caused 6 deaths) in Hong Kong. Since 2003, over 320 human H5N1 cases (and over 190 deaths; with a mortality rate of approximately 60 percent) have been confirmed (by WHO) in a growing list of countries, including Thailand, Vietnam, Cambodia, Indonesia, China, Egypt, Turkey, Azerbaijan, Djibouti, Iraq, Laos, and Nigeria.

Additional concerns that the H5N1 virus currently circulating has significant potential in triggering a pandemic because:

  • The avian H5N1 virus is becoming more deadly in a growing number of bird species and mammals.
  • Wild birds and domestic ducks may be infected asymptomatically, providing a reservoir for infection of other domestic poultry species.
  • The virus is able to transmit directly from birds to some mammals and to people.
  • There is sporadic spread directly from animals to humans with suspected human-to-human transmission in rare instances (with close contact of family members caring for those ill).
  • Genetic studies confirm that H5N1, like other influenza viruses, is continuing to change and evolve.

While H5N1 is the greatest current pandemic threat, other avian influenza subtypes have also infected people in recent years. In 1999, H9N2 infections were identified in Hong Kong; in 2003, H7N7 infections occurred in the Netherlands; and in 2004, H7N3 infections occurred in Canada. Such outbreaks/strains also have the potential to give rise to the next pandemic, reinforcing the need for continued surveillance and ongoing vaccine development efforts against these strains.

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C. Impact of Influenza and Influenza Pandemics

Pandemic strains of influenza viruses have demonstrated their ability to spread worldwide within months, or weeks, and will cause illness and deaths in all age groups. The pandemic will likely spread around the world in multiple waves (periods during which community outbreaks occur across the country) of illness with each wave taking 2 - 3 months to spread the globe. As each wave is spreading, it is estimated that epidemics of influenza will remain in affected communities for 6 – 8 weeks.

While the ultimate number of infections, illnesses, and deaths is unpredictable, it is nonetheless certain that without adequate planning and preparations, an influenza pandemic in the 21st Century has the potential to cause enough illnesses to overwhelm current public health and medical care capacities at all levels, despite the vast improvements made in medical technology during the 20th Century.

Certain modern trends will likely increase the potential for pandemics to cause more illnesses and deaths than in previous pandemics:

  • The global population is larger and increasingly urbanized, allowing viruses to be transmitted within populations more easily
  • Levels of international travel are much greater than in the past, allowing viruses to spread globally more quickly than in the past
  • Populations in many countries consist of increasing numbers of elderly persons, and those with chronic medical conditions; thus, increasing the potential for more complicated illnesses and deaths to occur.

This combination of factors suggests that the next pandemic may lead to influenza spreading among the population and around the world faster than in the past, overwhelming countries and health systems that are not adequately prepared.

The 1957 pandemic, during an era with much less globalization, spread to the U.S. within 4 - 5 months of its detection in China, and the 1968 pandemic spread to the U.S. from Hong Kong within 2 - 3 months. As the Severe Acute Respiratory Syndrome (SARS) outbreak in 2003 demonstrated, modern travel patterns may significantly reduce the time needed for pandemic influenza viruses to spread globally to a few months or even weeks. The major implication of such rapid spread of an infectious disease is that many, if not most, countries will have minimal time to implement preparations and responses once pandemic viruses have begun to spread. While SARS infections spread quickly to multiple countries, the epidemiology and transmission modes of the SARS virus greatly helped to contain the spread of this infection in 2003, along with quarantine, isolation, and other control measures. By contrast, because influenza spreads more rapidly between people and can be transmitted by those who are infected but do not yet have symptoms, the spread of pandemic influenza to multiple countries is expected to lead to the near simultaneous occurrence of multiple community outbreaks in an escalating fashion. No other infectious disease threat, whether natural or engineered, poses the same current threat for causing increases in infections, illnesses, and deaths so quickly in the U.S. and worldwide.

Table C-1. Effects of Past Pandemics on the U.S.
Pandemic Estimated U.S. Death Influenza A Strain Populations at greatest risk




Young, healthy adults




Infants, elderly




Infants, elderly

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D. Interim Pre-pandemic Planning Guidance: Community Strategy for Pandemic Influenza Mitigation in the United States - Early, Targeted, Layered Use of Nonpharmaceutical Interventions

Non-pharmaceutical interventions (NPIs) will likely reduce influenza transmission, and therefore impact the severity of the pandemic, by reducing contact between sick and uninfected persons, thereby decreasing the number of those who become infected. This will, in turn, lessen the need for healthcare services and minimize the impact of a pandemic on the economy and society. To have the greatest effect, NPI measures should be implemented in the early stages of the pandemic. NPIs described in the Community Strategy for Pandemic Influenza Mitigation guidance document include:

  • Isolation and treatment (as appropriate) with influenza antiviral medications of all persons with confirmed or probable pandemic influenza.
  • Voluntary home quarantine of members of households with confirmed or probable influenza case(s) and consideration of combining this intervention with the prophylactic use of antiviral medications, providing sufficient quantities of effective medications exist and that a feasible means of distributing them is in place.
  • Dismissal of students from school (including public and private schools as well as colleges and universities) and school-based activities and closure of childcare programs, coupled with protecting children and teenagers through social distancing in the community to achieve reductions of out-of-school social contacts and community mixing.
  • Use of social distancing measures to reduce contact between adults in the community and workplace, including, for example, cancellation of large public gatherings and alteration of workplace environments and schedules to decrease social density to the greatest extent possible without disrupting essential services.
  • All such community-based strategies should be used in combination with individual infection control measures, such as hand washing and cough etiquette.

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Decisions about what tools should be used during a pandemic should be based on the observed severity of the event, its impact on specific subpopulations, the expected benefit of the interventions, the feasibility of success in modern society, the direct and indirect costs, and the consequences on critical infrastructure, healthcare delivery, and society. The most controversial elements (e.g., prolonged dismissal of students from schools and closure of childcare programs) are not likely to be needed in less severe pandemics, but these steps may save lives during severe pandemics. Just as communities plan and prepare for mitigating the effect of severe natural disasters (e.g., hurricanes), they should plan and prepare for mitigating the effect of a severe pandemic.

To further assist in pre-pandemic planning, the Community Strategy for Pandemic Influenza Mitigation also introduces the Pandemic Severity Index (PSI), which is five discrete categories of increasing severity (Category 1 - 5), similar to the Saffir-Simpson Scale for measuring hurricane strength/intensity (which is also Category 1 – 5). All future pandemics will be assigned to one of these categories. The PSI is based on the case fatality ratio, and other key epidemiological characteristics that define outbreaks/pandemics of disease, as shown below.

Table C-2. Pandemic Severity Index Categories
Characteristics Pandemic Severity Index

Case Fatality Ratio (percentage)

0.1 -
0.5 -
1.0 -

Excess Death Rate (per 100,000)

30 -
150 -
300 -

Illness Rate (percentage of the population)


Potential Number of Deaths (based on 2006 U.S. population)

90,000 -
450,000 -
900,000 -
≥1.8 million

20th Century U.S. Experience

(Illness rate
1957, 1968 Pandemic
1918 Pandemic

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The timing of initiation, which PSI Categories to implement, and length of duration of the NPIs outlined in the Community Strategy for Pandemic Influenza Mitigation will influence their effectiveness; and Table C-3 below provides guidance.

Table C-3. When to Implement Various NPIs Based on PSI Category Scheme
  Pandemic Severity Index
Interventions * by Setting 1 2 and 3 4 and 5
Footnotes for Table C3      



Voluntary isolution of ill at home (adults and children); combine with use of antiviral treatment as available and indicated


Voluntary quarantine of household members in homes with ill persons (adult and children), consider combining with antiviral prophylaxis if effective, feasible, and quantities sufficient

Generally not recommended

Child social distancing


-dismissal of students from schools and school based activities, and closure of child care programs

Generally not recommended
Consider ≤4 weeks††
Recommend ≤12 weeks§§

- reduce out-of-school social contacts and community mixing

Generally not recommended
Consider ≤4 weeks††
Recommend ≤12 weeks§§

Workplace / Community
Adult social distancing


-decrease number of social contacts (e.g., encourage teleconferences, alternatives to face-to-face meetings)

Generally not recommended

-increase distance between persons (e.g., reduce density in public transit, workplace)

Generally not recommended

-modify, postpone, or cancel selected public gatherings to promote social distance (e.g., stadium events, theater performances)

Generally not recommended

-modify work place schedules and practices (e.g., telework, staggered shifts)

Generally not recommended

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Historical data from the 1918 pandemic suggests that the duration of implementing NPIs significantly impacts mortality rates. Stopping or limiting the intensity of NPIs while pandemic virus was still circulating within the community caused increases in mortality due to pneumonia and influenza in some communities. As long as susceptible individuals are present in large numbers in a particular area, spread of influenza will continue. While NPIs may limit or slow community transmission, persisting pandemic virus circulating in a community with a susceptible population is a risk factor for re-emergence of the pandemic.

Therefore, while the duration of NPI implementation will depend on the severity of the pandemic, and the duration of the pandemic wave (about 6 – 8 weeks) within the particular area, communities should be prepared to maintain these measures for up to 12 weeks in a Category 4 or 5 pandemic, as shown above in Table C-3. Monitoring of excess mortality, case fatality ratios, or other surrogate markers over time will be important for determining both the optimal duration of implementation and the need for resumption of these measures.

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E. Pandemic Influenza Planning Checklists

In the event of an influenza pandemic, many sectors play a key role in protecting the health and safety of their employees, as well as limiting the negative impact to the economy and society. Planning for pandemic influenza is a critical aspect of this. HHS and CDC developed guidelines, including checklists, to assist businesses, industries, and other employers in planning for a pandemic. These checklists cover multiple key sectors, including:

  • Business
  • Child Care
  • Colleges/Universities
  • Emergency Medical Services (EMS)
  • Faith-Based
  • Health Insurer
  • Law Enforcement
  • Correctional Facilities
  • Home Health
  • Hospitals
  • Individual/Families
  • Long-Term Care
  • Medical Offices and Clinics
  • Schools (Kindergarten -12th Grade)
  • State/local

Appendix O contains the checklists that are most applicable to DOI and its equities; these are the EMS, Law Enforcement, Correctional Facilities, Individual/Families, Schools (Kindergarten -12th Grade), and Colleges/Universities checklists. Those checklists that are not included in this appendix can be found at

The checklists identify key areas that DOI equities should address, in addition to the guidance laid out in the DOI Pandemic Influenza Plan, in their pandemic planning. The checklists also enable those responsible for pandemic influenza planning to assess and improve their preparedness efforts.

Given the differences within sectors, individual facilities/equities should adapt the checklists to meet their unique needs. In addition, when developing pandemic plans and preparations, sectors and individual facilities/equities should also utilize information and guidance from State, regional, and local health departments and emergency management agencies/authorities.

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F. Internet Resources on Influenza

The links listed below were active as of November 2007. However, because web sites can change without notice, no site can be guaranteed active or accurate indefinitely.

U.S. Government:

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Nongovernmental Organizations:

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Background & Historical Information:
  • World Health Organization – Defines influenza pandemic, explains how a new influenza virus can cause a pandemic, presents the consequences of an influenza pandemic, explains the global surveillance systems, and provides links to other pandemic plans from other nations.
  • FAO – Offers updates/news related to avian influenza (including the FAOAIDEnews: Avian Influenza Disease Emergency Situation Updates), guidance documents, background, Q&As, slide show, and videos, as well as a presentation on the socio-economic impact of AI.
  • CDC – Presents information on the symptoms, treatment, and complications of the disease, prevention and control, the types of influenza viruses, questions and answers on symptoms, vaccination and myths.
  • Federal Occupational Health (FOH) – Serves as a resource for fact sheets, articles and other background information on pandemic influenza – including Frequently Asked Questions fact sheets for Federal managers and employees, and Top 10 Tips for Pandemic Preparedness and Prevention.
  • Department of Defense (DoD) Pandemic Influenza Watchboard - Provides general information about avian and pandemic influenza, including recent outbreaks, as well as information about how it could affect service member and civilian health.
  • DOI – Describes the Department's and the Federal Government's role in pandemic/avian influenza planning and response, information for DOI employees, as well as press releases, Q&As, fact sheets and speeches on the topic.
  • U.S. Department of Agriculture (USDA) – Offers a variety of resources on avian influenza, including fact sheets, news items/transcripts, and information on reporting sick/dead birds.
  • NIOSH - Worker safety and health issues related to pandemic/avian influenza.
  • American Red Cross – Provides information for individuals and families related to pandemic influenza planning, including a list of what necessities to stockpile in the home (in case the second and third order effects of the pandemic impact critical infrastructure), what people should ask related to their community's planning for a pandemic, personal hygiene tips, and how to care for sick family member and prevent the illness from spreading to other family members.
  • Department of State – Travel guidance to U.S. citizens traveling abroad to areas impacted by avian influenza.
  • National Vaccine Program Office – Presents background information about influenza pandemics, a historical overview of pandemics (Spanish Flu of 1918, Asian Flu of 1957, Hong Kong Flu of 1968) and three influenza threats (Swine Flu of 1976, Russian Flu of 1977, and Avian Flu of 1997) that occurred during the past century, and preparations for the next pandemic.
  • U.S. Food and Drug Administration (FDA) – Provides a wide variety of influenza-related topics in the area of FDA's focus (including anti-virals, vaccines and diagnostics)
    • FDA – FDA and Pandemic Flu.
    • FDA, Center for Biologics Evaluation and Research – Discussion of influenza vaccines (for both seasonal and pandemic influenza) and related information.
    • FDA, Center for Drug Evaluation and Research – Discussion of influenza anti-viral drugs and related information.
  • CDC - Workshop Proceedings from Pandemic Influenza—Past, Present, Future: Communicating Today Based on the Lessons from the 1918–1919 Influenza Pandemic. Descriptive information on what how the 1918 pandemic impacted society, critical infrastructure, government, and individual/family life.
  • U.S. Federal GovernmentThe Great Pandemic of 1918: State-by-State provides stories and anecdotes of the impact of this pandemic on individual States.
  • Public Broadcasting Service (PBS)The American Experience: 1918 Pandemic
  • The National ArchivesThe Deadly Virus: The Influenza Epidemic of 1918 contains a library of electronic files of documents and photos from 1918 Pandemic.

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Additional Preparedness & Response Resources:
  • HHS Hospital Preparedness Program – Provides information about HHS' programs for enhancing the ability of hospitals and health care systems to prepare for and respond to bioterrorism and other public health emergencies.
  • CDC Emergency Preparedness and Response – Offers extensive information on bioterrorism, chemical emergencies, radiation emergencies, recent disease outbreaks/incidents, mass casualties, and natural disasters/severe weather. In addition, provides additional resources on surveillance, laboratories (Laboratory Response Network (LRN)), disaster mental health, clinician resources, planning, and training/education.
  • CDC's Public Health Emergency Response Guide for State, Local, and Tribal Public Health Directors – Provides an all-hazards reference tool for health professionals who are responsible for initiating the public health response during the first 24 hours of an emergency or disaster. It provides useful information on the activation and integration of a jurisdiction's public health system into the existing overall emergency response structure during the acute phase of an incident. It also contains guidance that may be unique to specific types of incidents, such as floods, earthquakes, and acts of terrorism. In addition, it is consistent with the doctrine, concepts, principles, terminology, and organizational processes in the National Response Plan (NRP) and the National Incident Management System (NIMS).
  • CDC Cooperative Agreements on Public Health Preparedness – Provide funding to State and local public health jurisdictions for preparedness for and response to bioterrorism, other outbreaks of infectious diseases, and other public health threats and emergencies (including pandemic influenza).
  • Centers for Public Health Preparedness Program– Describes and provides information on the CDC-funded Centers for Public Health Preparedness (CPHP) program. CPHP is a national network of academic institutions working in collaboration with State and local public health departments and other community partners to provide life-long learning opportunities to the public health workforce, in order to handle the next public health crisis. 
  • Epidemic Information Exchange – Provides a secure, web-based communications network for information exchange among CDC, State and local health departments, and other public health professionals.
  • Strategic National Stockpile – Provides information on the availability and rapid deployment of life-saving pharmaceuticals, antidotes, other medical supplies, and equipment necessary to counter the effects of nerve agents, biological pathogens (including influenza), and chemical agents.
  • Smallpox Response Plan and Guidelines (Version 3.0) – Presents the most current criteria for implementation of CDC smallpox response plan, notification procedures for suspected smallpox cases, CDC and State/local responsibilities and action in the event of a smallpox outbreak, vaccine mobilization and deployment, and CDC personnel mobilization and deployment.

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Footnotes for Table C3

Generally Not Recommended = Unless there is a compelling rationale for specific populations or jurisdictions, measures are generally not recommended for entire populations as the consequences may outweigh the benefits.

Consider = Important to consider these alternatives as part of a prudent planning strategy, considering characteristics of the pandemic, such as age-specific illness rate, geographic distribution, and the magnitude of adverse consequences. These factors may vary globally, nationally, and locally.

Recommended = Generally recommended as an important component of the planning strategy.

* All these interventions should be used in combination with other infection control measures, including hand hygiene, cough etiquette, and personal protective equipment such as face masks. Additional information on infection control measures is available at

This intervention may be combined with the treatment of sick individuals using antiviral medications and with vaccine campaigns, if supplies are available

§ Many sick individuals who are not critically ill may be managed safely at home

The contribution made by contact with asymptomatically infected individuals to disease transmission is unclear. Household members in homes with ill persons may be at increased risk of contracting pandemic disease from an ill household member. These household members may have asymptomatic illness and may be able to shed influenza virus that promotes community disease transmission. Therefore, household members of homes with sick individuals would be advised to stay home.

** To facilitate compliance and decrease risk of household transmission, this intervention may be combined with provision of antiviral medications to household contacts, depending on drug availability, feasibility of distribution, and effectiveness; policy recommendations for antiviral prophylaxis are addressed in a separate guidance document.

†† Consider short-term implementation of this measure—that is, less than 4 weeks.

§§ Plan for prolonged implementation of this measure—that is, 1 to 3 months; actual duration may vary depending on transmission in the community as the pandemic wave is expected to last 6-8 weeks.

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