Homeland Security

Flu Pandemic Timeline / Event Dynamics

In response to cases of swine influenza A(H1N1), reported in Mexico and the United States of America, on 25 April 2009 the World Health Organization Director-General convened a meeting of the Emergency Committee to assess the situation and advise her on appropriate responses. The Director-General determined that the current events constitute a public health emergency of international concern. There was a decision not to go into Phase Four.

On 25 April 2009 Dr. Anne Schuchat, Interim Deputy Director for Science and Public Health Program at the Centers For Disease Control, said "It's clear that this is widespread and that is one reason we have to let you know, we don't think we can contain the spread of this virus. If we found only swine flu -- a new influenza virus in one place, in a small community, we might be able to quench and contain it. In many different communities as we're seeing, we don't think that containment is feasible." The infectious window for this particular strain of influenza is usually within a few days but can be as long as nine days at this point.

This is not routine seasonal flu ioccurring at the tail end of the season of regular influenza. This is very late for seasonal influenza.

A pandemic may not occur for many years, and when it occurs it may or may not be severe. In 1999, the World Health Organization (WHO) Secretariat published guidance for pandemic influenza and defined the phases of a pandemic. Updated guidance was published in 2005 to redefine these phases. This schema is designed to provide guidance to the international community and to national governments on preparedness and response for pandemic threats and pandemic disease. Compared with the 1999 phases, the new definitions place more emphasis on pre-pandemic phases when pandemic threats may exist in animals or when new influenza virus subtypes infect people but do not spread efficiently.

    Interpandemic Period
    • Phase 1. No new influenza virus subtypes have been detected in humans. An influenza virus subtype that has caused human infection may be present in animals. If present in animals, the risk of human infection or disease is considered to be low
    • Phase 2. No new influenza virus subtypes have been detected in humans. However, a circulating animal influenza virus subtype poses a substantial risk of human disease

    Pandemic Alert Period

    • Phase 3. Human infection(s) with a new subtype but no human-to-human spread or at most rare instances of spread to a close contact
    • Phase 4. Small cluster(s) with limited human-to-human transmission but spread is highly localized, suggesting that the virus is not well adapted to humans
    • Phase 5. Larger cluster(s) but human-to-human spread is still localized, suggesting that the virus is becoming increasingly better adapted to humans but may not yet be fully transmissible (substantial pandemic risk)

    Pandemic Period

    • Phase 6. Pandemic phase: increased and sustained transmission in the general population

    Postpandemic Period

    • Return to the Interpandemic Period (Phase 1)

Viruses are known to survive on non-porous surfaces such as steel and plastic, for up to 24 to 48 hours after inoculation and from cloth, paper, and tissues for up to 8 to 12 hours. Viable virus can be transferred from non-porous surfaces to hands for 24 hours and from tissues to hands for 15 minutes.

The patterns of infection for pandemic influenza may or may not resemble those of seasonal flu. The incubation period (the time between acquiring the infection until becoming ill) for annual epidemic influenza is 1-4 days, with an average of 2 days. Adults typically are infectious the day before symptoms begin through approximately 5 days after illness onset. The exact incubation period for bird flu in humans is unclear, though illness seems to develop within one to five days of exposure to the virus. Viral shedding and the risk for transmission are normally greatest during the first 2 days of illness. Viral shedding, and the period during which a person may be infectious to others, generally peaks on the second day of symptoms. Children will shed the greatest amount of virus and, therefore are likely to pose the greatest risk for transmission. Children can be infectious for more than 10 days, and young children can shed virus for up to 6 days before their illness onset. The length of time of viral shedding may be prolonged during initial infection with a new influenza subtype. Severely immuno-compromised persons can shed virus for weeks or months.

A patient's close contacts may include family, friends, work colleagues, classmates, fellow passengers, and/or healthcare providers. Management of contacts might include passive or active monitoring without activity restrictions and/or quarantine at home or in a designated facility. Quarantine may be lifted as soon as the exposed contact has remained without signs or symptoms of disease for a complete incubation period for influenza disease. Experience with seasonal influenza suggests the incubation period is 1-4 days, with an average length of 2 days. However, the clinical behavior of a novel influenza virus may be different and could potentially be as long as 10 days. Pandemic influenza preparedness activities should plan for containment measures that may last between 1-10 days. For the purposes of the HHS plan, 10 days was referred to as the incubation period; however, public health authorities should be prepared to adjust the time frame as more is known about the virus.

Antiviral medications can be used both to prevent infection (prophylaxis) and to reduce complications in persons who have been infected. To prevent infection they would need to be taken as long as the virus is circulating. To be effective in treating persons who have been infected they need to be started ideally within 48 hours of onset of symptoms. *Treatment requires a total of 10 capsules and is defined as 1 course. Post-exposure prophylaxis (PEP) also requires a single course. Prophylaxis (P) is assumed to require 40 capsules (4 courses) though more may be needed if community outbreaks last for a longer period.

While the typical flu season predictably occurs from November through March, during pandemics, flu can vary from this script with outbreaks at any time of the year. The seasonality of a pandemic cannot be predicted with certainty. The largest waves in the US during 20th century pandemics occurred in the fall and winter. Experience from the 1957 pandemic may be instructive in that the first US cases occurred in June but no community outbreaks occurred until August and the first wave of illness peaked in October.

The scope and pace of an influenza pandemic may defy accurate prediction. The disease may appear in many different parts of the Nation almost simultaneously, or disease may occur in only one or a few communities, and if not contained there, proceed to affect other communities.

When planning and preparing for the next influenza pandemic, there are two equally important timelines: how fast the disease spreads, and how quickly a vaccine can be created and distributed. Due to the rapid spread of the influenza pandemic and the time required to develop, test, produce, and distribute an effective vaccine, the disease will likely arrive in the United States before a "significant" number of people can be vaccinated. The implication of this is that, as part of any pandemic influenza preparation and response plan, there must be a mechanism for allocating the vaccine among the population.

If the virus gradually improves its transmissibility among humans through adaptive mutation, clusters of cases would be indicative, and sensitive surveillance might detect them. In the alternative scenario, in which a fully transmissible pandemic virus emerges following a reassortment event, the resulting explosion of cases would be difficult for any surveillance system to miss. The doubling time for an epidemic curve was about three days in the 1918 pandemic. The high attack rate and exceptionally high case fatality proportion (CFP) of 1918 influenza caused an unusual and massive increase in the number of pneumonia and influenza (P&I) deaths.

In the previous century, pandemics traveled from continent to continent along sea lanes, with global spread complete within six to eight months. The 1957 pandemic, during an era with much less globalization, spread to the US within 4-5 months of its detection in China, and the 1968 pandemic spread to the US from Hong Kong within 2-3 months. As was amply demonstrated by the SARS outbreak, 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 there will be only minimal time to implement preparations and responses once pandemic viruses have begun to spread. The speed of international spread has no direct effect on mortality, but could compromise response capacity should large parts of the world experience almost simultaneous outbreaks. Many of the public health interventions that successfully contained SARS will not be effective against a disease that is far more contagious, has a very short incubation period, and can be transmitted prior to the onset of symptoms.

Most experts believe that there will be one to six months between the identification of a novel influenza virus and the time that widespread outbreaks begin to occur in the US. Outbreaks are expected to occur simultaneously throughout much of the US, preventing relocation of human and material resources. Because populations will be fully susceptible to an H5N1-like virus, rates of illness could peak fairly rapidly within a given community. The effect of influenza on individual communities will be relatively prolonged - six to eight weeks, though possibly up to twelve weeks.

A pandemic will last much longer than most public health emergencies and may include "waves" of influenza activity separated by months. In 20th century pandemics, a second wave of influenza activity occurred 3 to 12 months after the first wave. In 1957 the second wave began 3 months after the peak of the first wave, while in 1968 the second wave began 12 months after peak of the first wave. The first wave of the 1918 flu occurred in the spring of that year ending in March. That flu was very severe by usual standards but the second wave beginning 6 months later in September was the most fatal. During the 1918 pandemic, the deadly second wave was responsible for more than 90% of the deaths for the entire pandemic. The third wave occurred more than a year later, during the following 1919-1920 winter/spring, and was the mildest of all.

Once a potential pandemic strain of influenza virus is identified, it takes several months before a vaccine will be widely available. With the current technologies, it will take at least five or six months before vaccines based on a new influenza strain can be produced on a large scale. After inoculation, it takes about 2 weeks for adults and up to 6 weeks for children to achieve optimal protection under a one-dose regimen, with an additional 4 weeks if a booster shot is needed a month later. Production of a vaccine would take a minimum of six months after isolation of the circulating strain, and given the capacity of all the current international vaccine manufacturers, supplies during the following six months would be limited to fewer than a billion monovalent doses. Pandemic vaccine production using current technologies might not be available before the second pandemic wave.

Globally and nationally, a pandemic might last for at least one year and up to three years, while disease outbreaks in local communities may last 5 to 10 weeks. Pandemics end simply because all or most susceptible persons within the population have contracted the infection and have either died, developed immunity or been vaccinated.

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