Saturday, June 17, 2017

HHS 2017 Influenza Pandemic Update: Planning Assumptions


For those under about the age of 40, an influenza pandemic probably feels like a rare,  once-in-a-lifetime, event.  And we had `our pandemic' in 2009 - and while it was bad - it wasn't nearly as bad as the tabloid media hyped it would be.
But as the chart above - and anyone over the age of 60 - can attest, pandemics come along a little more frequently than that.  At the age of 63, I've been through 3 of them; 1957, 1968, and 2009.
And that doesn't count the pandemic scare of 1976 (see Deja Flu, All Over Again), the 1977 H1N1 `Russian Flupseudo-pandemic (see Pseudo Pandemics And Viral Interlopers), or the SARS outbreak of 2003 (technically a coronavirus, not influenza.If you lived in China, Hong Kong, Toronto or Singapore . . it felt like a pandemic). 

Of the three legit pandemics I've seen, the 2009 H1N1 was the mildest, with 1957 the worst (which I don't really remember), and 1968 coming in the middle.  While I wasn't around for the Great Pandemic of 1918, as a young paramedic I had occasion to meet, and talk to a number of survivors.
The stories they told were sobering, to say the least (see HHS Video: We Heard The Bells).
The next pandemic could be as moderate as 2009, worse than 1918, or somewhere in between.  We won't know until we are well into it, or perhaps not until long after it is over. Researchers are still arguing over the death toll from 1918, although most put it over 50 million people.

For that reason, the HHS has to plan for a wide range of scenarios, and that is reflected in the following excerpts (bolding mine) from their updated 2017 Pandemic Plan, published this week.

Appendix A
Pandemic Scenarios
An emerging influenza pandemic virus can place extraordinary and sustained demands on public health and health care systems and on providers of essential community services across the United States and throughout the world. By some estimates, and based on experience with past pandemics, 20 to 30 percent of the global population could develop clinical disease; a substantial proportion of these people could develop severe disease, and even die.
Although the 1918 pandemic resulted in an estimated 500,000 deaths in the United States (out of a total population then of about 105 million), the 1968 pandemic caused an estimated 34,000 US deaths (out of a total population then of about 201 million), and the 2009 A(H1N1) pandemic resulted in approximately 12,500 US deaths (out of a total population then of about 305 million).
The direct and indirect health costs alone (not including disruptions in trade and other costs to business and industry) have been estimated to approach $181 billion for a moderate pandemic (similar to those in 1957 and 1968) with no interventions. Faced with such a threat, the United States and its global partners will have to respond quickly and decisively to protect human health and preserve community functioning.
8 Shrestha SS, Swerdlow DL, Borse RH, Prabhu VS, Finelli L, Atkins CY, Owusu-Edusei K, Bell B, Mead PS, Biggerstaff M, Brammer L, Davidson H, Jernigan D, Jhung MA, Kamimoto LA, Merlin TL, Nowell M, Redd SC, Reed C, Schuchat A, Meltzer MI. Estimating the burden of 2009 pandemic influenza A (H1N1) in the United States. (April 2009-April 2010). Clin Infect Dis 2011;52 (Suppl 1):S75-82.

Planning Assumptions
Given the difficulty associated with estimating timing or impact, pandemic planning is based on the following assumptions about viral epidemiology and human susceptibility:
  • Delays in availability of vaccines and shortages of antiviral drugs are likely, particularly early in the pandemic.
  • The seasonality of a pandemic cannot be predicted with certainty. With seasonal influenza, peak disease usually occurs during December through March in the United States. During the 2009 A(H1N1) pandemic, the first cases were identified in April, and widespread US community outbreaks first began in August, with illness peaking in October 2009, months earlier than is routinely seen with seasonal influenza.
  • The novel virus will have the ability to spread rapidly worldwide.
  • If the pandemic is characterized by severe disease, it will have the potential to disrupt national and community infrastructures (including health care, transportation, commerce, utilities, and public safety) due to widespread illness, absenteeism, and death among workers and their families, as well as concern about ongoing exposure to the virus.
  • Not all jurisdictions will experience clusters of disease simultaneously; however, near-simultaneous clusters likely will occur in many communities across the United States, thereby limiting the ability of any jurisdiction to support and assist other jurisdictions.
  • During a pandemic, infection in a localized area can last about six to eight weeks. At least two pandemic disease waves will occur. Following the pandemic, the newly circulating virus is likely to become a regularly occurring seasonal influenza.
  • Immunity to the novel pandemic influenza subtype will vary based on the strain of the virus, but most people will likely be susceptible, depending on whether a similar strain has circulated in previous seasons.
  • The clinical disease attack rate could range from 20% to 30% of the overall population. Illness rates will likely vary by age group (and other epidemiologic characteristics) and could create selective pressures on segments of the community, such as nursing homes or schools.
  • The typical incubation period (the time between acquiring the infection and becoming ill) for influenza averages two days (range is one to four days).
  • Of those who become ill with influenza, up to 50% will seek outpatient medical care.
  • The number of hospitalizations and deaths will depend on the severity of the disease and the success of steps to mitigate its transmission. Nonetheless, estimates could differ by as much as a factor of 10 between more and less severe scenarios.
  • Risk groups for severe and fatal infections cannot be predicted with certainty. During annual fall and winter influenza seasons, infants and the elderly, people with certain chronic illnesses, people with morbid obesity, and pregnant women are usually at higher risk of complications from influenza infections than other groups. In contrast, in the 1918 pandemic, deaths were notably evident among young, previously healthy adults; in 2009, elderly people were disproportionately spared severe illness and death.
  • People who become infected will shed virus and transmit infection for up to one day before the onset of illness.
  • Viral shedding and the risk for transmission will be greatest during the first two days of illness and may persist for five to seven days.
  • Children will shed the greatest amount of virus and, therefore, are likely to pose the greatest risk for transmission.
  • The most severely ill people with influenza will shed the most virus for the longest period of time.
  • One or two secondary infections will occur as a result of transmission from someone who is ill. In contrast, some estimates from past pandemics have been higher, with up to three secondary infections per primary case.
Because the precise impact of a future pandemic is unpredictable, national, state, and local planners need multiple scenarios for pandemic planning to implement rapid response efforts. The timing and progression of a pandemic, as well as its severity and transmissibility, will determine the mitigation measures selected. HHS will use several possible scenarios for pandemic planning, including a future pandemic that is moderate in severity, a pandemic that is severe, and one that is very severe in its effects on human health. These scenarios will provide a way to plan for use of response measures scaled to different levels of pandemic severity.
Table A.1 provides estimates of illness; outpatient medical care; hospitalizations; intensive care unit care; and deaths for moderate, severe, and very severe influenza pandemics. These estimates are based on scenarios that are unmitigated, meaning that they do not account for public health interventions that would likely be implemented during a pandemic. For comparison, the 2014–2015 season was at the high end of severity for seasonal influenza, with an estimated 974,206 hospitalizations, due to a significantly drifted influenza A(H3N2) virus for which the vaccine was not effective.

What these numbers don't reflect, but was hinted at in bullet point #4 above (bolded), is that the collateral effects of a severe pandemic can be varied, and severe, and potentially more crippling than the influenza virus itself.

But those are scenarios we'll look at again in a series of future blogs.

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