H1N1 vaccine: Too little too late?

October 23, 4:35 PMMilitary Health ExaminerKaren Estrada

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According to a study at Purdue University conducted by professors Sherry Towers and Zhilan Feng (respectively, from Purdue's statistics and mathematics departments), the H1N1 vaccine will arrive too late to help most Americans who may possibly contract the virus during this flu season.

The study was published in Eurosurveillance (15 Oct) , a scientific journal devoted to epidemiology and the surveillance and control of communicable diseases.
The authors wrote in their study: "The model predicts that there will be a significant wave in autumn, with 63% of the population being infected, and that this wave will peak so early that the planned [U.S. Centers for Disease Control and Prevention] vaccination campaign will likely not have a large effect on the total number of people ultimately infected by the pandemic H1N1 influenza virus.

Even though the vaccination program has barely begun in the United States," the authors also stated during this time period (18 – 24 Oct-approx) the greatest number of people would become infected with H1N1" (also known as Swine Flu). However, the lead study author cautioned that people should still get vaccinated as the vaccine becomes available in case some of the study data is wrong." She also commented that the press is creating “a lot of hysteria” when “in reality, the overwhelming majority of people getting sick are going to have mild illness.” Study results indicated that while a majority of the U.S. population would become infected by the virus, previous studies "showed that up to 60% of seasonal influenza infections are asymptomatic." Meaning, those infected with the H1N1 virus will show little to no symptoms. "If the same is true of the current pandemic influenza, about a quarter of the population will fall ill," the authors wrote.

The following statistical assumptions and predictions + errors could lead the study to underestimate the number of infections.
• The study used data only though the week ending 22 Aug.
• The planned CDC vaccination program against pandemic H1N1 influenza would have begun with six to seven million doses being delivered by the end of the first full week in October. (In reality, according to the CDC, by 09 October about 6.8 million doses were available and 3.7 million doses had been ordered).
• Shipments of the vaccine were down by a quarter, because of a slowdown in production caused by difficulties of processing great numbers.
• That all adults would achieve full immunity shortly after receiving inoculations.
• That children and adults are equally responsive to the flu vaccine.
The study was based on a statistical analysis of weekly reports from the CDC for a model derived from the observed spread of the H1N1 virus during the summer (2009), to predict pandemic behavior during autumn (2009).

A White House released the following figures in late August that had predicted:
• Anywhere from 30 - 50 percent of the population would get the H1N1 virus strain.
• Estimated number of deaths from 30,000 to 90,000.
• The regular seasonal flu normally affects up to approximately 20 percent of the population, with approximately 36,000 deaths resulting.

Dr. Thomas Frieden, the Director of the CDC, contended that at the time the study was conducted, such a scenario was unlikely unless the virus changed (mutated) into another form. To date, this has not occurred. The study formula (SIR Model) used by the Purdue scientists predicted the higher rates of infection and illness because of how it adjusts the rates of infection spread per ‘seasonal forcing’ (a virus more infectious in the fall and winter in the northern hemisphere than it is in the summer).

Towers S, Feng Z. Pandemic H1N1 influenza: predicting the course of a pandemic and assessing the efficacy of the planned vaccination programme in the  United States. Euro Surveill. 2009;14(41):pii=19358. Available online: http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=19358 Date of submission: 08 October 2009 [Accessed 23 Oct 2009].  Washington Times-Online. Study: H1N1 vaccine too late to help most. by Ann Geracimos. Published 20 Oct 2009. http://www.washingtontimes.com/news/2009/oct/20/study-swine-flu-vaccine-too-late-to-help-most/ [Accessed 23 Oct 2009].

Resources:
Centers for the Disease Control. Seasonal Influenza (Flu). Questions & Answers: Seasonal Influenza. http://www.cdc.gov/flu/about/qa/disease.htm [Accessed 23 Oct 2009].

U.S. Department of Health & Human Services. HHS.gov. Image & Icon Library: HHS Flu Widgets (Eng/Spa). http://www.hhs.gov/web/library/hhsfluwidgets.html#flugovwidgetmini [Accessed 23 Oct 2009].

Centers for the Disease Control. Seasonal Influenza (Flu). Info for Health Professionals.  Flu Activity & Surveillance: Reports & Surveillance Methods in the United States. http://www.cdc.gov/flu/weekly/fluactivity.htm [Accessed 23 Oct 2009].