Vaccine and Infectious Disease Division

Modeling pandemic flu

New modeling and epidemiological data have revealed that vaccinating approximately 70 percent of the U.S. population may stem the tide of pandemic H1N1 flu, prioritizing children as first recipients of the vaccine as they are the most aggressive spreaders of the disease. However, the report by Center and University of Washington scientists, led by VIDI member Dr. Ira Longini, concluded that vaccinations would need to start in September to be most effective. H1N1 vaccines were not available to the public until mid-October in most parts of the U.S., and supply has been slow.

Using data from past flu outbreaks estimated to be similar to pandemic H1N1, including the spread of Asian influenza A in 1957-58, and early data from 2009 pandemic H1N1 spread, the scientists mathematically modeled the likely behavior of pandemic H1N1 in early fall 2009. The disease would likely begin spreading in September, peaking in October, their report said.  Although a 70 percent vaccination rate would avoid a pandemic this flu season altogether, vaccination rates of 30 or 50 percent would do significant good in dampening and delaying the disease peak.

If the disease peaked later than their predictions, starting vaccination in October may still be effective. The scientists also found that pandemic H1N1 is highly transmissible, with each infected person going on to infect between 1.3 to 1.7 other people on average, and has a short incubation time – once infected, a person will infect another in 2.7 to 3.2 days on average.

The Transmissibility and Control of Pandemic Influenza A (H1N1) Virus. Yang Y, Sugimoto JD, Halloran ME, Basta NE, Chao DL, Matrajt L, Potter G, Kenah E, Longini IM Jr. Science. 2009 Sep 10.

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