Flu Shot Targets Should Be Kids, Parents

Aug. 20, 2009— -- The best way to control a flu outbreak is to vaccinate children and their parents, researchers said -- at least according to a mathematical model.

The reasoning is that children are responsible for most transmission of influenza and their parents are the first adults to be infected, according to Jan Medlock and Alison Galvani of the Yale University School of Medicine in New Haven, Conn.

Once parents are infected, the outbreak begins to spread throughout the community, they argued in the online edition of the journal Science.

To avert such an outcome, they said, children ages 5 to 19 and adults 30 to 39 should be a priority for vaccination, they said.

The finding demonstrates the need to incorporate transmission patterns into influenza planning, the researchers said.

And one implication of the study is that vaccine recommendations from the U.S. Centers for Disease Control and Prevention -- which for seasonal flu emphasize children under five and the elderly -- "performed substantially worse than our optimal strategies for all outcome measures."

The agency's recommendations for the current pandemic flu -- to give priority to people ages six months to 25 years and exclude the elderly -- are better but still not optimal, the researchers said.

The study reports on a model that includes both contact and mortality data from influenza pandemics and looks for the best way to use vaccine to improve five outcome measures: deaths, infections, years of life lost, contingent valuation and economic costs.

The researchers considered the mortality pattern from the 1918 pandemic -- in which infants and young adults suffered the highest mortality -- as well as from the 1957 pandemic, in which the elderly bore the greatest burden.

The mortality pattern in 1957 was similar to that seen during seasonal flu outbreaks.

For both patterns -- as long as at least 37 million doses of vaccine were available -- targeting the two age groups gave the best results on all five outcome measures, the researchers said.

Vaccination Strategy Could Decrease Supply Needed

Currently, 85 million doses are administered annually for seasonal flu.

A key parameter for a flu outbreak is the so-called basic reproductive number, R0, which measures how efficiently the virus moves through a population.

For the current H1N1 pandemic strain, the number is estimated at 1.4, which is similar to those of the seasonal flu.

In the model, as long as at least 40 million vaccine doses are available for an outbreak with R0=1.4, the entire population would be better protected by vaccinating schoolchildren, the researchers said.

For example, they said, if the mortality pattern was similar to the 1957 pandemic, the new CDC policy for seasonal influenza led to 83 million infections, 153,000 deaths, and $86 billion in cost.

The CDC policy for swine-origin influenza resulted in 59 million infections, 139,000 deaths, and $67 billion in cost, according to the models.

In contrast, vaccinating kids and parents resulted in only 44 million infections, 108,000 deaths, and a total cost of $53 billion, they said.

For the 1918 mortality pattern, the new CDC seasonal-influenza policy led to 83 million infections, 1.2 million deaths, and $1.4 trillion in cost and the CDC policy for swine-origin influenza produced 59 million infections, 853,000 deaths, and $939 billion in cost.

"Our proposed optimal strategies resulted in 44 million infections, 645,000 deaths, and a cost of $703 billion," the researchers said.