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Is the CDC wrong about who should get the swine flu vaccine first?
Rebecca Goldin, Ph.D, September 22, 2009

Modeling how disease spreads shows that the most vulnerable to H1N1 are those who aren’t being urged to get the vaccine.

When the swine flu first hit the news media, it was as a menacing, deadly virus that was just about to getcha! People bought face masks, schools closed, and emergency rooms were inundated with people worried that they were a sneeze and a cough away from dying. The fear was that H1N1 Influenza – as swine flu is formally called – was going to be another Spanish Flu, bequeathing a pandemic similar to that of 1918, which killed more than 50 million people out of an estimated 500 million people infected. With one-in-ten infected people dying from the flu (and one in three people in the world infected), the 1918 flu pandemic was one of the deadliest viral outbreaks in modern history.

The original estimates of the virulence of the mortality rate of H1N1 were based on the belief that very few people had gotten swine flu. The deaths that occurred were thus attributed to very few infections, and that suggested a very high death rate. Since then, it was discovered that more people had been infected than previously thought, and that the death rate among those infected is approximately one in 200, rather than one in ten. It remains the case that younger, healthier people are more vulnerable to this virus than older people.

And then there is seasonal flu, which contributes to the death of approximately 36,000 people in the United States each year, according to the Centers for Disease Control (CDC), which works out at about one in 800 infections.

Immunization programs are our best defense against both kinds of flu. Currently, about 85 million doses of vaccine are distributed each year during flu season. Following the simple logic that those most vulnerable should be most protected by the vaccine, the Centers for Disease Control has come up with the following strategy to distribute the vaccine to the following people first:

  1. pregnant women
  2. household contacts and caregivers for children younger than 6 months old
  3. healthcare and emergency medical services personnel
  4. all people aged six months to 24 years old
  5. high-risk adults aged between 25-64.

After all these groups are taken care of, the next group should be adults aged 24 to 65, and finally older adults, aged 65 and up are of least priority. For the seasonal flu, the CDC recommendations are similar:

  1. Children aged six months to 18 years old
  2. pregnant women
  3. people age 50 or older
  4. high risk adults
  5. healthcare personnel
  6. caretakers of individuals of high risk, including infants under 6 months old.

From the point of view of who would die if they got the flu, the CDC’s strategy makes sense: for H1N1, young people are more at risk than older people, so we should vaccinate everyone under 25. Infants can’t be vaccinated, so we protect them by vaccinating their caregivers, and healthcare people need to stay healthy to take care of the rest of us, so they should get it. For the seasonal flu, we vaccinate people over 50 before we vaccinate people aged between 19 and 49, because older folk are at higher risk of complications if they become infected.

But the CDC recommendations ignore how these viruses spread. We should not be asking, “how likely is someone to die if they get the virus?” but rather, “how can we minimize the deaths due to the flu?” And for that, it is important to look at the way the virus spreads through the population.

Suppose healthy adults will die from a flu infection in about 1 in 1,000 cases, but an immuno-depressed senior will die in about 1 in 100 cases. The CDC reasons we should protect our elderly. But suppose by immunizing older people that flu spreads widely among younger adults due to social contact, infecting 100,000 people (with an expectation of 100 deaths). On the other hand, suppose if we immunize the younger adults, older people don’t spread it much, and there are only 5,000 infections, with approximately 50 deaths. Even if the death rate among those infected is higher among older people, the total deaths will be fewer if there are fewer people infected.

This was the reasoning of the authors of a recent article in Science after examining survey data and mortality data from the 1918 and 1957 flu pandemics. Like H1N1, the 1918 flu affected young, healthy people more, while the 1957 flu was similar the seasonal flu that tends to target people with compromised immune systems. They employed a statistical technique to find the best vaccination strategy for a variety of outcome measurements, using the survey data to figure out how transmission actually occurs. They also varied how likely you are to infect someone you come in contact with, in case the model depends on how spreadable the disease is.

The findings? The people most likely to spread the disease to others in their age groups are children aged 5-19. These kids should be the main targets for immunization. They are already on the “first serve” list of the CDC, along with other groups that are not the main viral-dispensers. But who do children aged 5-19 spread H1N1 to? Adults aged 30-39. And through these adults, the virus spreads to other adults aged 25 and older. In other words, as a group, adults aged 30-39 are among the most vulnerable to the virus. This is directly in contrast with CDC recommendations, which place young children and immuno-depressed adults of all ages before this group.

If we truly want to prevent a viral outbreak, we should not be worrying about young children, unless perhaps they come into contact with a lot of other kids. We should be worrying first and foremost about our school-aged children, and those they come directly in contact with: their parents. Keep the virus from spreading, and the death rate stays low.


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