Do Women Need Such Big Flu Shots?

By Sabra L. Klein

The emergence of the H1N1 swine flu has added urgency to what has become an annual ritual for millions of Americans: getting a flu shot. The good news is that scientists have developed a vaccine against the H1N1 virus. But it is taking much longer than expected to produce the hundreds of millions of doses the government had planned to distribute. And it is still too soon to know how effective the vaccine will be in preventing swine flu.

In all likelihood, we’d have a better H1N1 vaccine — and more of it — if in our preparations we had accounted for the biological differences between men and women.

Under the current guidelines, men and women are to get equal-sized doses of the H1N1 vaccine. Yet women’s bodies generate a stronger antibody response than men’s do, research shows, so less vaccine may be needed to immunize them. If we could give women a smaller dose, there would be more vaccine to go around. And we might also spare them the mild side effects that vaccines can cause, like injection site pain, inflammation and fever. All of these are more common in women than in men.

To be clear, it is essential that women get their flu shots, even if the current dose is more than they need. Pregnant women in particular should make sure they are protected, since those who get the swine flu are at least four times as likely as others with the virus to be hospitalized. (We don’t know whether pregnant women are more susceptible to the H1N1 virus, but it’s clear that once they are infected, they have a higher-than-average risk of complications.)

But ultimately we should do everything we can to vaccinate as many people as possible.

Many clinical studies have shown that men and women differ in their responses to several viral vaccines. A recent one demonstrated that women produce as many antibodies in response to a half dose of the seasonal flu vaccine as men make in response to a full dose. Other studies have revealed similar sex differences in response to vaccines for yellow fever virus, measles, mumps and rubella, hepatitis A and B viruses and herpes simplex virus.

Whether vaccines work differently in males than in females is not known.

Clearly, more research on sex-dependent immune responses is needed.

In planning for the swine flu, however, public health authorities ignored the evidence that vaccines affect women more strongly than men. To determine the proper dose of H1N1 vaccine, the National Institutes of Health set up studies involving 600 children, from babies to teenagers — but neglected to investigate whether males and females should get the same dose.

Although our public health authorities remain confident they will eventually have enough vaccine for all Americans who want it, there almost certainly won’t be enough for all the vulnerable populations in poorer countries. Only a handful of countries have plants to manufacture influenza vaccine, and the world’s wealthiest countries have locked up most of what these plants can produce with signed purchasing contracts. We could make much more — and potentially save millions of lives — if we stopped giving women larger doses than they need.

Sabra L. Klein is an assistant professor of molecular microbiology and immunology at the Johns Hopkins Bloomberg School of Public Health. Society for Women’s Health Research is a national non-profit based in Washington, D.C.