“Do Women Need Such Big Flu Shots?” suggests that we would have more doses of influenza 2009 H1N1 vaccine if we accounted for the biological differences between men and women. The idea is that women generate a stronger antibody response than men, and therefore require less vaccine. Does this idea have scientific support?
The opinion is based in part on a study carried out in 2004-05, in which adults were immunized with full (15 micrograms) or half-doses of trivalent inactivated influenza vaccine. This vaccine, made by Aventis Pasteur, contains influenza H3N2, H1N1, and B strains. Serum samples obtained before immunization and 21 days later were assayed for antibody response to each strain of influenza by hemagglutination-inhibtion. I’ve taken the data on geometric mean serum HI titers according to age, sex, and dose and plotted them on a graph:
Based on the results the authors conclude that “Significantly higher geometric mean titer responses in women were identified for all ages, regardless of dose or influenza strain. Half-dose vaccination may be an effective strategy for healthy adults younger than 50 years in the setting of an influenza vaccine shortage.” But are these immune responses protective?
HI titers of 1:40 or more (which would be reported as 40 or higher in the graph) are believed to indicate levels of antibody that would protect against infection with influenza virus. By this criteria, the full and half dose of vaccine would provide protection agains the influenza H3N2 and B viruses in both men and women. The results confirm that females respond more strongly to the same dose of vaccine than men. But look at the results with the H1N1 strain – in all subjects, no matter the dose of vaccine or gender, the antibody response would not be sufficient to protect against infection. Furthermore, the response is only slightly better than in women.
I interpret these observations to mean that the antibody response to inactivated influenza virus vaccine is not universally more robust in women compared with men – it appears to depend on the virus strain. Clearly clinical studies are required to address this question. Even after spending millions of dollars to decide whether to give women less influenza vaccine, a new strain of influenza virus might come along that induces no better antibody response in women than in men.
My conclusion is that it would not be possible to determine conclusively that women could receive half the amount of inactivated influenza virus vaccine as men. I would rather spend money on developing new ways to produce as much influenza vaccine as needed as quickly as possible – such as by making virus-like particles in plants.
Engler RJ, Nelson MR, Klote MM, VanRaden MJ, Huang CY, Cox NJ, Klimov A, Keitel WA, Nichol KL, Carr WW, Treanor JJ, & Walter Reed Health Care System Influenza Vaccine Consortium (2008). Half- vs full-dose trivalent inactivated influenza vaccine (2004-2005): age, dose, and sex effects on immune responses. Archives of internal medicine, 168 (22), 2405-14 PMID: 19064822