Several studies have assessed gender differences in susceptibility to a variety of infectious diseases, including malaria, HIV, parasitic disease, and influenza. While some early studies suggest that rates of invasive pneumococcal disease (IPD) may be higher in males than in females, few have attempted to characterize the role of gender in the incidence of IPD. Immunologic and environmental factors contributing to IPD risk may also differ by gender and age. For a study published in The Journal of Infectious Diseases, William Schaffner, MD, and colleagues examined gender differences in rates of IPD as well as trends in these rates by age and race following introduction of pneumococcal conjugate vaccines (PCVs).


Characterizing IPD Rates

Following introduction of the 7-valent PCV (PCV7) in 2000, overall IPD and pneumonia rates substantially decreased for vaccinated and unvaccinated children, and racial disparities in pneumococcal diseases declined. Studies in recent years indicate a further decrease in IPD and pneumonia rates as well as racial disparities following introduction of the 13-valent PCV (PCV13) in 2010. Dr. Schaffner and colleagues used the CDC’s Active Bacterial Core surveillance data from 1998 to 2013 for more than 8,000 patients in Tennessee to better understand the impact of these vaccines on potential gender differences in IPD rates.

While introduction of PCV7 nearly eliminated differences in IPD rates by geography, age, and race, gender differences have persisted, although in a muted fashion, according to Dr. Schaffner. Throughout the study period, IPD rates were higher among males than females, especially among children younger than 2 and adults aged 40 to 64, with males having IPD rates 1.5 to 2 times higher than females. Although the introductions of PCV7 and PCV13 were associated with declines in IPD rates in both genders, IPD rates following PCV13 introduction were still significantly higher in males among children, adults aged 40 to 64, and adults older than 74. Similarly, while racial differences in IPD rates diminished substantially with the introduction of PCV7, they still existed (Figure). “For almost all race/age categories we studied, males had higher rates than females,” adds Dr. Schaffner.

“Children have many pneumococcal types in their nasopharynx and are the great distributors of these types among themselves and to their elders,” says Dr. Schaffner. “Eliminating these seven types through use of PCV7 essentially eliminates nasopharyngeal carriage among children and indirectly protects their elders. Even though only children are vaccinated, invasive pneumococcal disease due to those seven types has also diminished in middle-aged and older adults, reinforcing the notion that children, with their nasopharyngeal carriage, are the great distributors of pneumococci.”

Males tend to be the weaker gender for a variety of illnesses and mortality during infancy and early childhood, explains Dr. Schaffner. “However, because women tend to have closer relationships with children than do men in our society,” he says, “I anticipated that adult women might be more affected with IPD than adult men, but that clearly wasn’t the case. It’s not the epidemiologic association that’s causing this disparity; there must be a biological reason why men continue to be more susceptible to IPD, but there’s no clear explanation. Whether adult women have more immunity to IPD because of their earlier, and more intense, exposure to children when compared with men is a reasonable hypothesis that ought to be pursued.”


Important Implications

Dr. Schaffner hopes that epidemiologic studies like this will stimulate investigations into the basic biological reasons for gender differences in IPD susceptibility. “Like anything else, if we can understand these differences, it might be a route to developing further preventative measures, or even therapeutic measures, that can help us improve overall quality of life,” he says. “Clinicians shouldn’t get lost in the nuances of our findings and overlook the success of PCV7 and PCV13 in reducing IPD rates, not only in immunized children but, indirectly, in older persons. Pediatricians and family physicians are universally vaccinating children with PCV13 and need to continue doing so to solidify and continue this public health benefit.”

In children, rates of PCV13 use are over 90%, says Dr. Schaffner. “In adults, depending on the risk group and race, rates are about 25% to 40%. And only a small percentage of those aged 65 and older have received the currently recommended schedule of two pneumococcal vaccinations. It’s a complicated schedule, so clinicians should refer to the CDC’s recommendations. A lot of attention should be directed to appropriate pneumococcal vaccination of adults.”