Vaccination against infectious diseases is perhaps the most important reason why millions of additional persons do not succumb with morbidity and mortality from viral and bacterial infections in the modern world. Vaccines are most effective when they are administered with sufficient distribution and frequency to protect as many people as possible.
In the July 23, 2009 issue of the New England Journal of Medicine (N Engl J Med 2009;361:335-44, there appeared an article reporting a study by Dipika Sur, MD and colleagues entitled “A Cluster-Randomized Effectiveness Trial of Vi Typhoid Vaccine in India.” The premise of the study was that typhoid fever, caused by infection with the bacteria Salmonella enterica serotype typhi (S. typhi), causes up to 600,00 deaths per year, mostly in developing countries. Injectable Vi polysaccharide vaccine has up to this time been used in a limited fashion in public health programs, and there have been unanswered questions about its effectiveness in children (ages 2 to 5 years) and in particular its use to cause “herd” immunity (e.g., if it is given to a large population living in close proximity, will it promote immunity in the nonimmunized “herd” of people).
The trial was performed by administering a dose of either Vi typhoid vaccine or hepatitis A vaccine (the latter was the “control” group) to 37,673 subjects ages 2 years or older. The vaccines were administered to subjects living in discrete geographic clusters. The results showed that the immunized (with Vi typhoid vaccine) persons, including young children, were protected from typhoid to a significant degree by vaccination. In addition, non-immunized (with Vi typhoid vaccine) persons were also protected to a significant degree, thereby demonstrating the presence of herd immunity. So, the Vi typhoid vaccine was effective in young children and also protected unvaccinated neighbors of Vi vaccinees. Furthermore, side effects were deemed to be minimal. The protection was greatest in children under the age of 5 years.
The authors concluded that the Vi vaccine provides a certain degree of herd immunity, so that future public health deliberations that involve consideration of herd protective effects with this particular vaccine should be informed by this particular study.
This post, Vi Typhoid Vaccine: Safe And Effective For Young Children, was originally published on Healthine.com by Paul Auerbach, M.D..