Even if you are not vaccinated against a particular illness, you might still be protected, as long as your friends are. This idea, known as herd immunity to infectious disease specialists, is the premise for mass-vaccination campaigns: If enough people are vaccinated, bacteria and viruses will have a hard time spreading, because they will not find people to infect.
A study done by Katherine O'Brien, an associate professor at the Bloomberg School of Public Health, shows that indirect immunity, or herd immunity, significantly decreases the rate of a pneumococcal disease spreading among American Indians living on the Navajo and White Mountain Apache reservations.
Pneumoccus is a common bacterium that causes pneumonia and other ailments. Before the pneumococcal vaccine (PCV) became widely available in 2000, pneumococcal disease was a major cause of pneumonia, meningitis, ear infections and blood stream infections, particularly in children and the elderly.
"Navajo and White Mountain Apache are known to have had high rates of pneumococcal disease compared with the general U.S. population prior to the introduction of PCV," O'Brien said.?"We only work on diseases of specific relevance to the tribes."
O'Brien's current study was carried out in the wake of a separate long-term study of the efficacy of the pneumococcal vaccine among the Navajo and White Mountain Apache tribes. In the previous study, PCV was administered to one group of children and meningococcal vaccine (MCC) was administered as a control to another group of children. O'Brien then tested the family and household members of the two different groups for colonization - an infection that may or may not cause symptoms - by pneumococcal bacteria.
"We had two major findings. The risk of carrying a vaccine serotype pneumococcal strain was 43 percent lower among adults living with PCV-vaccinated children than adults living with children who had not received PCV," O'Brien said.
"Even if a child carries a vaccine serotype strain, the likelihood ?that an adult will also carry that strain is reduced among the households where the children were vaccinated with PCV than in households where the children have not received PCV."
In other words, adults who had not been vaccinated but lived with vaccinated children were approximately half as likely to be infected by the bacteria as adults who lived with children who had not been vaccinated. This herd immunity has been well documented.
For example, even if you have not had chicken pox or have not been vaccinated against it, getting the disease is probably lower on your list of worries than contracting this year's flu. You are protected because almost everyone around you is immune; there is no one who can get chicken pox and pass it on to you.
This means that in order to significantly lower the morbidity rate of a disease, it isn't necessary to inoculate every member of a given community. Just making sure all children receive the vaccine as part of routine vaccinations, for instance, will have a dramatic effect.
PCV is part of the routine childhood immunization schedules in most developed countries and is spreading to developing countries. In the future, O'Brien plans to look further into how vaccination affects the rate and pattern of infection in a community, as well as study at different strains of pneumococcal disease.
Her studies will still focus on the Navajo and Apache living in the southwest. "They continue to have rates of disease and colonization rates that are higher than the general U.S. population," O'Brien said.
However, this study was definitely part of a bigger global picture. "If the global community can realize the successful introduction of PCVs," O'Brien said, "these vaccines have the potential to save millions of lives of children who otherwise would die from pneumococcal disease."
Please note All comments are eligible for publication in The News-Letter.