Abstract
Vaccine-induced Immunity Not Permanent
Generally, 1 to 2 weeks after a person is vaccinated (or has completed a vaccine series: some diseases require several inoculations to build full immunity), his or her immune system will have produced enough antibodies to ward off future attacks from that organism. For example, if a person who has received the hepatitis B vaccine series is exposed to hepatitis B virus, he or she cannot be infected by the virus, because the antibodies generated by the vaccine block the ability of the virus to infect.
Over time, however, the immunity of the vaccinated person may begin to decrease as the time from the last vaccination increases, and, in some cases, the person's immunity may fall below a critical level, thus opening the door for an infection to occur. The time frames vary for different conditions, but this lessening of immunity usually occurs over the course of several years.
Remaining partial immunity may protect a person from contracting a severe case of the illness, but that individual is still able to serve as a host for the microbe and may be able to pass it on to others who are susceptible because they are unvaccinated for any of a number of reasons, including age (ie, being too young to be fully immunized) or personal or parental choice.
Whooping Cough
In recent months headlines have announced the resurgence of whooping cough (or pertussis) in several states. While most cases are not severe, several children in California have died from the disease, and several other states are seeing record numbers of cases. 1 Pertussis is a disease that many people consider a relic of history made irrelevant by modern vaccination. However, pertussis is still with us, and several factors explain its resurgence.
100 Days of Cough
Whooping cough is a contagious disease that is contracted from other people by inhaling the bacterium Bordetella pertussis, which is expelled when a person with pertussis coughs. Both children and adults can be infected with the bacteria and can develop symptoms. 2
Whooping cough is characterized by a protracted illness whose most common feature is a chronic cough, which can last so long it is often referred to as a “hundred day cough.” At times, the cough can be so violent that ribs can be fractured during coughing spells. Vomiting after coughing spells also is common.
The more severe manifestations of whooping cough—including pneumonia, which can be fatal—are primarily restricted to the very young. The classic “whoop”—a sound emitted during inhalation after a fit of violent coughing—may or may not be heard and is more characteristic of the disease in children and infants than in adults. 3
Vaccination to Combat Pertussis
Pertussis immunity requires a series of 3 initial vaccinations, followed by 3 booster vaccinations before age 12. The first is administered to children aged 2 to 6 months in the same shot with tetanus and diphtheria vaccines (the DPT shot). After completing the series, the child is presumed to be immune. Recently, however, it has been found that the immunity induced by the childhood series of vaccinations does not endure through adulthood and can fall below the level needed to protect against infection. Thus, beginning in 2007 it was recommended that adults under age 65 should receive a booster shot against pertussis with their next tetanus/diphtheria shot, which is usually given every 10 years. 3 More recently, clinicians have been advised to move beyond the age 65 cap and offer the vaccine to all their patients to fully augment adult immunity to pertussis. 4
A Perilous Combination
When adults are not fully immunized and contract the illness, they serve as a means for the pathogen to find new victims who are not immune. These nonimmune individuals include babies too young to be fully immunized—especially those younger than 2 months of age who have not yet received any pertussis vaccinations—as well as those with only partial immunity because they have not yet received the full series of vaccinations, other adults with lapsing immunity, and those who have not been vaccinated for personal/parental reasons. If pertussis finds an infant as its new host, it can cause an illness that may ultimately be lethal.
The Resurgence of Mumps
The viral disease mumps is characterized by swollen salivary glands, and its most serious complications include sterility and meningitis. Mumps, like pertussis, is a fully vaccine-preventable illness, and full immunity was thought to occur after a series of 2 vaccinations by age 6. However, several recent outbreaks in the U.S. have raised the question of whether a third dose would lessen the chance of outbreaks caused by waning immunity and gaps in vaccination. In one outbreak, 75% of those who contracted mumps had received the recommended 2-dose regimen. Although some locales have offered a third dose of vaccine, the official recommendation remains at 2 doses. 5
Herd Immunity
Not everybody has to have been vaccinated to prevent sustained spread of a contagious disease. When vaccination levels in a population reach about 70%, and there are too few susceptible members left to be infected by a specific microorganism, it is said that the population has “herd immunity.” Herd immunity prevents large outbreaks from occurring because the likelihood that a pathogen will find someone who it can infect is small as the majority of people it comes into contact with are already immune. However, if herd immunity has faltered—through waning immunity or failure to vaccinate—susceptible hosts will abound, as is the case with pertussis.
Vaccination Rates Must Remain High
In order to ensure that diseases that have been controlled by vaccination do not recur and spark new epidemics, it is essential to understand that the effects of vaccines administered in childhood have limits and may need boosting. It is equally important to realize that vaccination is a crucial part of promoting the health of the public and that any pockets of the populace that are not immunized will be exploited by pathogens. Therefore, a personal decision to refuse vaccination has implications beyond the individual. When a person declines vaccination, it increases the risk of infection for everybody else, including those who are not or cannot be vaccinated.