Perverse effects of vaccination
Encyclopedia
Perverse effects of vaccination occur when a vaccination
program causes more harm than it cures. This can happen if too few are vaccinated, allowing the disease to spread, although more slowly than in an unvaccinated population. This raises the average age of infection, which in some cases can increase the number of serious health problems associated with the disease.
In mathematical modelling in epidemiology
, there is a critical threshold value (denoted qc) at which enough people are immune
to the disease that its spread through the population (even to unvaccinated susceptible individuals) is stopped. This effect is commonly known as herd immunity
. If a vaccination program does not attain qc, its effect is not to prevent the spread of the disease across the unvaccinated population; instead, it delays the spread and so increases the average age at which individuals are infected. This is called an epidemiological shift. As an extreme example, suppose a disease spreads so rapidly that everybody is typically exposed to it once a day; people of all ages will contract the disease. In contrast, if everybody is typically exposed to the disease only once every 30 years, most children will not have the disease. In diseases like rubella
and mumps
, which have an increased severity or risk of complications with increased age, a vaccination program that causes an epidemiological shift can in some cases have the unintended consequence
of increasing the number of deaths and problems caused by the disease, even if it protects vaccinated individuals.
Perverse effects arose in congenital rubella syndrome
(CRS) cases in Greece following the introduction in 1975 of rubella
vaccination for young children. This vaccination program failed because it did not attempt to protect adolescents and young women, and did not attempt to obtain high coverage. The resulting epidemiological shift caused rubella to infect more pregnant women and cause more CRS, showing that rubella vaccination programs should not be halfhearted. A claim has been made that similar perverse effects occurred in the U.S. in the early 1970s, but this misrepresents the overall pattern of U.S. CRS incidence, which fell from an estimated 20,000 in the 1964 epidemic to 7 in 1983, with a large drop in CRS incidence the early 1980s and with rubella eliminated in the U.S. by 2004.
Theoretical models for vaccination programs have found that perverse effects of vaccines are possible in other circumstances. One example can occur if a vaccine targets one tuberculosis
(TB) strain that provides cross-immunity against a non-targeted TB strain that in turn does not provide much immunity.
Vaccination
Vaccination is the administration of antigenic material to stimulate the immune system of an individual to develop adaptive immunity to a disease. Vaccines can prevent or ameliorate the effects of infection by many pathogens...
program causes more harm than it cures. This can happen if too few are vaccinated, allowing the disease to spread, although more slowly than in an unvaccinated population. This raises the average age of infection, which in some cases can increase the number of serious health problems associated with the disease.
In mathematical modelling in epidemiology
Mathematical modelling in epidemiology
It is possible to mathematically model the progress of most infectious diseases to discover the likely outcome of an epidemic or to help manage them by vaccination...
, there is a critical threshold value (denoted qc) at which enough people are immune
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
to the disease that its spread through the population (even to unvaccinated susceptible individuals) is stopped. This effect is commonly known as herd immunity
Herd immunity
Herd immunity describes a form of immunity that occurs when the vaccination of a significant portion of a population provides a measure of protection for individuals who have not developed immunity...
. If a vaccination program does not attain qc, its effect is not to prevent the spread of the disease across the unvaccinated population; instead, it delays the spread and so increases the average age at which individuals are infected. This is called an epidemiological shift. As an extreme example, suppose a disease spreads so rapidly that everybody is typically exposed to it once a day; people of all ages will contract the disease. In contrast, if everybody is typically exposed to the disease only once every 30 years, most children will not have the disease. In diseases like rubella
Rubella
Rubella, commonly known as German measles, is a disease caused by the rubella virus. The name "rubella" is derived from the Latin, meaning little red. Rubella is also known as German measles because the disease was first described by German physicians in the mid-eighteenth century. This disease is...
and mumps
Mumps
Mumps is a viral disease of the human species, caused by the mumps virus. Before the development of vaccination and the introduction of a vaccine, it was a common childhood disease worldwide...
, which have an increased severity or risk of complications with increased age, a vaccination program that causes an epidemiological shift can in some cases have the unintended consequence
Unintended consequence
In the social sciences, unintended consequences are outcomes that are not the outcomes intended by a purposeful action. The concept has long existed but was named and popularised in the 20th century by American sociologist Robert K. Merton...
of increasing the number of deaths and problems caused by the disease, even if it protects vaccinated individuals.
Perverse effects arose in congenital rubella syndrome
Congenital rubella syndrome
Congenital rubella syndrome can occur in a developing fetus of a pregnant woman who has contracted rubella during her first trimester. If infection occurs 0–28 days before conception, there is a 43% chance the infant will be affected. If the infection occurs 0–12 weeks after conception, there is a...
(CRS) cases in Greece following the introduction in 1975 of rubella
Rubella
Rubella, commonly known as German measles, is a disease caused by the rubella virus. The name "rubella" is derived from the Latin, meaning little red. Rubella is also known as German measles because the disease was first described by German physicians in the mid-eighteenth century. This disease is...
vaccination for young children. This vaccination program failed because it did not attempt to protect adolescents and young women, and did not attempt to obtain high coverage. The resulting epidemiological shift caused rubella to infect more pregnant women and cause more CRS, showing that rubella vaccination programs should not be halfhearted. A claim has been made that similar perverse effects occurred in the U.S. in the early 1970s, but this misrepresents the overall pattern of U.S. CRS incidence, which fell from an estimated 20,000 in the 1964 epidemic to 7 in 1983, with a large drop in CRS incidence the early 1980s and with rubella eliminated in the U.S. by 2004.
Theoretical models for vaccination programs have found that perverse effects of vaccines are possible in other circumstances. One example can occur if a vaccine targets one tuberculosis
Tuberculosis
Tuberculosis, MTB, or TB is a common, and in many cases lethal, infectious disease caused by various strains of mycobacteria, usually Mycobacterium tuberculosis. Tuberculosis usually attacks the lungs but can also affect other parts of the body...
(TB) strain that provides cross-immunity against a non-targeted TB strain that in turn does not provide much immunity.