What Is a Vaccine?
The following is adapted from Immunization Information by Martin G. Myers.
Vaccination is a deliberate attempt to interfere in our interactions with pathogens (microorganisms that cause infections). This differs in a big way from just guessing that something might work, such as eating nutmeg to prevent bubonic plague, as was done in the Middle Ages. As we began to understand our complex relationships with specific pathogens — how the pathogens get to us, how they get past our initial defenses, how our bodies try to defend us against them, how they try to avoid our immune systems, and so on — it became feasible for us to intervene, preventing many illnesses and the epidemics they caused. For example, sanitation, clean water, and the pasteurization of milk greatly reduced our exposure to the bacteria that cause plague, typhus, cholera, typhoid fever, brucellosis (undulant fever), tuberculosis of the gut, and many, many other infections.
Vaccines not only prevent infection in the person who has been immunized, but some vaccines can prevent society’s exposure to a pathogen by interrupting the transmission of the pathogen from person to person — we call that community immunity. Some vaccines prevent the manifestation of the infection when it occurs, while other vaccines prevent the infection from occurring in the first place. Some vaccines interrupt many of the steps of our interaction with the pathogen.
What Should a Vaccine Do?
With each infectious disease, there may be a number of ways to try to tilt the balance in our favor. Once we understand the dynamics of exposure to a pathogen, how infection progresses, and how our immune systems respond, we can consider whether we want to prevent exposure to the pathogen, infection of the individual, illness in the individual (or someone else), and/or the spread of the infecting organism between individuals.
Take the hepatitis A vaccine for example. That vaccine is given to young children largely to prevent transmission of the virus to older children and adults. Hepatitis A is a food-borne virus common in many parts of the world. It usually infects young children, who generally have no symptoms when infected. However, they often transmit the virus to older family members, who may develop severe hepatitis illness. Tetanus vaccine, however, does not reduce our exposure to the bacterium (which is in soil and does not spread between people), but it protects against the toxin the bacterium makes. Haemophilus influenzae type b vaccine, on the other hand, protects against disease by preventing infection (and therefore illness) of the child, but, as a bonus, it also decreases the spread of the bacterium from person to person.
Teaching the Immune System: How Vaccines Work
Every day, our bodies are exposed to a multitude of microorganisms in the air we breathe, the food we eat, and the objects we touch. When one of those microorganisms is a potential disease-causing pathogen, our innate immune system activates to mount an immediate defense. We recognize the “wars” between some of these pathogens and our innate immune system. During this initial battle, the adaptive immune system begins to learn and better prepare for the next encounter with that pathogen. This is why someone who has recovered from measles is immune when exposed again. The immune system acquires memory so that the next time it encounters the virus that causes measles, the virus is quickly destroyed. Influenza A viruses, however, alter their surface antigens so they will be different for the next encounter with our immune systems, which is why the components of the influenza vaccine have to be changed so frequently.
Vaccines are a deliberate attempt to prevent disease by “teaching” the immune system to recognize an antigen, or the whole pathogen, so that when that pathogen makes an appearance, the acquired immune mechanisms are already in place. The vaccine is intended to make the immune system respond as if it has previously encountered this pathogen — the adaptive immune system has already been armed.
A vaccine may contain an inactivated (killed) pathogen, a weakened pathogen (called attenuated), or pieces of the pathogen (one or more antigens). These are intended to trigger an adaptive immune response without causing the disease. Then, when the vaccinated person encounters the real pathogen, the adaptive immune system is ready to attack.
Community Immunity
Community immunity, the indirect protection of people susceptible to the pathogen, is a critical aspect of how some vaccines protect us against some infections. When people have adaptive immunity to a pathogen, they are much less likely to develop the illness; plus, they are less likely to transmit the pathogen to others. Reduced pathogen transmission protects those in the community who are not immune, such as a child who is too young to have responded to the vaccine or someone whose immune system is unable to respond to that pathogen, by preventing the transmission of the pathogen to them.
Community immunity is much more important for some infections than others, which means a neighbor’s decision not to immunize their child could impact your family differently for each of the infectious diseases that vaccines prevent. For example, whether or not everyone in school has been immunized against tetanus will not affect the other students because tetanus does not spread person to person, but it is important for the health of every student that they are immune to pathogens such as measles. The role that community immunity plays in protecting your family differs for each pathogen due to factors such as their developmental age.
Generally, community immunity becomes more effective as the proportion of immune people in the community increases because as more people in the community become immune, the likelihood of the pathogen spreading to a susceptible person decreases. The proportion that must be immune to prevent an outbreak varies from pathogen to pathogen. For example, prevention of a measles outbreak — the most communicable of all the pathogens — requires a very high proportion of the community (95 percent or higher) to be immune. In contrast, mumps virus is somewhat less communicable, requiring about 85 percent to be immune for community protection. For tetanus, only the person with a wound must be immune. Since everyone gets a wound through their skin at some point in life, everyone needs tetanus immunity.
For more information on immunization you can find Immunization Information on Amazon.
Dr. Martin Myers is a retired professor of pediatric infectious diseases. The former Director of Pediatric Infectious Diseases at the University of Iowa and then the Cincinnati Children’s Hospital, he spent much of his career working with and teaching about vaccines.
He served as Chairman of Pediatrics at Northwestern University, Director of the National Vaccine Program Office in the US Department of Health and Human Services, and Director for Education and Policy of the Sealy Center for Vaccine Development at the University of Texas Medical Branch. From 2003 until 2015, he ran a vaccine information website for the National Network for Immunization Information.
