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The College of Physicians of Philadelphia
1.How do vaccines work? Do they work against viruses and bacteria?
Vaccines work to prime your immune system against future “attacks” by a particular disease. There are vaccines against both viral and bacterial pathogens, or disease-causing agents.
When a pathogen enters your body, your immune system generates antibodies to try to fight it off. Depending on the strength of your immune response and how effectively the antibodies fight off the pathogen, you may or may not get sick.
If you do fall ill, however, some of the antibodies that are created will remain in your body playing watchdog after you’re no longer sick. If you’re exposed to the same pathogen in the future, the antibodies will ”recognize” it and fight it off.
Vaccines work because of this function of the immune system. They’re made from a killed, weakened, or partial version of a pathogen. When you get a vaccine, whatever version of the pathogen it contains isn’t strong or plentiful enough to make you sick, but it’s enough for your immune system to generate antibodies against it. As a result, you gain future immunity against the disease without having gotten sick: if you’re exposed to the pathogen again, your immune system will recognize it and be able to fight it off.
Some vaccines against bacteria are made with a form of the bacteria itself. In other cases, they may be made with a modified form of a toxin generated by the bacteria. Tetanus, for example, is not directly caused by the Clostridium tetani bacteria. Instead, its symptoms are primarily caused by tetanospasmin, a toxin generated by that bacterium. Some bacterial vaccines are therefore made with a weakened or inactivated version of the toxin that actually produces symptoms of illness. This weakened or inactivated toxin is called a toxoid. A tetanus immunization, for example, is made with tetanospasmin toxoid.
2.Why aren’t all vaccines 100% effective?
Vaccines are designed to generate an immune response that will protect the vaccinated individual during future exposures to the disease. Individual immune systems, however, are different enough that in some cases, a person’s immune system will not generate an adequate response. As a result, he or she will not be effectively protected after immunization.
That said, the effectiveness of most vaccines is high. After receiving the second dose of the MMR vaccine (measles, mumps and rubella) or the standalone measles vaccine, 99.7% of vaccinated individuals are immune to measles. The inactivated polio vaccine offers 99% effectiveness after three doses. The varicella (chickenpox) vaccine is between 85% and 90% effective in preventing all varicella infections, but 100% effective in preventing moderate and severe chicken pox.
3.Why are there so many vaccines?
Currently, the U.S. childhood vaccination schedule for children between birth and six years of age recommends immunizations for 14 different diseases. Some parents worry that this number seems high, particularly since some of the diseases being vaccinated against are now extremely rare in the United States.
Each disease for which vaccinations are recommended, however, can causes serious illness or death in unvaccinated populations, and might quickly begin to appear again if vaccination rates dropped. The United States has seen mumps outbreaks in recent years since vaccination rates have dropped, with severe complications and hospitalizations required for some patients. And before the introduction of the Hib (Haemophilus Influenzae Type b) vaccine, Hib meningitis affected more than 12,000 American children annually, killing 600 and leaving many others with seizures, deafness, and developmental disabilities. After introduction of the vaccine, the number of deaths from Hib dropped to fewer than 10 per year.
Each vaccine on the schedule continues to be recommended because of the risks posed by wild infection.
4.Is natural immunity better than vaccine-acquired immunity?
In some cases, natural immunity is longer-lasting than the immunity gained from vaccination. The risks of natural infection, however, outweigh the risks of immunization for every recommended vaccine. For example, wild measles infection causes encephalitis (inflammation of the brain) for one in 1,000 infected individuals. Overall, measles infection kills two of every 1,000 infected individuals. In contrast, the combination MMR (measles, mumps and rubella) vaccine results in a severe allergic reaction only once in every million vaccinated individuals, while preventing measles infection. The benefits of vaccine-acquired immunity extraordinarily outweigh the serious risks of natural infection. (For more on this topic, see our Understanding Risks activity.)
Additionally, the Hib (Haemophilus influenzae type b) and tetanus vaccines actually provide more effective immunity than natural infection.
5.Why do some vaccines require boosters?
It’s not completely understood why the length of acquired immunity varies with different vaccines. Some offer lifelong immunity with only one dose, while others require boosters in order to maintain immunity. Recent research has suggested that the persistence of immunity against a particular disease may depend on the speed with which that disease typically progresses through the body. If a disease progresses very rapidly, the immune system’s memory response (that is, the “watchdog antibodies” generated after a previous infection or vaccination) may not be able to respond quickly enough to prevent infection—unless they’ve been “reminded” about the disease fairly recently and are already watching for it. Boosters serve as a “reminder” to your immune system.
Research is continuing on the persistence of immunity generated by vaccines
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