Last updated 22 April 2022
Our most frequently asked questions. Expand for detailed answers from experts.
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.
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. Therefore, 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.
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 this number seems high, particularly since some vaccine-preventable diseases are now extremely rare in the United States.
Each disease for which vaccinations are recommended can cause serious illness or death in unvaccinated populations, and could 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 the vaccine was introduced, 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.
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 .)
Additionally, the Hib (Haemophilus influenzae type b) and tetanus vaccines actually provide more effective immunity than natural infection.
It is unclear why the length of acquired immunity varies with different vaccines. Some offer lifelong immunity with only one dose, while others require boosters 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 rapidly, the immune system’s memory response (that is, the “watchdog antibodies” generated after a previous infection or vaccination) may not 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 continues on the persistence of immunity generated by vaccines.
The idea of “pox parties” is generally tied to the perception of chickenpox as a harmless illness. Before the varicella vaccine became available, however, chickenpox infections required 10,000 hospitalizations and caused more than 100 deaths each year in the United States. Exposing a child to wild chickenpox puts him at risk for a severe case of the disease.
Even uncomplicated cases of chickenpox cause children to miss a week or more of school, with a caregiver missing work to care for the sick child. Natural infection also means a risk of infecting others: while successful vaccination protects a child against chickenpox without this risk, children infected with chickenpox naturally are contagious. They can spread the disease to other people—not just other children, but also adults, who have a higher risk of complications from the disease.
Meanwhile, vaccination for chickenpox typically prevents future infection with the disease. In rare cases where individuals do not develop adequate protection from vaccination to prevent future infection, chickenpox infection is typically mild, results in fewer symptoms, and ends more quickly than natural infection. (People with this mild form are contagious, however, and should take care not to expose others to the virus.)
Vaccines made with killed versions of pathogens—or with only a part of the pathogen—are not able to cause illness. When a person receives these vaccines, it is impossible for him or her to become ill with the disease.
Live, attenuated (or weakened) vaccines are theoretically capable of causing illness: because they can still replicate (though not well), mutation is possible, which can lead to a virulent form of the pathogen. However, they are designed with this in mind, and attenuated to minimize this possibility. Reversion to virulent form is a problem with some forms of the oral polio vaccine (OPV), which is why only the inactivated form (IPV) is now used in the United States.
It is important to note that attenuated vaccines can cause serious problems for individuals with weakened immune systems, such as cancer patients. These individuals may receive a killed form of the vaccine if one is available. If not, their doctors may recommend against vaccination. In such cases, individuals rely on herd immunity for protection.
Why some vaccines contain live pathogens and others contain killed pathogens, the reasons vary by illness. However, live, attenuated vaccines generally generate longer-lasting immunity than killed vaccines. Thus, killed vaccines are more likely to require boosters to maintain immunity. Killed vaccines, however, tend to be more stable for storage purposes, and can’t cause illness. The medical community must weigh these trade-offs in deciding which approach to use against a particular disease.
Yes. Studies demonstrate that infants’ immune systems can handle receiving many vaccines simultaneously—more than the number currently recommended. The immunization schedule is based on infants’ ability to generate immune responses, as well as when they are at risk of certain illnesses. For example, the immunity passed from mother to child at birth is only temporary, and typically does not include immunity against polio, hepatitis B, Haemophilus influenzae type b, and other diseases that can be prevented by vaccination.
Unlike most vaccines, which contain the most common strains of a given pathogen (if more than one exists) and are rarely changed, the seasonal flu vaccine changes frequently, though one or more flu strains in the vaccine may be retained from one year to the next. This is because the strains of influenza viruses that circulate are constantly changing. Each year, researchers choose viruses for the vaccine based on which ones are likely to circulate over the coming flu season, providing protection against the most prevalent strains. So when you get a seasonal flu vaccine, you’re usually not getting another “dose” of the same flu vaccine you were given before. Instead, you’re usually getting protection against a whole new batch of flu viruses.
Herd immunity, also known as community immunity, refers to the protection offered to everyone in a community by high vaccination rates. With enough people immunized against a given disease, it’s difficult for the disease to gain a foothold in the community. This offers some protection to those who are unable to receive vaccinations—including newborns and individuals with chronic illnesses—by reducing the likelihood of an outbreak that could expose them to the disease.
Some vaccines, including most vaccines against influenza, are cultured in chicken eggs. During the process of creating the vaccine, most egg protein is removed, but there is some concern that these vaccines could generate an allergic reaction in individuals with an egg allergy.
A recent report found that most children with egg allergies who were given a flu shot had no adverse reactions. About 5% of children in the studied group developed relatively minor reactions, such as hives, which resolved without treatment. Additional research is underway to study this issue further.
In most cases, only people with a severe (life-threatening) allergy to eggs are recommended against receiving egg-based vaccines. Your doctor can provide specific information.
No. Vaccines do not cause autism. This possibility was publicized after a 1998 paper by a British physician who claimed to have evidence that the MMR (measles, mumps and rubella) vaccine was linked to autism. The potential link has been thoroughly explored; study after study has found no such link, and , which had originally published it. Studies were also done regarding the possibility of a link between the preservative thimerosal, which is used in some vaccines, and autism; again, no such link was found.
It’s likely that this misconception persists because of the coincidence of timing between early childhood vaccinations and the first appearance of symptoms of autism.
All vaccines have possible side effects. Most, however, are mild and temporary. Adverse effects from vaccines are thoroughly monitored via multiple reporting systems, and there is no evidence from these systems to support these claims.
Every vaccine has potential side effects. Typically they are mild: soreness at the injection site (for a vaccine delivered via a shot), headaches, and low-grade fevers are examples of common vaccine side effects. Serious side effects are possible, including severe allergic reactions. However, these side effects are rare. (Your doctor can explain the risks for individual vaccines in detail; .)
When considering possible side effects from vaccination, it’s important to do so in context. While some possible side effects are serious, they are rare. It’s important to remember that choosing not to vaccinate also has serious risks. Vaccines protect against potentially fatal infectious diseases. Avoiding vaccination raises the risk of contracting those diseases and spreading them to others.
Vaccines are tested repeatedly before being approved, and continue to be monitored for adverse reactions after their release. See our article on vaccine testing and safety for more information and details about this topic.
No. The rubella vaccine virus included in the MMR (measles, mumps and rubella) shot is cultured using human cell lines. The vaccine material is carefully separated from the cells in which it was grown before being used.
Some of these cell lines were generated from fetal tissue obtained in the 1960s from legal abortions. No new fetal tissue is required to generate the rubella vaccine.
Improved hygiene and nutrition, among other factors, can certainly lower the incidence of some diseases. Data documenting the number of cases of a disease before and after the introduction of a vaccine, however, demonstrate that vaccines are responsible for the largest drops in disease rates. Measles cases, for example, numbered anywhere from 300,000 to 800,000 a year in the United States between 1950 and 1963, when a newly licensed measles vaccine went into widespread use. By 1965, U.S. measles cases were beginning a dramatic drop. In 1968, about 22,000 cases were reported (a drop of 97.25% from the height of 800,000 cases in just three years). By 1998, the number of cases averaged about 100 per year or less. A similar post-vaccination drop occurred with most diseases for which vaccines are available.
Perhaps the best evidence that vaccines, not hygiene and nutrition, are responsible for the sharp drop in disease and death rates is chickenpox. If hygiene and nutrition alone were enough to prevent infectious diseases, chickenpox rates would have dropped long before the introduction of the varicella vaccine, which was not available until the mid-1990s. Instead, the number of chickenpox cases in the United States in the early 1990s, before the vaccine was introduced in 1995, was about four million a year. By 2004, the disease incidence had dropped by about 85%.
In theory, nearly any infectious disease for which an effective vaccine exists should be eradicable. With sufficient vaccination levels and coordination between public health organizations, a disease can be prevented from gaining a foothold anywhere. Without anyone to infect, it must die off. (A notable exception is tetanus, which is infectious but not contagious: it’s caused by a bacterium commonly found in animal feces, among other places. Thus, tetanus could not be eradicated without completely removing the Clostridium tetani bacterium from the planet.)
Smallpox is unusual, however, in the characteristics that made it susceptible to eradication. Unlike many other infectious diseases, smallpox has no animal reservoir. That is, it can’t “hide” in an animal population and re-emerge to infect humans, while some diseases can do exactly that (yellow fever, for example, can infect some primates; if a mosquito bites an infected primate, it can transmit the virus back to humans).
Another obstacle to eradication for many infectious diseases is visibility. People with smallpox were highly visible: the smallpox rash was easily recognizable, so that new cases could be detected quickly. Vaccination efforts could be focused on the location of the cases and potential exposure to other individuals. Polio, by contrast, causes no visible symptoms in about 90% of the people it infects. As a result, tracking the spread of the polio virus is extremely difficult, making it a difficult eradication target.
Perhaps most importantly, smallpox patients generally did not reach their highest level of infectivity (that is, their ability to infect others) until after the appearance of the smallpox rash. As a result, quick action to quarantine infected individuals upon the eruption of the rash usually left enough time to vaccinate anyone already exposed, and prevent additional exposures. Many infectious diseases do not allow for this type of reaction time. Measles patients, for example, can become infectious up to four days before the appearance of the measles rash. As a result, they can pass the virus on to many, many other people before anyone even knows they are infected.
Many people still think eradication is possible for certain diseases. Efforts are ongoing to eradicate polio and Guinea worm disease (Dracunculiasis), with both eliminated in many regions, but remaining endemic in several countries. Meanwhile, the Carter Center International Task Force for Disease Eradication has declared additional diseases potentially eradicable: lymphatic filariasis (Elephantiasis), mumps, pork tapeworm, and yaws.
[For more about this topic, see our article on .]
The polio vaccines developed by Jonas Salk and Albert Sabin in the mid-20th century were made with monkey cells. Years later, microbiologist Maurice Hilleman found a monkey virus in both vaccines—the 40th monkey virus to be discovered, which he called Simian Virus 40 (SV40). (Salk’s killed vaccine, which was treated with formaldehyde, had very small amounts of the virus; Sabin’s live vaccine was heavily contaminated.) Worried about the potential effects the virus could have on humans, Hilleman injected it into hamsters, finding that nearly all of them developed massive cancerous tumors. But the initial panic this caused gave way in the face of future studies.
First, hamsters that ingested SV40 instead of being injected with it didn’t get cancer. Sabin’s live vaccine (which contained more SV40 than Salk’s) was given orally. Additional studies showed that children given Sabin’s vaccine did not develop antibodies to SV40; it simply passed through their digestive system, never causing infection.
That left only Salk’s vaccine, which contained very little SV40, but was given by injection. Studies performed eight years, fifteen years, and thirty years after SV40-contaminated vaccines were given to children found they had the same cancer incidence as unvaccinated groups. No credible evidence suggests SV40 has ever caused cancer in humans.
For a discussion on why the polio vaccine is not associated with HIV, read our article discussing this proposed association:
The mRNA vaccines developed in response to the COVID-19 pandemic caused concern among many people who claimed the mRNA technology was “too new” to be considered safe, or that it would be a while before we would know all the risks.
First, the clinical trials to show the vaccine safety and efficacy were carried out in the same manner as other clinical trials for vaccines. They had the same number of participants, same steps, and same oversight. The only aspect of those trials that was different was the timeframe in which they were done. Because of the need for new vaccines to counter to the COVID-19 pandemic, the studies and vaccine development/manufacturing were done simultaneously, not subsequently. Those clinical trials showed that the mRNA vaccines were safe and effective in preventing severe disease.
Second, mRNA technology has been around since the 1990s. So why have they not been used in vaccines? Two reasons: Lack of funding to use them as vaccines against infectious agents, and lack of interest, since the existing licensed vaccines worked well. In 2020, the Trump Administration authorized the use of funds for “Operation Warp Speed” to fund the rapid development of vaccines, taking care of the funding part. Once the mRNA vaccines were shown to be safe and effective against COVID-19, other infectious diseases were targeted for mRNA vaccine development.
- Gever, J. . MedPage Today. (2010) Accessed 01/25/2018.
- Carroll-Pankhurst, C., Engels, E.A., Strickler, H.D., Goedert, J.J., Wagner, J., Mortimer Jr, E.A. . British Journal of Cancer. 2001 Nov;85(9):1295. Accessed 01/25/2018.
- The Carter Center. . (20 KB PDF) Accessed 01/25/2018.
- Centers for Disease Control and Prevention. . Accessed 01/25/2018.
- CDC. . Accessed 01/25/2018.
- Sharp, P.M., Hahn, B.H. . Philosophical Transactions of the Royal Society of London. Biological Sciences. 2010 Aug 27;365(1552):2487-94. Accessed 01/25/2018.
- Worobey, M., Santiago, M.L., Keele, B.F., Ndjango, J.B., Joy, J.B., Labama, B.L., Dhed'a, B.D., Rambaut, A., Sharp, P.M., Shaw, G.M., Hahn, B.H. Origin of AIDS: contaminated polio vaccine theory refuted. Nature. 2004 Apr 22;428(6985):820-.