question: How do vaccines still work after an initial disease exposure?
comment: Ok, first, I'm no physician or anything like that, so take what I say with precautions. There are two situations that could be covered by your question, so let's tackle both. If you're talking about rabbies-like deseases, they have a quite long incubation time (15d) during which the immune system isn't active. That's why if you get bit by a wild animal, you will get a shot which will have time to prepare your body for when the desease will really start spreading. If you're talking about common deseases, the first infection does not always create immunity. Vaccines are enhanced to force as most as possible the process. For most people it's not really useful, but for the rest of the population it will allow them to have an immunity. Vaccines aren't perfect either, they sometimes fail, but we know the statisyics linked to that fact. That's why you still get vaccinated, to increase the herd immunity to a known effective value. Edit: spelling
passages: Section::::Developing immunity.
  
  The immune system recognizes vaccine agents as foreign, destroys them, and "remembers" them. When the virulent version of an agent is encountered, the body recognizes the protein coat on the virus, and thus is prepared to respond, by (1) neutralizing the target agent before it can enter cells, and (2) recognizing and destroying infected cells before that agent can multiply to vast numbers.
   When an individual is vaccinated against a disease, the individual's immune system is triggered and memory B cells store the specific antibody response. These cells remain in circulation the pathogen infection is cleared. Because the telomeres in genes degenerate after each successive cell division, lymphocytes, including memory B cells are not capable of proliferating indefinitely. Typically, the cells live for multiple decades, but there is variation in the longevity of these cells depending on the type of vaccine they were stimulated with and vaccine dosage. The reason for the differences in the longevity of memory B cells is currently unknown. However, it has been proposed that the differences in memory B cell longevity are due to the speed at which a pathogen infects the body and, accordingly, the number and type of cells involved in the immune response to the pathogen in the vaccine.
   Vaccination of humans with AVA induces an immune response to PA. More than 1/3 of subjects develop detectable anti-PA IgG after a single inoculation; 95% do so after a 2nd injection; and 100% after 3 doses. The peak IgG response occurs after the 4th (6 month) dose. The potency of AVA vaccine lots is routinely determined both by the survival rates of parenterally challenged guinea pigs and their anti-PA antibody titres as measured by an enzyme linked immunosorbant assay (ELISA). The shelf-life of AVA is reported to be three years when stored between 2 °C and 8 °C (36 °F and 46 °F) and never frozen.
   First generation vaccines are whole-organism vaccines – either live and weakened, or killed forms. Live, attenuated vaccines, such as smallpox and polio vaccines, are able to induce killer T-cell (T or CTL) responses, helper T-cell (T) responses and antibody immunity. However, attenuated forms of a pathogen can convert to a dangerous form and may cause disease in immunocompromised vaccine recipients (such as those with AIDS). While killed vaccines do not have this risk, they cannot generate specific killer T cell responses and may not work at all for some diseases.
  
  Section::::History.:Subunit vaccines.
   Section::::Manufacturer.:Preparation.
  
  A weakened strain of bovine tuberculosis bacillus, "Mycobacterium bovis" is specially subcultured in a culture medium, usually Middlebrook 7H9.
  
  Section::::Manufacturer.:Dried.
  
  Some BCG vaccines are freeze dried and become fine powder. Sometimes the powder is sealed with vacuum in a glass ampoule. Such a glass ampoule has to be opened slowly to prevent the airflow from blowing out the powder. Then the powder has to be diluted with saline water before injecting.
  
  Section::::History.
   Long-term "active" memory is acquired following infection by activation of B and T cells. Active immunity can also be generated artificially, through vaccination. The principle behind vaccination (also called immunization) is to introduce an antigen from a pathogen in order to stimulate the immune system and develop specific immunity against that particular pathogen without causing disease associated with that organism. This deliberate induction of an immune response is successful because it exploits the natural specificity of the immune system, as well as its inducibility. With infectious disease remaining one of the leading causes of death in the human population, vaccination represents the most effective manipulation of the immune system mankind has developed.
   In most infants, maternal antibodies disappear 12–15 months after birth, so vaccines administered outside this window are not compromised by maternal antibody interference.
  
  Section::::Characteristics.:Biological Causes.:Longevity of Memory B Cells.
   The process of serial passage yields a live vaccine. There are both advantages and disadvantages to this. Most notably, live vaccines are sometimes more effective and more long-lasting than inactivated or other types of vaccines. However, just as the virus evolved to become attenuated, it may reverse-evolve in the host, leading to infection.
  
  Section::::Experiments.
   There are several types of vaccines in use. These represent different strategies used to try to reduce the risk of illness while retaining the ability to induce a beneficial immune response.
  
  Section::::Types.:Inactivated.
  
  Some vaccines contain inactivated, but previously virulent, micro-organisms that have been destroyed with chemicals, heat, or radiation. Examples include the polio vaccine, hepatitis A vaccine, rabies vaccine and some influenza vaccines.
  
  Section::::Types.:Attenuated.
   It would, however, not explain effects occurring shortly after vaccination, as for instance the rapidly occurring beneficial effects of BCG vaccine, as the heterologous effect would only be expected to be present after some weeks, as the adaptive immune response need time to develop. Also, it is difficult to explain why the effect would vanish once a child receives a new vaccine.
  
  Section::::Immunological mechanisms.:Trained innate immunity.
   J.T. van Oirschot and colleagues at the Central Veterinary Institute in Lelystad, The Netherlands.
  
  They found that some existing vaccines against pseudorabies (also termed Aujeszky's disease) had deletions in their viral genome (among which the gE gene). Monoclonal antibodies were produced against that deletion and selected to develop an ELISA that demonstrated antibodies against gE. In addition, novel genetically engineered gE-negative vaccines were constructed.
  
  Along the same lines, DIVA vaccines and companion diagnostic tests against bovine herpesvirus 1 infections have been developed.
  
  Section::::DIVA vaccines.:Use in practice.
   Section::::Administration.
  
  In an attenuated vaccine, live virus particles with very low virulence are administered. They will reproduce, but very slowly. Since they do reproduce and continue to present antigen beyond the initial vaccination, boosters are required less often. These vaccines are produced by growing the virus in tissue cultures that will select for less virulent strains, or by mutagenesis or targeted deletions in genes required for virulence. There is a small risk of reversion to virulence; this risk is smaller in vaccines with deletions. Attenuated vaccines also cannot be used by immunocompromised individuals.
  
  Section::::Advantages.
   The PfSPZ Vaccine candidate has granted fast track designation by the U.S. Food and Drug Administration in September 2016.
  
  Section::::Mechanism.
  
  CD8+ T cells play a key role in killing "Plasmodium" developing in liver. Mice or monkeys which received monoclonal antibody to the CD8 lost protection by this type of vaccine. Once the antibody application was stopped, the protection was returned.
   BULLET::::- Bacterial: typhoid vaccine, cholera vaccine, plague vaccine, and pertussis vaccine
  
  Inactivated vaccines are contrasted with attenuated vaccines, or "live" vaccines.
  
  Section::::Mechanism.
  
  The pathogen particles are destroyed and cannot divide, but the pathogens maintain some of their integrity to be recognized by the immune system and evoke an adaptive immune response. When manufactured correctly, the vaccine is not infectious, but improper inactivation can result in intact and infectious particles. Because the killed pathogens in a properly produced vaccine do not reproduce, booster shots are required periodically to reinforce the immune response.
   During a primary infection, long-lived memory B cells are generated, which remain in the body, and provide protection from subsequent infections. These memory B cells respond to specific epitopes on the surface of viral proteins in order to produce antigen-specific antibodies, and are able to respond to infection much faster than B cells are able to respond to novel antigens. This effect shortens the amount of time required to clear subsequent infections.
   Some vaccines contain live, attenuated microorganisms. Many of these are active viruses that have been cultivated under conditions that disable their virulent properties, or that use closely related but less dangerous organisms to produce a broad immune response. Although most attenuated vaccines are viral, some are bacterial in nature. Examples include the viral diseases yellow fever, measles, mumps, and rubella, and the bacterial disease typhoid. The live "Mycobacterium tuberculosis" vaccine developed by Calmette and Guérin is not made of a contagious strain but contains a virulently modified strain called "BCG" used to elicit an immune response to the vaccine. The live attenuated vaccine containing strain "Yersinia pestis" EV is used for plague immunization. Attenuated vaccines have some advantages and disadvantages. They typically provoke more durable immunological responses and are the preferred type for healthy adults. But they may not be safe for use in immunocompromised individuals, and on rare occasions mutate to a virulent form and cause disease.
   Preconditioning (adaptation)
  
  Preconditioning occurs when an animal is exposed to a stressor or stimulus in order to prepare it for a later encounter with a similar stressor or stimulus. 
  
  For example, in vaccinations, a human is exposed to an artificially weakened virus in order to stimulate the body's immune system to produce antibodies that fight the virus. Then, when the live virus is encountered, the body can vigorously defend against it, already having produced the relevant antibodies. See pre-exposure prophylaxis. 
   Vaccine production has several stages. First, the antigen itself is generated. Viruses are grown either on primary cells such as chicken eggs (e.g., for influenza) or on continuous cell lines such as cultured human cells (e.g., for hepatitis A). Bacteria are grown in bioreactors (e.g., "Haemophilus influenzae" type b). Likewise, a recombinant protein derived from the viruses or bacteria can be generated in yeast, bacteria, or cell cultures. After the antigen is generated, it is isolated from the cells used to generate it. A virus may need to be inactivated, possibly with no further purification required. Recombinant proteins need many operations involving ultrafiltration and column chromatography. Finally, the vaccine is formulated by adding adjuvant, stabilizers, and preservatives as needed. The adjuvant enhances the immune response of the antigen, stabilizers increase the storage life, and preservatives allow the use of multidose vials. Combination vaccines are harder to develop and produce, because of potential incompatibilities and interactions among the antigens and other ingredients involved.
   Vaccine delivery is a very common application of bacterial surface display. There are two types of live bacterial vaccines that can be made:
  
  BULLET::::1. Normally pathogenic bacteria are weakened so they are no longer pathogenic.
  
  BULLET::::2. Commensal or food-grade bacteria which are not pathogenic.
   After the 48 hour incubation period, the top of the egg is cracked and the 10 milliliters of allantoic fluid is removed, from which about 15 micrograms of the flu vaccine can be obtained. At this point, the viruses have been weakened or killed and the viral antigen is purified and placed inside vials, syringes, or nasal sprayers. Done on a large scale, this method is used to produce the flu vaccine for the human population.
  
  Section::::Manufacturing.:Annual reformulation.
   Inactivated vaccine
  
  An inactivated vaccine (or killed vaccine) is a vaccine consisting of virus particles, bacteria, or other pathogens that have been grown in culture and then lose disease producing capacity. In contrast, live vaccines use pathogens that are still alive (but are almost always attenuated, that is, weakened). Pathogens for inactivated vaccines are grown under controlled conditions and are killed as a means to reduce infectivity (virulence) and thus prevent infection from the vaccine. The virus is killed using a method such as heat or formaldehyde. 
   After vaccination, IgG+ antibody-secreting plasma cells (ASCs) increase rapidly and reaches a maximum level at day 7 before returning to a minimum level at day 14. The influenza-specific memory B-cells reach their maxima at day 14–21. The secreted antibodies are specific to the vaccine virus. Further, most of the monoclonal antibodies isolated have binding affinities against HA and the remaining demonstrate affinity against NA, nucleoprotein (NP) and other antigens. These high affinity human monoclonal antibodies can be produced within a month after vaccination and because of their human origin, they will have very little, if any, antibody-related side-effects in humans. They can potentially be used to develop passive antibody therapy against influenza virus transmission.
   Vaccines rely on immune modulation or augmentation. Vaccination either excites or reinforces the immune competence of a host to ward off infection, leading to the activation of macrophages, the production of antibodies, inflammation, and other classic immune reactions. Antibacterial vaccines have been responsible for a drastic reduction in global bacterial diseases. Vaccines made from attenuated whole cells or lysates have been replaced largely by less reactogenic, cell-free vaccines consisting of purified components, including capsular polysaccharides and their conjugates, to protein carriers, as well as inactivated toxins (toxoids) and proteins.
  
  Section::::Research.:Phage therapy.
   When B cells and T cells are activated by a pathogen, memory B-cells and T- cells develop, and the primary immune response results. Throughout the lifetime of an animal, these memory cells will "remember" each specific pathogen encountered, and can mount a strong secondary response if the pathogen is detected again. The primary and secondary responses were first described in 1921 by English immunologist Alexander Glenny although the mechanism involved was not discovered until later.This type of immunity is both active and adaptive because the body's immune system prepares itself for future challenges. Active immunity often involves both the cell-mediated and humoral aspects of immunity as well as input from the innate immune system.
   The use of vaccine is the only way to ensure that gilts develop active immunity before conception. Both inactivated and modified live-virus (MLV) vaccines have been developed. An inactivated vaccine has been tested under field conditions, and both types of vaccines were effective when tested under controlled laboratory conditions.
  
presuppositions: If you are exposed to a disease, getting a vaccine will not work.
corrections: Infections don't always cause immunity, and some infections take a long time to be recognized by the immune system, therefore vaccines do have use after a person has already been infected.
raw_presuppositions: If you are exposed to a disease, getting a vaccine will not work. If you are exposed to a disease, getting a vaccine will not work.
raw_corrections: Infections don't always cause immunity, and some infections take a long time to be recognized by the immune system, therefore vaccines do have use after a person has already been infected. Vaccines can still work if the disease has an incubation period, or to increase herd immunity.