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Vaccine types
Under elaboration, looking for reliable sources

The regulatory approval process aims to ensure that the risks of vaccinating a population are much lower than the risks of letting the disease run uncontrolled in an unvaccinated population. Then, postmarketing surveillance sometimes leads to changes in use recommendations in order to minimize the chance of adverse events. Assessing the risks of approved vaccines is often not straightforward due to the combination of generally extremely low frequency of severe outcomes with the high individual impact of these occurrences. Pregnant women, old people and people with health conditions (particularly those with immunodeficiency or autoimmune disease) generally benefit from vaccination but should seek medical advice before doing so in order to ensure efficacy and to manage risks, typically by carefully choosing an appropriate vaccine when there are alternatives, by monitoring possible adverse effects, or by timing vaccination correctly (eg. before, during or after pregnancy).

Safety is usually related to the vaccine type, the chosen adjuvant, route of administration, age and previous immunizations. Sometimes, a combination of those properties is unsafe, for example, replicating agents and some adjuvants are not safe for vaccination through a respiratory mucosal route. Also, adverse effects tend to be more intense on a booster dose.

Innate, humoral, cell-mediated and mucosal immune responses are often considered correlates of immunity and protection, which are especially useful in deciding when a vaccine candidate should advance in a clinical trial and in providing some rapid response to a sudden pandemic. Antibodies are easy to detect in the blood, and neutralizing antibodies in particular play a major role in achieving sterilizing immunity, preventing infection and ensuring immunization against rapidly invasive pathogens (against which immunological memory alone does not confer immunity), while cytotoxic T cells are necessary to clear established infections but are difficult to access as they reside in tissues like lymph nodes. Sterilizing immunity is not a requirement for disease control, and in some cases (eg. tuberculosis) antibodies are not the main protective mechanism. Herd immunity can be achieved when enough individuals in the population receive a vaccine that protect against infection, stopping transmission and resulting in protection for those people that for some reason cannot be vaccinated. There are general trends within each vaccine type regarding how much each of these responses are stimulated, but the complexity of the immune system sometimes breaks these expectations, highlighting the need for postmarketing surveillance. For example, while attenuated vaccines tend to generate a long immune memory and be more effective than other conventional types of vaccines (eg. inactivated, subunit, toxoid, conjugate), one attenuated cholera vaccine showed very low effectiveness when deployed while a subunit vaccine was effective.

Duration of immunity depends on the strength of the initial immune response, the decay rate of antibodies (also T-cell response, which is not as easy to measure), the threshold of protection of the specific disease, the mutation rate of the pathogen, and the place of infection in the body. When the threshold of protection is known, the durability of the vaccine can be estimated from the other factors.