Correlates of immunity or correlates of protection to a

virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsky's 1 ...

or other infectious

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...

are measurable signs that a person (or other potential host) is

immune In biology, immunity is the capability of multicellular organisms to resist harmful microorganisms. Immunity involves both specific and nonspecific components. The nonspecific components act as barriers or eliminators of a wide range of pathogens ...

, in the sense of being protected against becoming infected and/or developing

disease A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...

. For many viruses,

antibodies An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...

and especially

neutralizing antibodies A neutralizing antibody (NAb) is an antibody that defends a cell from a pathogen or infectious particle by neutralizing any effect it has biologically. Neutralization renders the particle no longer infectious or pathogenic. Neutralizing antibod ...

serve as a correlate of immunity. Pregnant women, for example, are routinely screened in the UK for

rubella Rubella, also known as German measles or three-day measles, is an infection caused by the rubella virus. This disease is often mild, with half of people not realizing that they are infected. A rash may start around two weeks after exposure and ...

antibodies to confirm their immunity, which can cause serious

congenital A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can ...

abnormalities in their children. In contrast, for

HIV The human immunodeficiency viruses (HIV) are two species of ''Lentivirus'' (a subgroup of retrovirus) that infect humans. Over time, they cause acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the immune ...

, the simple presence of antibodies is not a correlate of immunity/protection since infected individuals develop antibodies without protection against the disease. The fact that the correlates of immunity/protection remain unclear is a significant barrier to HIV vaccine research. There is evidence that some highly exposed individuals can develop resistance to HIV infection, suggesting that immunity and therefore a

vaccine A vaccine is a biological Dosage form, preparation that provides active acquired immunity to a particular infectious disease, infectious or cancer, malignant disease. The safety and effectiveness of vaccines has been widely studied and verifie ...

is possible. However, without knowing the correlates of immunity, scientists cannot know exactly what sort of immune response a vaccine would need to stimulate, and the only method of assessing vaccine effectiveness will be through large

phase III trials The phases of clinical research are the stages in which scientists conduct experiments with a health intervention to obtain sufficient evidence for a process considered effective as a medical treatment. For drug development, the clinical phases ...

with clinical outcomes (i.e. infection and/or disease, not just laboratory markers). Multiple studies used predictive markers to validate higher levels of neutralizing antibodies corresponding with lower likelihood of breakthrough infection after vaccination in

COVID-19 Coronavirus disease 2019 (COVID-19) is a contagious disease caused by a virus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The first known case was COVID-19 pandemic in Hubei, identified in Wuhan, China, in December ...

See also

Sources and notes

Further reading