Correlates of immunity/correlates of protection

Correlates of immunity/protection to a virus or other infectious pathogen are measurable signs that a person (or other potential host) is immune, in the sense of being protected against becoming infected and/or developing disease.

For many viruses, antibodies serve as a correlate of immunity. So for example, pregnant women are routinely screened in the UK for rubella antibodies to confirm their immunity to this infection which can cause serious congenital abnormalities. In contrast for HIV, the simple presence of antibodies is clearly not a correlate of immunity/protection since infected individuals develop antibodies without being protected against 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 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 with clinical outcomes (i.e. infection and/or disease, not just laboratory markers).