COVID-19: Correlates of Protection Explained

What is meant by ‘correlates of protection’?

Correlates of protection define the immune response a vaccine or natural viral infection would need to trigger so that an individual is protected from a virus or infectious disease in the future.

The aim of measuring immune response to vaccination or natural viral infection is to understand the level at which a person can fight off a future infection.1

How are correlates of protection determined?  

The immune system acts as a biological defence to protect against infection caused by foreign invaders, such as viruses. As a virus infects the body, it will be detected by the immune system. There are a range of immune system cells involved in protection against disease and these can be measured to determine the correlates of protection:

  • B-cells produce antibodies which bind to the outside of the virus, stop it from infecting cells and help to fight off future infections.
  • T-cells act as a second line of defence that can recognise and destroy infected cells.

Distinct viruses can generate different immune responses from the body.

How long does protection last?

Ideally, we want to protect against disease in the long-term. Some B-cells and T-cells can work as ‘memory cells’ to enable the immune system to respond quickly and effectively next time the body encounters the intruder. This can offer lasting protection. However, in other diseases, protection may diminish over time or drop to a level that leaves people susceptible to disease once again.

How are correlates of protection measured?

Blood tests can determine the presence and quantity of antibodies that might provide protection following vaccination, or after infection. These levels are often closely related to a person’s future protection against infection, symptomatic disease, or severe disease. It can take time to establish how long an individual remains protected and to what level of immune response is required to do so. 

What do we know so far about correlates of protection from COVID-19?

This is an area of research focus given human infection of the SARS-CoV-2 virus was previously unknown before the pandemic. Immune response measurements depend on when samples are taken following vaccination or infection.

Scientists continue to learn more about the levels of antibodies and T-cells that are related to a person’s protection against COVID-19. 

Why is it important to establish correlates of protection for COVID-19?

Establishing correlates of protection against the SARS-CoV-2 virus is important for several reasons:

  • It may help to assess individual susceptibility to the virus.
  • It may help evaluate population immunity and inform interventions for infection control like social distancing and mask wearing.

How is immune response studied in relation to vaccines?

Randomised, controlled clinical trials, often involving many thousands of people, are used to study immune response to vaccinations and determine their efficacy. Scientists and researchers also explore the type and level of immune response vaccines can provide over time, looking at complex measures. Together these data may help determine correlates of protection against infections to inform vaccination programmes.


1. Jin P, Li J, Pan H, et al. Immunological surrogate endpoints of COVID-2019 vaccines: the evidence we have versus the evidence we need. Available at Last accessed May 2021.