Right now, we are able to test an individual to determine whether or not they have anti-SARS-CoV-2 antibodies – the amount of antibodies and T cells that can be measured more or less accurately. depending on the method used. However, what are the numbers that can tell with virtual certainty whether a person is immune or not? At the moment, we don’t know, but with the latest advancements in the field, we may well be on the verge of getting this valuable response that will help us tackle this pandemic.
This threshold of immunity is called a “correlate of protection”. It is a simple surrogate assessment of the entire immune response that indicates whether a person is protected against disease or infection. ” For example, you measure the number of antibodies in the blood and find that if you have a certain number of them, you are protected Explains Christine Dahlke of the University Medical Center Hamburg-Eppendorf, Germany. This number is therefore the correlate of protection (or CoP). We haven’t defined it for SARS-CoV-2 yet, and according to Dahlke, we urgently need it.
CoPs are a standard tool in vaccinology, and although they are difficult to define, we have established them for many diseases, including measles, influenza and hepatitis. Getting one for COVID-19 would be of significant help to our efforts to end the pandemic. It would allow us, for example, to avoid large vaccine trials that compare a vaccine candidate to a placebo to measure differences in infection rates. Instead, we could perform simpler and faster tests to determine whether a vaccine achieves CoP or not.
Finding CoPs for SARS-CoV-2 and its variants, an urgent matter
Finding the CoP for SARS-CoV-2 (and its variants) is an urgent matter because, despite the unprecedented success of COVID-19 vaccine development through large-scale clinical trials, there are growing concerns that this approach has had its day. As the pandemic progresses, these tests become increasingly difficult to perform, for two main reasons.
First, it is difficult to find volunteers who have not been vaccinated or infected and who are therefore immunologically neutral. Second, it is unethical to give placebos to unprotected people when effective vaccines are available. ” We need a better approach Says Dahlke.
A CoP would also help us quickly measure the impact of the new variants, says Salim Abdool Karim, co-chair of the South African Ministerial Advisory Committee on COVID-19. Blood serum would be collected from a vaccinated person who crosses the CoP threshold, and it could be determined whether they neutralize the new variant. If not, that would mean that the vaccine would probably need to be modified. This saves considerable time and money, as reformulating vaccines for each new variant of concern would be literally unnecessary if in fact they still prove to be adequate.
According to Fengcai Zhu, of the Center for Disease Control and Prevention of Jiangsu Province, Nanjing, China, the CoP can also help estimate immunity levels in the population. We have known for some time that it is possible to identify a CoP for SARS-CoV-2, notably thanks to a study carrying the crew of a ship, the American Dynasty, which left Seattle on the 13th. May 2020. Although everyone was screened for COVID-19 before departure, 104 of the 122 crew members contracted the virus at sea.
No one knows who brought the virus on board, but blood tests by a team from the University of Washington in Seattle revealed that three of the people who did not contract COVID-19 had antibodies to the virus from from a previous infection.
Clinical trials aimed at defining the CoP
This lack of new infection in those people who already had antibodies was the first direct evidence that antibodies can effectively protect against COVID-19. Researchers now increasingly believe the discovery of a CoP is imminent. ” The data we need come from different clinical trials of vaccines Says Amit Srivastava of Pfizer’s vaccine development unit in Pennsylvania. Many of these tests consist of immunoassays on volunteers, as the manufacturers also want to get CoPs as quickly as possible.
While we do not have the definitive answer yet, it is likely that a CoP involves measuring the levels of antibodies and T cells. It should be noted that antibodies bind to pathogens outside of cells and kill them, while T cells destroy cells infected with a virus. Ideally, any immunity test should be based on the simplest possible measurement and be capable of being performed by a family doctor.
In addition to vaccine trials, an ongoing experiment that will help us reach this stage is a ‘challenge study’ by the University of Oxford, which will expose volunteers who have contracted COVID-19 to SARS-CoV- 2 in order to re-infect them and study their reaction.
” The objective of this study is to determine what type of immune response prevents reinfection », Explains Helen McShane, chief researcher. ” We’ll be looking at antibodies, T cells, every aspect of immunity that we can study. At its simplest, if we find that it is not possible to reinfect volunteers who have a certain level of antibodies, then we will finally have a correlate of protection. “. It would be a big step forward, according to Dahlke. ” The world doesn’t have enough vaccines, we need new vaccines She adds. “ There is an urgent need for correlates of protection “.