MRC warns healthcare workers not to use antibody tests to check if vaccine worked
The council says, among other reasons, testing too soon after vaccination could yield a false negative and should be avoided
The Medical Research Council (MRC) has advised healthcare workers vaccinated against Covid-19 not to take antibody tests to check if the jab has worked, as the tests may give a false negative.
The MRC is overseeing the Sisonke implementation study, which aims to provide Johnson & Johnson’s (J&J) shot to 500,000 healthcare workers. In a statement issued on Tuesday night, it said it had received numerous enquiries from vaccinated health workers who had obtained antibody tests from private laboratories, and were concerned about receiving negative results with prompts suggesting “vaccine failure”.
The MRC said this was no cause for alarm as most commercially available antibody tests in SA detected a response to the SARS-CoV-2 nucleocapsid protein, which is not included in the vaccine. The J&J vaccine is based on the SARS-CoV-2 spike protein.
Even if the tests were designed to measure spike protein antibodies, testing too soon after vaccination could yield a false negative as it takes several weeks for antibodies to be generated, and some commercial tests are not sensitive enough to detect low levels of antibodies, said the MRC.
“However, even low levels of antibodies will help combat Covid-19 and are complemented by cell-mediated immunity, which is not tested outside limited clinical studies,” it said. The cell-mediated part of the immune system helps create T-cells, which also fight viruses.
Researchers have found participants in clinical trials for vaccines have produced a T-cell response, but this cannot be detected with commercially available tests.
“A negative antibody test after vaccination should therefore be interpreted with caution and we are not advising repeat testing nor vaccination at this time. Clinical efficacy is the most important indicator, and trial data shows that the vaccine provides 82% protection against severe disease,” said the MRC.
To date 292,600 healthcare workers have been vaccinated with J&J’s shot in SA. The study was paused last week by the SA Health Products Regulatory Authority (Sahpra) to investigate reports of rare and unusual blood clots in a few people who received the vaccine in the US.
The EU’s medicines regulator said on Tuesday that the vaccine should come with a warning about the risk of these clots, but it should not be withdrawn. Sahpra is expected to announce its final decision by the end of the week.
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