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Use of Johnson & Johnson’s COVID-19 vaccine in the United States was halted after blood clots formed in six of the 6.8 million vaccinated people. These cases appear to be similar to the rare cases of blood clots with the Oxford / AstraZeneca vaccine, which has led some countries to discontinue its use.
Blood clot syndrome is associated with an unusual type of clot that often forms in the brain, called cerebral venous sinus thrombosis, CVST, along with low levels of blood clots that adhere to clots.
The syndrome was reported primarily in people under the age of 60, more often in women than men. But gender inequality may be due to more women getting vaccinated, as they make up a higher proportion of home and health care staff. An analysis of 79 cases in the UK after the Oxford / AstraZeneca vaccine found no difference between men and women, says Munir Primohamed, director of the UK Commission for Human Medicine. The average incidence was four out of a million vaccinated in the UK.
It is not known why younger people are at higher risk, but this is partly due to the age distribution in some countries, where people under a certain age are vaccinated. Another reason is that older people are at higher risk for COVID-19, so the benefits of the vaccine should outweigh the risks.
Six recently reported cases of CVST in the Johnson & Johnson vaccine, one of which was fatal, occurred in women ages 18 to 48.
Johnson & Johnson has announced a delay in the distribution of its product in Europe. “We have worked closely with medical experts and healthcare officials, and we strongly support the communication of this information to healthcare professionals and the public,” the firm said in a statement.
What could cause clots?
In the Oxford / AstraZeneca cases, most of the antibodies that reacted with platelet-derived platelet factor 4 (PF4) were positive. The vaccine may have somehow caused the production of these antibodies, which resulted in tiny blood clots and therefore the consumption of platelets, says Andreas Greinacher of the University of Greifswald in Germany. It is reminiscent of a syndrome in which similar antibodies can be activated by heparin, an anticoagulant drug, says the scientist, who is an expert on the side effects of heparin.
Recent guidelines in some countries recommend a platelet antibody test if CVST symptoms develop within two weeks of the COVID-19 vaccine. If the test is positive, the same treatment that is normally used should be used for people who experience rare side effects from heparin treatment that are different from standard treatment for thrombosis.
Greinacher says that a previous study in mice showed that DNA could bind to PF4, cause the formation of antibodies and stimulate the formation of blood clots. He hypothesizes that this is why this effect has only been seen with DNA-containing adenovirus-based vaccines.
The Oxford / AstraZeneca vaccine contains a gene encoding a coronavirus needle protein that is carried by harmless chimpanzee adenovirus DNA. The Johnson & Johnson vaccine works on a similar principle, but uses human adenovirus DNA.
Meanwhile, the mRNA-based Pfizer / BioNTech and Moderna vaccines provide the body with instructions for the production of needle protein. No cases of low platelet counts have been reported for any of these vaccines, Peter Marx of the US Food and Drug Administration (FDA) noted at a news conference.
“We see that what is happening with the Johnson & Johnson vaccine is very similar to the AstraZeneca vaccine,” Marx said. “We can assume that the probable cause is a mechanism similar to another adenovirus vector vaccine.”
However, the clot syndrome is so rarely registered that those who experience it have other factors in its manifestation, considers A. Greinacher. “There must be individual cofactors causing this. Otherwise, we would see this problem with a lot more people and fortunately it doesn’t happen.”
Marks did not say whether antiplatelet antibodies had been detected in six cases related to Johnson & Johnson. But the FDA noted that one of the reasons for stopping vaccination was to give doctors time to learn how to properly diagnose and treat unusual blood clot syndrome. Additionally, the agency should fully investigate those six cases, says Anne Schuchat of the US Centers for Disease Control and Prevention.
Prepared by New Scientist.
