Anna Hartz
When new medical findings emerge, they often help explain events that once seemed confusing or even alarming. This is especially true when it comes to rare side effects linked to vaccines.
Researchers have spent years trying to understand what caused a small number of severe cases connected to early COVID-19 vaccines.
The rare condition VITT
In March 2021, Norwegian doctor and professor Pål André Holme said that the cause of a group of serious cases had been identified. His team had found a link to a COVID-19 vaccine, according to Videnskab. Around the same time, similar cases were reported in Austria and Germany. These cases involved blood clots, low platelet counts, and bleeding.
The condition was later named VITT. This stands for vaccine-induced immune thrombotic thrombocytopenia. It describes a rare reaction where the immune system behaves in a harmful way.
In response to these findings, some countries, including Denmark, stopped using the AstraZeneca vaccine in 2021. The decision came after studies showed more cases of severe blood clots than expected. Researchers agreed that the risk was very small, but serious enough to take action.
At the time, scientists could not explain why only some people were affected. That question remained open for years. Now, new research offers a clearer answer.
The latest studies suggest that the condition was caused by a mix of genetic factors and the way the vaccine was built. The vaccine used a harmless adenovirus to deliver instructions to the body. This helped the immune system learn how to fight the virus.
In rare cases, the immune system reacted in an unexpected way. It produced antibodies that mistakenly targeted a protein in the blood called PF4. This protein helps the blood clot when needed. But when these antibodies attached to PF4, the result was dangerous.
The body began forming blood clots in the wrong places. At the same time, platelet levels dropped. This combination led to serious complications, and in some cases, death.
Genetic traits in common
Researchers also found that people who developed VITT shared certain genetic traits. These traits are fairly common in some populations. However, only a small number of people developed the harmful reaction.
Another factor was previous exposure to adenoviruses. Many people have been infected with similar viruses in the past. In rare cases, this may have influenced how the immune system responded to the vaccine.
Experts now say the reaction was the result of an unusual and unlikely combination of factors. It cannot be predicted in advance. This makes it difficult to identify who might be at risk.
Scientists continue to study adenovirus-based vaccines. They are used in some treatments today and are being developed for other diseases. Researchers hope that future versions can be made safer.
The findings help explain a rare but serious side effect. They also show how complex the human immune system can be.
