covid-19 vaccine

How the ‘Extremely Rare' J&J Reaction Differs From Other Blood Clots, According to Chicago's Top Doctor

Chicago's top doctor said the six cases of clots that prompted the pause of the Johnson & Johnson vaccine are what's called cerebral venous sinus thrombosis

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U.S. regulators on Tuesday recommended a nationwide pause on administering the Johnson & Johnson COVID vaccine amid an investigation into six reports of "extremely rare" but potentially dangerous blood clots.

Those clots occurred in six women between the ages of 18 and 48, six to 13 days after vaccination, the Centers for Disease Control and Prevention and the Food and Drug Administration said. All six experienced clots in the veins that drain blood from the brain - known as cerebral venous sinus thrombosis - and occurred together with low platelets, health officials said.

More than 6.8 million doses of the J&J vaccine have been administered in the U.S., the vast majority with no or mild side effects.

So why did these cases necessitate a pause on the J&J vaccine and how do they differ from other blood clots? Chicago's top doctor explained the reasoning and what experts are watching for in a Facebook Live broadcast on Tuesday.

"So the six cases that have been reported at this point are across the whole country," Chicago Department of Public Health Commissioner Dr. Allison Arwady said. "None of them are in Chicago, none of them are in Illinois, and we've been looking at our records and not seeing anything of concern at this point related to blood clots or to low platelets locally here."

"But one of the reasons why the decision was made to put this pause and to announce this was so that clinicians, you know, doctors who are treating patients, know to look for this as a potentially rare effect and then to report it so that it can be fully investigated," she said.

Arwady said the complication is a "rare but severe" form of blood clot that happened in combination with low blood platelets, explaining that platelets are a component of blood that helps control clotting to stop bleeding if you get a cut or a bruise.

"And so, number one, there are rare cases not associated with vaccines of people having low platelets for all kinds of reasons," Arwady said. "People will get low platelets because they have cancer, people will get low platelets because they have a reaction to a medication, people will get low platelets for sometimes reasons we don't understand at all, we call it idiopathic, and sometimes you will get low platelets as a result of an immune response."

Arwady said the U.S. sees somewhere between 500,000 and 600,000 people with blood clots each year.

"Blood clots can be caused by a whole range of things," Arwady said. "They're known to be associated with cancer, with smoking, with long car rides where people are not moving around, you know, and again there's other things that can put you at risk."

Arwady said the six cases of clots that prompted the pause of the J&J vaccine are what's called cerebral venous sinus thrombosis.

"If we break that down, cerebral means head, so in the head, you know, in and around the brain. Venous sinus actually means the part where blood is draining from your brain and then thrombosis means a clot," Arwady said. "So these people were getting blood clots, not just in their leg, but they were getting blood clots, you know, that was draining from their brain and that can be really serious."

Arwady said that even though the number of cases is low relative to how many doses of the vaccine have been administered, "having seen six cases of people having this blood clot in a severe area, combined with having low platelets, these two rare events together was unusual enough that the CDC and FDA made the decision to put further vaccination on hold while they investigate further."

She said that the decision to pause the vaccine was made also to inform doctors about the potential reaction and how to identify and treat it - noting that a medicine typically given to patients with blood clots could actually be dangerous in these cases if the person does have low platelets as well.

"The other reason why they made the decision to make this announcement is that typically, if somebody has a blood clot, like if they come in with a clot, there's a common medication called Heparin that you would start people on with a clot to help break up that clot and help get the blood flowing again," Arwady said.

"But when somebody has low platelets, you can't give them Heparin, it actually could be dangerous and so it was important, and I think this was right too, that they get the information out for doctors so that if someone is coming in with either low platelets - again which we see not related to vaccine, or with a blood clot, which we see not related to vaccine - that they made sure to ask about the history of vaccinations and then test potentially for both of those," she continued. "And then if it was in the setting of having received the Johnson and Johnson vaccine, the FDA obviously wants to hear about it, the CDC wants to hear about it, so that we assess whether this is a more significant problem, whether this remains very rare, and we just want to share some information."

Because all six of the cases were reported among women, Arwady was asked Tuesday if the clots may have something to do with estrogen, which she conceded it "might."

"At this point, you know, six cases is really tiny compared to, again, the almost 7 million that have gotten this vaccine, but I don't think it's a surprise to see something that could be related to an immune reaction affecting women," Arwady said.

"We know that autoimmune diseases generally are much more prevalent in women. And that's because women and men do have differences in their immune system. Many of the immune modulating genes live on the X chromosome and if you remember back to high school biology, women have two X chromosomes and men only have one, they have X and Y, and so there's more opportunity for sort of different genetic things on X chromosomes in women as compared to men," she continued.

"But also, estrogen does tend to be a bit of an immune modulator in a positive way and then testosterone in a bit of a negative way, and because women need to, you know, evolutionarily be able to have babies, there are reasons that you don't, you know, you don't want their immune system rejecting a fetus, that kind of thing," Arwady said.

Officials are recommending that people who were given the J&J vaccine who are experiencing severe headache, abdominal pain, leg pain, or shortness of breath within three weeks after receiving the shot contact their health care provider.

If you received the J&J shot and have not developed any of the side effects associated with signs of blood clots within three weeks after vaccination, the risk of an adverse reaction is unlikely. Health officials urge patients to continue monitoring for symptoms.

Arwady also noted that the Moderna and Pfizer vaccines use different mechanisms than the Johnson & Johnson shot and that this pause in no way affects either of those vaccines, which have made up the vast majority of the vaccinations in Chicago, in Illinois and across the U.S. thus far.

She conceded that these cases of blood clots are "scary to hear about" but reiterated that they are "extremely rare" and that the city has seen "so many bad outcomes from COVID itself," noting that vaccinations remain the way to "protect yourself, your family and Chicago."

Chicago has received a total of 91,000 doses of the single-shot Johnson & Johnson vaccine in five shipments since February, according to the city's data. Those 91,000 doses make up less than 6% of the 1,645,470 doses of all three available vaccines that the city has received from the federal government thus far.

Arwady said she expected to have more information on how the federal government and the city approach the J&J vaccine after the CDC's Advisory Committee on Immunization Practices meets to discuss the investigation on Wednesday.

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