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The shots were celebrated, not necessarily for their efficacy, Johnson and Johnson was a bit less than other vaccines, but people loved the idea of being one and done. Now they may be having second thoughts after six women were diagnosed with cerebral venous sinus thrombosis.

Dr. Webster Crowley is a Rush University Medical Center Neurosurgeon.

“It is a blockage on the vein side, so instead of blocking the blood going to the brain, what it does is it backs it up,” he said. “It backs up the pipes where the veins draw blood away from the brain. And when those clot off there is a buildup of pressure, a back up of blood.”

Those who suffered had something else in common besides the Johnson and Johnson vaccine. They also had been diagnosed with COVID-19.

Six out of 6.8 million got clots two weeks after their vaccine.

“I think, if anything, it falls a little lower than what we would expect to see, but it certainly falls within the realm,” Crowley said. “And we have seen that number at least triple with people who had Covid. So I do think … people should not hesitate to vaccinate against this disease.”

Dr Alfonso Tafur is a vascular medicine physician with NorthShore University HealthSystem.

“Covid is strongly associated with blood clotting in itself,” he said. “So if anything I would say this is an opportunity not to ignore the other symptoms of clotting that may happen regardless of vaccine or not  – which are way more frequent and potentially lethal.”

Another similarity was young women ages 18 to 48 were the ones who suffered. Again doctors say that is common.

“That is also the exact population we see with this type of clot when it happens spontaneously,” Tafur said.

All vaccines kick the immune system into high gear. And, in the case of Johnson and Johnson and AstraZeneca, use an adenovirus as a vector. It’s what doctors suspect may be causing a platelet reaction in the blood.

“We have not seen anything like this on Pfizer or Moderna,” Tafur said. “Is there a link or correlation that we know how the adenovirus is exposing or presenting PF4, then to create a platelet reaction? There is a missing link on that still. .. Another signature of this clotting is paradoxically your platelets go lower, so you have a clot in an unusual place. With lower platelets and it seems to be an immunological reaction.”

Here’s how you might know you are having a negative reaction.

“Anybody who has gotten the vaccine be cautious, look for symptoms,” Tafur said. “I think it’s fair to look for headaches, nausea, vomiting  regardless of your age and whether you got Johnson and Johnson. (It is) reason to consider following up with your physician.”

But headaches are normal following a vaccine. The key is when they last.

Crowley advised to think of the worst headache you’ve ever had.

“We may see them start kind of dull and then build up overtime. But it would be rare for a headache associated with this to go away without medical intervention,” he said. “Particularly if there’s a bleed in the brain associated with it. It can cause increased pressure in the head that pressure isn’t just going to go away over an hour.”

In Europe there have been 169 brain clots in AstraZeneca vaccine recipients out of 34 million who got the shot. Doctors advise anyone who thinks they are having a serious reaction within the first two weeks of their vaccine to seek medical attention.