Americans who received the one-dose J&J Covid vaccine were 3.5 times likely to develop blood clots


Johnson & Johnson‘s COVID-19 vaccine increases a person’s risk of developing rare blood clots, a new study suggests.

Researchers from the Mayo Clinic in Rochester, Minnesota, compared data from the general population before the pandemic to data gathered from reported vaccine side-effects suffered by Americans.

They found that a person who received the vaccine was 3.5 times as likely to develop brain blood clots as an average person before the pandemic.

Blood clotting, and specifically cerebral venous sinus thrombosis (CVST) is a well known side-effect of the J&J vaccine, and the discovery of this risk was the reason usage of the vaccine was paused in April.

However, the team insists the risk is rare and that the findings must be looked at in the context of the effectiveness of the vaccine in preventing severe cases COVID-19.

Researchers found that recipients of the Johnson & Johnson COVID-19 vaccine (pictured) were 3.5 times as likely to develop CVST – a potentially deadly blood clotting condition – than the average person 

Cerebral venous sinus thrombosis (CVST) is a rare but potentially deadly condition that restricts the brains ability to drain blood and could cause a stroke

Cerebral venous sinus thrombosis (CVST) is a rare but potentially deadly condition that restricts the brains ability to drain blood and could cause a stroke

Researchers, who published their findings in JAMA Internal Medicine on Monday, gathered data from Olmstead, County, Minnesota – a county of around 158,000 people 90 miles southeast of Minneapolis – from 2001 to 2015.

They then used the Centers for Disease Control and Prevention’s Vaccine Adverse Event Reporting System (VAERS) to find diagnoses of blood clots in people who received the J&J vaccine between the jab’s approval date at the end of February 2021 to May 7. 

During the 14-year period, there were 39 Olmstead residents who developed CVST – a rare, potentially deadly, blood clotting condition that can form in a person’s brain.

Of that group, 29 had a risk factor within the 92 days proceeding development of the blood clot, such as infection, active cancer, or oral contraceptives for women.

Adjusted for population, there were 2.46 cases of CVST out of every 100,000 person-years of the residents of Olmstead County developed blood clotting during that period.


The CDC and the FDA recommended that rollout of the J&J vaccine be paused in April after multiple reports of rare, but serious, blood clots.

Nine people between the ages of 18 and 59 had developed cerebral venous sinus thrombosis (CVST) by April 13.

CVST is a rare type of blood clot that blocks the brain’s sinus channels of draining blood, which can cause hemorrhages.

One of the nine patients died and two are in critical condition. 

With more than seven million people who had gotten the vaccine by that point, this means just 0.00012% developed CVST.

That is less that the five out of one million people – 0.0005% – who develop the condition in the general population.

The pause was lifted on April 23 after the CDC determined the risk was extremely low. 

Around 8.7 million doses of the J&J vaccine had been administered in the U.S. between February and May.

They found 46 reports of CVST to VAERS after receiving the J&J vaccine, though eight were removed from the pool for either being duplicate reports or not being professionally diagnosed.

In total, 38 cases tied to the J&J vaccine were detected – with over 70 percent being among women.

When adjusted for population, there were 8.65 cases out of every 100,000 person-years among people who received the vaccine – a rate 3.5 times higher than the general population.

They also found that vaccine recipients are at most risk of developing the condition within the first 15 days after receiving the jab.

Women are also at most risk between ages 30 to 64.

Researchers note that the risk is rare, and that the small risk of the vaccine is worth the potential benefits of avoiding complications from COVID-19. 

While this study does give the level of increased risk from receiving the J&J vaccine, that existence of the risk itself has been known for some time.

On April 13, the Food and Drug Administration (FDA) paused its authorization of the Covid vaccine after six women were found to have developed blood clotting after receiving the shot.

All had pre-existing conditions that put them at an increased risk of developing the clots.

Ten days later, on April 23, usage of the vaccine was resumed, though the label was advised with a warning to women under the age of 50 about the blood clotting.

The J&J vaccine is the least popular of the three Covid jabs available in America, though its one-shot nature has increased its potential to be used internationally in areas with less resources available to give a jab twice.

As of Monday morning, 15.6 million doses of the jab have been administered, CDC data show.

The clotting, CVST, is rare but potentially dangerous.

Blood clots will form within the veins in the outer layers of the brain, and it can block the blood from draining from the brain to the heart.

In the worst cases, it can cause a person to suffer a stroke, that could lead to death. 

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