Here's what's known about why women seem more susceptible to rare vaccine blood clots

Two leading COVID-19 vaccines are being linked to rare, serious blood clots, with the bulk of cases being women. But medical experts say with so few reports after millions of doses administered, determining what's actually behind these cases will take time and research. Here's what we do know.

Also no evidence linking post-vaccine blood clots to hormonal birth control

Two leading COVID-19 vaccines are being linked to rare, serious blood clots, with the bulk of cases being women. But medical experts say with so few reports after millions of doses administered, determining what's actually behind these cases will take time and research. (Evan Mitsui/CBC)

It was a report that likely prompted alarm for many women: rare, serious blood clots following vaccination for six Americans. All women, all of child-bearing age.

That update this week also came on the heels of similar global blood clot reports among mostly women following AstraZeneca-Oxford vaccinations, sparking fears that younger women — including those on hormonal birth control — may be more at risk.

But leading medical experts say it's not that simple.

More research is needed, several of them told CBC News, in order to confirm what role each vaccine may play in causing clots, and whether women truly face higher risks.

Serious clots tied to 2 vaccines

When it comes to the blood clot reports following Johnson & Johnson vaccinations, information is limited.

The U.S. Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA) made a joint announcement on Tuesday, revealing that out of nearly seven million doses administered, the half-dozen women between the ages of 18 and 48 all started having symptoms up to two weeks post-vaccination.

In each case, the potentially deadly type of blood clot in the brain — called cerebral venous sinus thrombosis, or CVST — was seen in combination with low levels of blood platelets.

The news followed weeks of controversy over AstraZeneca-Oxford's two-dose shot, which has also been linked to a similar type of clot dubbed vaccine-induced prothrombotic immune thrombocytopenia, or VIPIT.

WATCH | Health Canada's chief adviser talks about risk of blood clots:

Health Canada's chief adviser breaks down risk of blood clots

3 years ago
Duration 2:54

"Obviously, there's going to be speculation," said Dr. Menaka Pai, a clinical hematologist at McMaster University in Hamilton and a member of Ontario's COVID-19 Science Advisory Table.

"Is there something about these two vaccines?"

Both options are approved in Canada and use similar mechanisms, though only AstraZeneca's shot has been used here so far, with the country's first shipment of Johnson & Johnson doses expected at the end of April.

One VIPIT case has also been confirmed in this country to date from that vaccine — also in a woman, though her age hasn't been revealed by public health officials. 

There are reports among men in other countries as well, but so far the global trend appears to be skewing toward women making up the bulk of the growing number of cases.

Not clear women at higher risk

But while it seems women may be more at risk of developing these severe, post-vaccination blood clots, medical experts warn it's not that cut and dried — with far more research and data needed to draw any firm conclusions.

"Right now, not only is the information really preliminary, this is a really small number of cases," said Pai.

Currently, Health Canada doesn't have any restrictions on the AstraZeneca-Oxford vaccine and deems it safe for use, though advisory committee recommendations followed by the provinces stipulate it should only be used for people 55 and older.

Alyson Kelvin, a vaccinologist with VIDO-InterVac, a vaccine developer affiliated with the University of Saskatchewan, said the recent recommendation to pause use of the Johnson & Johnson vaccine south of the border made sense to allow for further research to see if the vaccine actually caused the reported clots.

"There's a difference between an association and a causal link — and a causal link, if there is one, is going to take time," she said.

That hasn't stopped speculation over the potential causes, with theories circulating online over whether there could be ties to women's hormonal birth control.

No known links to birth control

But medical experts say it's highly unlikely there's a connection or any layered risk between vaccines and hormonal contraceptives — even though birth control contains a clotting risk of its own.

"Those are not the same types of clots that we're seeing with the vaccine," said Dr. Supriya Sharma, Health Canada's chief medical adviser, while speaking to reporters at a technical briefing earlier this week. 

Notably, none of the women with CVST cases tied to the Johnson & Johnson vaccine were on birth control, a U.S. CDC panel on Wednesday heard from the chief medical officer for Johnson & Johnson's vaccine division. Though a joint presentation from the CDC and FDA did note one of the women used estrogen or progesterone for undisclosed reasons.

Most also had pre-existing health issues, including hypertension, hyperthyroidism, asthma and obesity, according to U.S. media reports, and the cases were severe, including one death.

Toronto-based family physician Dr. Tali Bogler, chair of family medicine obstetrics at St. Michael's Hospital, stressed the mechanism behind blood clots tied to birth control appears to be completely different from what may be happening post-vaccination.

Blood clots that occur while taking hormonal birth control — or during pregnancy, or for older women on hormone replacement therapy — are linked to the hormone estrogen, Bogler explained, which ramps up the body's clotting system. 

That's typically beneficial during pregnancy so a woman doesn't bleed at dangerous levels while giving birth, but it does increase someone's blood clot risk.

Medical experts say it's highly unlikely there's a connection or any layered risk between vaccines and hormonal contraceptives — even though birth control contains a clotting risk of its own. (iStock)

Something else is likely going on when it comes to the two rare-but-similar forms of clots linked to these vaccines.

The clots raising serious concerns are occurring in the veins that drain blood from the brain — and, somewhat strangely, happened at the same time as people were experiencing low levels of platelets, the tiny cell fragments in our blood that help form clots in the first place to stop us from bleeding.

More research is needed to understand the exact mechanism, though there's speculation among some members of the medical community that it could be tied to someone's immune system going haywire.

It's already well known that women are more likely to develop autoimmune conditions than men, but again, Bogler says ties to post-vaccination symptoms aren't yet clear.

Reports remain 'vanishingly rare'

All the medical experts who spoke to CBC News agreed far more research is needed to understand the exact mechanism at play, who's most at risk, and whether there's any clear causation playing out at all between these vaccines and certain individuals, women included.

What's challenging now is that until there's more clarity on these potentially deadly clots, experts simply can't answer some of the burning questions around these vaccines, which could increase vaccine hesitancy during the pandemic.

"We're not trying to gloss over the real concerns of women," Pai said, "but we really have to be honest about the science."

Community health volunteer Menusha Manokumar, 20, gets her first dose of the Moderna COVID-19 vaccine at a clinic run by Humber River Hospital in Toronto on April 14. (Evan Mitsui/CBC)

What is clear are the risks associated with COVID-19.

From severe blood clotting to lung damage to other serious symptoms, the infection remains dangerous and stubborn to treat, with women who are pregnant or anyone with autoimmune disorders even more at risk of dire outcomes, including death.

"Whether they're women between the ages of 18 to 48, or otherwise, everybody should keep in mind that your risk of having a severe complication of any kind from a vaccine is much, much, astronomically much lower than having a complication from getting COVID-19," said Dr. Angela Rasmussen, a virologist at Georgetown University's Center for Global Health Science and Security in Washington, D.C. 

That prompted Rasmussen to recently get the Johnson & Johnson vaccine herself, and she stands by that decision despite being in the age range for the clotting issues later reported in the U.S.

"Rest assured that, even if this is related to the vaccine, it is vanishingly rare — and it's very unlikely that it's going to happen to you," she said.


Lauren Pelley

Senior Health & Medical Reporter

Lauren Pelley covers health and medical science for CBC News, including the global spread of infectious diseases, Canadian health policy, pandemic preparedness, and the crucial intersection between human health and climate change. Two-time RNAO Media Award winner for in-depth health reporting in 2020 and 2022. Contact her at:

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