Why the U.S. recommended suspending Johnson & Johnson's vaccine, and what it means for Canada
Canada is expecting 10 million doses of the vaccine starting this month
On Tuesday, the U.S. recommended putting on hold the use of the single-dose Johnson & Johnson COVID-19 vaccine.
The U.S. Centers for Disease Control and Prevention and the Food and Drug Administration (FDA) said in a joint statement they are investigating blood clots in six women in the days after vaccination, in combination with reduced platelet counts.
Canada has approved the vaccine but hasn't yet received any doses. Here's what we know about what led the U.S. to make this decision, and what it could mean for Canada.
What do we know about the cases?
In the U.S., the only country using the vaccine, all six cases were among women between the ages of 18 and 48, and symptoms occurred six to 13 days after vaccination.
The clots occurred in veins that drain blood from the brain and occurred together with low platelets. One person died, and another patient is in critical condition. It isn't yet known how the clotting may have contributed.
Dr. Janet Woodcock, the acting commissioner of the FDA, stressed in a briefing Tuesday that the potential risks remain small and that the decision was made "out of an abundance of caution."
More than 6.8 million doses of the J&J vaccine have been administered in the U.S.
U.S. federal distribution channels, including mass vaccination sites, will pause the use of the J&J shot, and states and other providers are expected to follow.
What is causing the clots?
It's not entirely clear. "We don't have a definitive probable cause," Dr. Peter Marks, director of the Center for Biologics Evaluation and Research at the FDA.
The leading hypothesis, he said, is "this is an immune response that occurs very, very rarely after some people receive the vaccine. That immune response leads to activation of the platelets and extremely rare blood clots."
This adverse reaction appears to have similarities to a well-known condition called heparin-induced thrombocytopenia (HIT), where the blood thinner heparin paradoxically triggers the formation of an antibody that directly activates platelets.
When platelets are activated, they change from being round to spiny in order to stimulate blood clots.
Heparin binds to a protein that forms a compound, known as a complex. But some people produce antibodies against the complex, leading to a clotting reaction.
Are these the same kind of clots seen with AstraZeneca?
Marks said he believes there is a "similar mechanism" leading to the clots in the case of Johnson & Johnson and the AstraZeneca-Oxford vaccine. Both are non-replicating viral vector vaccines, which use a modified adenovirus to deliver genetic instructions for a key coronavirus protein needed to protect against COVID-19.
There have been no reports of any post-vaccination clots in people who have received the Pfizer or Moderna shots, which are based on mRNA technology, Marks said.
Canada's vaccine advisory committee recommended the suspension of the AstraZeneca vaccine for people under 55, following reports of rare but potentially fatal blood clots in Europe.
On Tuesday, Canada reported its first case of the clotting condition linked to the AstraZeneca vaccine. Quebec's health minister said the woman is recovering at home.
Angela Rasmussen, a virologist at Georgetown University's Center for Global Health Science and Security in Washington, said there appear to be similarities between the blood clotting cases linked to the two vaccines but "cautioned it's still too early to jump to conclusions."
Ultimately, she said, "we may see tailored recommendations about Johnson & Johnson use for only low-risk groups if it does turn out to be related to the vaccine."
What does this mean for Canada?
The first doses of the Johnson & Johnson vaccine, which has been approved for use in Canada, are expected to be shipped here later this month.
With 10 million on order, Prime Minister Justin Trudeau said Canadian officials are closely monitoring the decision by U.S. health authorities. "We can assure everyone that Health Canada will, every step of the way, put the health of Canadians first and foremost around any decisions we make around the distribution of the vaccine," Trudeau said.
A suspension of the vaccine in Canada would strike a blow to the country's vaccination campaign as it contends with a third wave, said Dr. Isaac Bogoch, an infectious diseases physician and member of Ontario's COVID-19 vaccine task force.
"We have millions and millions of doses of this that would arrive in Canada as early as late April, and of course this vaccine is phenomenal," he said, pointing out that it's a single dose and requires only basic refrigeration, in contrast to the Pfizer vaccine.
"This is something where you can really dole this out quickly to a large number of people."
By the time it arrives in Canada, he said, the National Advisory Committee on Immunization could have some "modified guidance as to who should get this and who should avoid this."
WATCH | Dr. Theresa Tam reacts to the U.S. decision:
Are women in particular at risk?
In the case of Johnson & Johnson, the FDA cautioned there are too few cases to be able to draw any conclusions about whether women or any particular subgroup are more vulnerable to the clots nor — in response to a reporter's question — whether there's any clear association between the clotting and oral contraceptives.
Dr. Menaka Pai, an associate professor at McMaster University and a hematologist and thrombosis medicine physician at Hamilton Health Sciences, said it appears women may be more likely to be affected, but the reasons for that remain unclear.
"We know that women in general are more prone to getting blood clots. It's because there's an interaction between estrogen and the blood clotting system," she said. "So is gender a predisposing factor to these very serious clots? Again, lots of speculation because the numbers are very small. There might be a true association here, but it might also be because women are more likely to be getting these vaccines."
Rasmussen, who was recently inoculated with the Johnson & Johnson vaccine herself, said she's not worried, despite being a woman between the ages of 18 and 48.
She pointed out the risk of getting a blood clot as a complication of COVID-19 is statistically far greater than from the Johnson & Johnson vaccine.
"If this is related to the vaccine, this is six cases out of nearly seven million doses administered. That's one in more than a million rate of occurrence, that means it's extremely rare," she said.
"I may as well buy a lottery ticket if I think that I'm going to overcome those odds and be another person who has this type of side effect."
WATCH | Canadian lab looks for biological marker of vaccine clotting risk:
With files from Adam Miller, Thomas Daigle and The Associated Press