(CNN) It was just about a year ago that doctors started noticing Covid-19 patients showing up in emergency rooms with strokes, and complained that blood clots were clogging up dialysis machines and other equipment being used to keep coronavirus patients alive.
Frantic intensive care unit specialists reported "dramatic" blood clots in the heart, liver and other organs. Autopsies of coronavirus victims in New Orleans showed their lungs were jammed with clots.
Some young, seemingly healthy patients were suffering massive strokes from Covid-19.
"As a blood clot expert, I can tell you it's the most blood-clotting disease we have ever seen in our lifetimes," said Dr. Alex Spyropoulos, a professor at the Feinstein Institutes for Medical Research in New York.
"I have been doing this for a quarter century. I have never seen these levels of blood clots."
Spyropoulos and colleagues have done a series of studies showing that treating Covid-19 patients with blood thinners can dramatically reduce or even prevent these clots. And getting vaccinated against Covid-19 can prevent them altogether by preventing infection in the first place.
So they feel it's ironic that fears about a much, much rarer type of blood clot may now scare people off getting vaccinated.
Last week, the US Centers for Disease Control and Prevention and the US Food and Drug Administration recommended a pause in giving out Johnson & Johnson's Janssen coronavirus vaccine while experts investigate whether it can cause blood clots and, if so, what to do about it.
The European Medicines Agency said Tuesday it had found a possible link, but said the overall benefits of the vaccine outweigh the risks. For use in the EU, the agency said the vaccine must include a warning about "unusual blood clots with low blood platelets" as "very rare side effects."
The CDC's Advisory Committee on Immunization Practices met last week to discuss the issue, and decided to wait for more information after discussing the cases of six women who developed a very unusual type of blood clot after getting J&J's vaccine, as well as other possible cases. They'll meet again Friday to come up with recommendations, which could include an extra warning to help vaccine recipients and doctors look for the symptoms, or restrictions on who should get the J&J vaccine.
Whatever the risk might be from vaccines, experts agree it's extremely low.
"You have as much risk of getting struck by lightning as you have of getting one of these rare blood clots," Spyropoulos told CNN.
Blood clots in general are extremely common -- affecting 900,000 Americans a year, according to the CDC. They kill an estimated 100,000 people every year. Especially common are clots in the brain. About 795,000 people suffer strokes every year in the US, according to the American Heart Association. The group estimates 10-15% of these are in adults under the age of 45.
Risk factors for ordinary blood clots include surgery, accidents, cancer treatments and even sitting too long, noted Dr. Mark Crowther, a hematologist and thrombosis expert for the American Society of Hematology.
"There are some weak risk factors -- for example, airline travel if you fly from Hawaii to Los Angeles," Crowther told CNN. "Prolonged car trips are risk factors," added Crowther who is chair of the department of medicine at Canada's McMaster University.
And being infected with coronavirus greatly raises this already common risk.
"The vaccine, without question, dramatically reduces the risk of any of the Covid-associated blood clots," Crowther said.
The underlying mechanism that may be involved in the blood clots linked to the Johnson & Johnson vaccine in the US and AstraZeneca's vaccine in the UK and Europe is extremely rare, and appears to involve a little-understood immune response.
"The chances of getting what we are calling vaccine-induced immune thrombotic thrombocytopenia or VITT is one in a million," Spyropoulos said.
"The chances of you being hospitalized with Covid is roughly one in 100 for the adult population. The chances of having a blood clot once you are hospitalized is probably one in five or one in six." That risk rises to 1 in 3 for people in the ICU, Spyropoulos said.
"The benefits of any vaccine far outweigh the risks -- period," he said.
"This complication from the vaccine is excruciatingly rare. Certainly more people are going to be killed by handgun in the United States than by these complications," agreed Crowther.
Many ordinary blood clots are never noticed -- something the CDC wants to change. It's taking part in an awareness campaign about blood clots which, if untreated, can cause mini-strokes, or lung-damaging pulmonary embolisms.
The rare blood clots being linked with vaccines are a type that become obvious. They block the veins leading from the brain and cause intense headaches or severe abdominal pain.
But they develop slowly, giving people the chance to get the right treatment -- if they seek it in time.
Now, media reports are starting to pop up about people suffering more ordinary blood clots after having been vaccinated. It's unlikely they are caused by the vaccine but it's hard for people who aren't trained in medicine to know the difference, Crowther said.
Getting vaccinated, especially against Covid, is a memorable event, and it's natural if people suffer a health problem soon after having been vaccinated for them to associate it with that vaccine.
"Vaccine hesitancy is a real problem. There is no way an average person can understand the magnitude of the risks," Crowther said. The results can be more deadly than the blood clots.
The good news is that it's not complicated to treat VITT, the experts agreed.
"The blood thinner that is most likely to be used for many of these patients is exactly the one you would use for standard, run-of-the-mill blood clots," said Crowther. Choices include pills such as apixaban, sold under the brand name Eliquis and rivaroxaban, sold under the brand name Xarelto. The American Society of Hematology also says delivering an antibody-based product called intravenous immune globulin can help restore a healthy balance of platelets.
"Any hospital in the United States would be well positioned to manage these blood clotting complications," Crowther said.
What's important for the time being is for doctors to avoid using another common blood thinner known as heparin. Heparin itself can cause the same antibody reaction being seen in VITT -- and in fact, it was doctors familiar with the heparin-related reaction who noticed what might be going on with the vaccines and blood clots.
The tell-tale signs of this rare reaction include not only blood clots, but a low level of platelets, which help blood to clot.
That seems like a contradiction, but Spyropoulos says what's going on is an immune reaction in which antibodies target platelets. "They form a complex causing the platelets to clump," he said.
The platelets disappear from circulation as they get stuck together. "You see a kind of gunk," Spyropoulos said.