What do we know about Johnson & Johnson's COVID-19 vaccine and rare clots?

Here’s a look at what we know about the vaccine and the unusual blood clots.

Why are these clots different?

These are not typical blood clots. They're weird in two ways.

First, they're occurring in unusual parts of the body, such as veins that drain blood from the brain. Second, those patients also have abnormally low levels of platelets - cells that help form clots - a condition normally linked to bleeding, not clotting.

Scientists in Norway and Germany first raised the possibility that some people are experiencing an abnormal immune system response to the AstraZeneca vaccine, forming antibodies that attack their own platelets. That's the theory as the US now investigates clots in J&J vaccine recipients, Dr. Peter Marks, the Food and Drug Administration's vaccine chief, said Tuesday, AP reported.

Why suspect immune response?

The first clue: A widely used blood thinner named heparin sometimes causes a very similar side effect. Very rarely, heparin recipients form antibodies that both attack and overstimulate platelets, said Dr. Geoffrey Barnes, a clot expert at the University of Michigan, AP reported

“It kind of can cause both sides of the bleeding-clotting spectrum,” Barnes said.

Because heparin is used so often in hospitals, that reaction is something “that every hospital in America knows how to diagnose and treat.”

There also are incredibly rare reports of this weird clot-low platelet combination in people who never took heparin, such as after an infection.

Health officials said one reason for the J&J pause was to make sure doctors know how to treat patients suspected of having these clots, which includes avoiding giving heparin.

The Centres for Disease Control and Prevention later Tuesday provided advice on how to spot and treat the unusual clots.

What does research show?

Among possible causes being investigated are that the vaccine triggers an unusual antibody in rare cases. So far, risk factors like age or gender have not been singled out.

In two studies in the New England Journal of Medicine last week, research teams from Norway and Germany found platelet-attacking antibodies in the blood of some AstraZeneca vaccine recipients who had the strange clots. The antibodies were similar to those found with the heparin side effect even though the patients had never used that blood thinner.

It's not yet clear if there's a similar link to the J&J vaccine.

Who experienced the rare side effects?

In J&J's case, all six recipients were women between the ages of 18 and 48, and the symptoms occurred six to 13 days after vaccination. In the six cases, a type of blood clot called cerebral venous sinus thrombosis (CVST) was seen in combination with low levels of blood platelets, or thrombocytopenia.

In total, more than 6.8 million doses of the Johnson & Johnson vaccine have been given in the United States through April 12.

What technology does it use?

The J&J vaccine uses a common-cold causing adenovirus, which has been genetically modified so that it can't replicate, to carry the gene for a key part of the coronavirus.

The part is known as the "spike protein" and it's what gives the virus its crown-like appearance.

The vaccine delivers the instructions to make this protein to human cells, and our immune systems then develop antibodies against it, preventing the virus from invading cells.

Apart from antibodies, the vaccine also elicits the production of immune T cells, which kill infected cells and help make more antibodies.

J&J's shot is known as an "adenovirus vector vaccine" and the company previously produced a European Union-approved Ebola vaccine using the same technology.

Oxford-AstraZeneca and Sputnik's shots are both adenovirus vector vaccines, too.

They all use double-stranded DNA molecules to carry genetic instructions, rather than single-stranded RNA used by Pfizer and Moderna.

DNA is more rugged, which allows these vaccines to be stored at warmer temperatures.

Are pauses like this common?

Fewer than 1 in 1 million Johnson & Johnson vaccinations are under investigation, and it still hasn't been determined that the blood clots were related to the vaccine.

Pauses like this are common even after vaccines go into wide use to investigate further if an unusually large cluster of a certain type of medical cases turns up among people who've been inoculated.

What about the other vaccines?

The most widely used COVID-19 vaccines in the US - from Pfizer and Moderna - are made with a completely different technology, and the FDA said there is no sign of a similar clot concern with those vaccines.

Should people be worried because they received the J&J vaccination?

Marks said it's important not to confuse the rare clot risk with normal flu-like symptoms people often feel a day or two after a COVID-19 vaccination. He said concerning symptoms, such as severe headache or severe abdominal pain, would occur a week to three weeks after the J&J vaccine.

How are other countries reacting?

Deliveries have already begun in some European countries. Authorities took differing approaches on whether to restrict use of the single-shot vaccine with Belgium and France saying they would go ahead, while Sweden, Greece and Italy put them on hold.

France is sticking to its plan to give over-55s the Johnson & Johnson vaccine suspended in the US and South Africa over rare blood clots, a government spokesman said Wednesday.

Gabriel Attal also reaffirmed the government's confidence in the AstraZeneca jab as an "essential tool" in the fight against Covid-19, hours after Denmark said it was stopping its use, also over rare incidents of clots in people who received the vaccine.

France has already been using the AstraZeneca jab among over-55s and had been planning to boost its campaign with the Johnson & Johnson vaccine, which is similar to the AstraZeneca shot.