On Tuesday morning, U.S. health officials recommended a break in using Johnson & Johnson’s Covid-19 vaccine while examining six reports of blood clots in women ages 18 to 48. One has died, another is in critical condition in the hospital.

As of Tuesday, 7.2 million people in the US had received the vaccine with no other serious side effects reported.

Experts are yet to determine to what extent the vaccine may be responsible for the clots. However, the investigation follows action by European regulators who concluded that a vaccine made by AstraZeneca may also be the cause of a similar, extremely rare, coagulation disorder.

US and European public health experts have emphasized that for most people, the benefits of Covid vaccines far outweigh the risks.

Several countries in Europe last week restricted the use of the AstraZeneca-Oxford vaccine to older adults due to the rare occurrence of a blood disorder in younger people. The AstraZeneca vaccine was not approved in the United States.

Fewer than one in a million J&J vaccinations are currently being studied. If there is indeed a risk of blood clots from the vaccine – which has yet to be determined – the risk is extremely small. The risk of contracting Covid-19 in the United States is much higher.

The FDA recommends people who have received the Johnson & Johnson vaccine within the past three weeks to see their doctor if they have severe headache, abdominal pain, leg pain, or difficulty breathing. Doctors who see people with these symptoms, especially if the patients are young women, should ask if they have recently had a Covid shot, said Dr. Anthony Fauci, senior scientific advisor to the Biden administration, at a press conference Tuesday.

People shouldn’t worry about mild headaches and flu-like symptoms for the first few days after vaccination. These are common, harmless side effects caused by the immune system’s defense against the coronavirus.

For people who got the vaccine a month or two ago, the blood clot problem means “nothing,” said Dr. Fauci. The six cases occurred within a “fairly narrow window” of six to 13 days after people got the shot, he said.

During clinical trials and after the widespread use of vaccines, experts track all of the medical problems that arise in people who receive them. If an unusually large number of cases arise, regulators may decide to stop a study or stop using a vaccine for further investigation.

Breaks are frequent, and tests usually show that the medical problems were incidental. If the investigation reveals that a vaccine poses a risk, regulators can write new guidance on who should or should not receive it. The break also gives them time to advise doctors on how to recognize and treat the disease.

In this case, the Food and Drug Administration and Centers for Disease Control and Prevention recommend a break, and states have already followed the agencies’ advice. Companies that are temporarily discontinuing use of the vaccine include CVS Health and Walgreens.

In a press conference Tuesday, federal officials said the government review would likely only take a few days. A CDC panel is expected to discuss the issue at a meeting on Wednesday.

Six women in the United States who received the Johnson & Johnson vaccine developed a rare blood clot disorder within about two weeks of being vaccinated. In people with this condition called cerebral venous sinus thrombosis, clots form in veins that drain blood from the brain. The results are “stroke-like,” said Dr. Anne Schuchat from the CDC.

Researchers studying a very similar disorder in AstraZeneca recipients in Europe say it appears to be caused by an intense immune system response to the vaccine, which makes antibodies that activate platelets, a blood component that helps to form normal blood clots in the Contributes to wound repair. In addition to clots, abnormal bleeding occurs. European researchers have named the disorder identified there as “vaccine-induced immune thrombotic thrombocytopenia”.

Dr. FDA’s Peter Marks said it was the unusual combination of clotting and bleeding that set a red flag for regulators as a possible safety signal.

So far, researchers have failed to find a way to predict who will develop the disorder and have not identified an underlying condition that could indicate a susceptibility.

Updated

April 13, 2021, 10:49 p.m. ET

Dr. Fauci said if scientists could identify a common core trait among the women who developed the blood clots, it could help identify who is at risk and allow regulators to pinpoint categories of people who are not receiving the J&J vaccine should.

A blood clot is a thickened, gelatinous clot of blood that can block blood flow. Clots form in response to injury and can also be caused by many diseases, including cancer and genetic disorders, certain medications, and prolonged sitting or bed rest. Covid itself can cause serious coagulation problems. Clots that form in the legs sometimes break off and travel to the lungs or, rarely, to the brain, where they can be fatal.

In the United States, 300,000 to 600,000 people each year develop blood clots in their lungs, leg veins, or other parts of the body, according to the CDC

Based on this data, approximately 1,000 to 2,000 blood clots occur daily in the US population. With millions of people being vaccinated every day now, some of these clots appear in those who just happened to receive the shots, regardless of the vaccine.

In the UK, about 1 in 1,000 people are affected by a blood clot in a vein each year, according to regulators.

However, the worrying coagulation disorder among vaccine recipients is much rarer and different from typical blood clots. In addition to clotting in the brain – cerebral venous sinus thrombosis for short, or CVST for short – all patients had remarkably low levels of platelets, which made them prone to abnormal bleeding.

On Tuesday, Johnson & Johnson announced that the company was aware of an extremely rare condition involving people with blood clots combined with low platelets in a small number of people who received the vaccine. “In addition, we have examined these cases with the European health authorities,” said the company in its statement. “We made the decision to proactively delay the launch of our vaccine in Europe.”

At the press conference on Tuesday, Dr. Marks from the FDA said the cases are “very, very similar.”

What You Need To Know About The Johnson & Johnson Vaccine Break In The United States

Both Johnson & Johnson and AstraZeneca use adenoviruses to transport DNA into human cells and begin the process of creating immunity to the coronavirus. It is not yet known if this technology is causing the problem.

German researchers have speculated that DNA from the vaccine could trigger the immune response in some people. However, the condition is so rare that researchers say patients likely also have an individual biological trait – which is still unknown – that predisposes them to the immune overreaction.

Last month, European regulators began investigating similar cases of cerebral vein thrombosis related to low platelets. They concluded that the disorder was a very rare side effect of the AstraZeneca vaccine. It’s too early to know if Johnson & Johnson’s vaccine is responsible for the same type of rare blood clot.

European regulators had recommended that recipients of the vaccine seek medical help for a number of possible symptoms, including leg swelling, persistent abdominal pain, severe and persistent headache or blurred vision, and tiny spots of blood under the skin outside of the area where the Injection was given was given.

However, these symptoms were so vague that the UK emergency departments almost immediately saw an increase in patients worried they were as they were described.

Still, German researchers say such symptoms need to be followed up in vaccine recipients. Blood tests can detect the antibodies.

Doctors in Germany and Norway have treated patients with blood-thinning drugs to stop the clot from growing and with intravenous immunoglobulin, which can help clear up the misdirected antibodies that are causing the problem.

The researchers there and the US federal health authorities advised against the use of a conventional blood thinner, heparin, on Tuesday and recommended that alternative drugs be chosen instead. The reason is that the disorder is very similar to a rare syndrome caused by heparin and it is possible that heparin could make the situation worse in these patients.

Heparin could “do a lot of damage,” said Dr. Marks.

German researchers have stressed that treatment should start as soon as possible as the condition can worsen quickly.

We do not know yet. The six cases studied in people who received the J&J vaccine all involved women. However, this number is so small that no clear conclusions can be drawn from it.

When asked on Tuesday whether the use of contraception could be a risk factor, US health officials said they knew no connection.

In Europe, it initially appeared that women were at a higher risk of blood clots associated with the AstraZeneca vaccine, although in some cases men who had received the vaccine were affected.

Upon closer inspection, it was found that in some countries more women overall received the vaccine because they were overrepresented among health workers. UK regulators now say they have no evidence as to whether men or women are more likely to be affected by blood clots.

The FDA did not find any similar cases in people who received the Moderna or Pfizer-BioNTech vaccines.

However, decreased platelet counts have been reported in a few patients who received the Moderna, Pfizer-BioNTech and AstraZeneca vaccines. One recipient, a doctor in Florida, died of a cerebral haemorrhage when his platelet counts failed to restore, and others were hospitalized. US health officials have stated that the cases are being investigated, but they have not reported the results of those reviews and have yet to advise that they are linked to the vaccines.

Benjamin Mueller contributed to the reporting.