The United States’ COVID-19 vaccination effort was dealt a setback Tuesday when the U.S. Centers for Disease Control and Prevention and the U.S. Food and Drug Administration recommended that states pause their administration of vaccines manufactured by Johnson & Johnson.

The reason for the pause was to give scientists more time to closely study reports that a small number of women developed blood clots after they received the one-dose vaccine. The CDC said the pause was recommended out of an abundance of caution and to communicate with health care providers to be on the lookout. It’s expected to last days, not weeks.

Here’s a closer look at what happened and what it means:

How many reports of blood clots have there been and how many doses of this vaccine have been administered?

According to the CDC, there have been six cases of a rare but severe blood clot that developed between 6 and 13 days following vaccination. All six cases were women between 18 and 48 and each had low platelet counts in their blood. One woman died.

More than 7 million doses of Johnson & Johnson vaccine have been administered across the country, or less than 5 percent of all vaccine doses to this point. That means the chance of developing a blood clot from the J&J vaccine is roughly 0.00009 percent, based on the existing data. In any given year, approximately 900,000 Americans experience blood clots, out of 328 million people. That’s approximately 0.3 percent, or over 3,000 times more likely than the blood clots reported among Johnson & Johnson vaccine recipients. For comparison, the odds of being struck by lightning in a given year are roughly one in 500,000, according to the U.S. CDC. That’s two times more likely than the blood clots reported among J&J recipients.


Blood clots also are far more likely to develop in people who contract COVID-19. Additionally, the risk of dying from COVID-19 is much greater. For every million cases of COVID-19 in the United States, 1,700 people have died, according to CDC data, although the risk is much greater among older individuals.

Do scientists know whether the vaccine caused the blood clots?

As of Tuesday, there was no causal link, which means no proof that the vaccine is what caused the blood clot. Blood clots can develop for any number of reasons and there are numerous factors that increase a person’s risk of developing them, including: smoking, obesity, prolonged bed rest due to surgery or hospitalization, use of birth control pills or hormone replacement therapy and cancer.

In fact, the risk of developing a blood clot in those who take birth control pills is between 1 in 1,000 and 1 in 3,000.

Any or all of the women who developed clots could have had underlying conditions. That’s one of the questions the CDC is hoping to answer.

What is a blood clot and how does it develop?


A blood clot occurs when blood coagulates, or becomes less liquid and more solid. Sometimes, it’s a necessary process to prevent people from losing too much blood if they have been injured. When you put pressure on a cut, for instance.

However, when clots develop in arteries or veins within the body they don’t always dissolve naturally, which makes it difficult for blood to flow. If undetected or untreated, they can cause heart attacks, stroke or even death in severe cases.

There are many different types of blood clots. The ones that are being studied in Johnson & Johnson vaccine recipients are cerebral venous sinus thrombosis (CVST), which means they occurred in veins that drain blood from the brain.

If I got the Johnson & Johnson vaccine, should I be worried?

Probably not. The instances of blood clots have been exceedingly rare. Both the CDC and FDA recommend that people who were given the Johnson & Johnson vaccine and who have experienced severe headache, abdominal pain, leg pain, or shortness of breath within three weeks after receiving the shot contact their health care provider.

Any minor symptoms are not cause for concern and in fact are common in the 24-48 hours after getting the vaccine.


One of the biggest reasons for the pause in administration of the vaccine was to communicate with health care providers the risk in using a typical treatment for blood clots – specifically blood-thinners like heparin.

“In this setting, administration of heparin may be dangerous and alternative treatments need to be given,” the FDA and CDC said in a statement.

How does the Johnson & Johnson vaccine work?

The COVID-19 vaccine developed by Johnson & Johnson is a traditional carrier vaccine. It involves injecting individuals with an inactive adenovirus, or common cold virus, that carries the genetic code of the coronavirus spike proteins. The body’s immune system is then trained to create antibodies and memory cells that protect against the actual virus.

The other two COVID-19 vaccines that have been granted emergency use authorization by the FDA – from Pfizer and Moderna – were the first vaccines ever developed using messenger RNA technology. That involves delivering a piece of genetic code from the coronavirus to host cells in the body. The body then makes copies of the virus’ spike proteins, which stimulates the immune system to produce antibodies and develop memory cells.

There have not been any reports of blood clots associated with either the Pfizer or Moderna vaccine.


However, another carrier vaccine, produced by AstraZeneca, also has seen extremely rare incidences of blood clots in younger people. That vaccine has not been authorized for use in the United States.

What happens next with the Johnson & Johnson vaccine?

The U.S. CDC said it will convene a meeting of the Advisory Committee on Immunization Practices on Wednesday to further review these cases and assess their potential significance. The FDA will review that analysis as it also investigates these cases. The pause will last until that process is complete.

Maine CDC Director Dr. Nirav Shah said although the pause is a “bummer,” it’s a good sign of how the process is supposed to work to ensure safety.

What happens to existing Johnson & Johnson vaccine doses that have been distributed to states like Maine?

Because most experts predict the pause will be temporary, the doses that have been sent to states for use will be shelved but not discarded. Unlike the other vaccine options, the Johnson & Johnson version does not need to be kept in cold storage.

Shah said the state has advised all vaccine providers to hang on to their Johnson & Johnson doses.

“I’m not too worried about them spoiling on the shelf,” he said.

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