On Tuesday, the U.S. Food and Drug Administration and the Centers for Disease Control and Prevention called for a pause in the use of Johnson & Johnsons COVID-19 vaccine out of an abundance of caution so the agencies can review extremely rare blood clots.

Six recipients of the one-dose vaccine all in relatively young women developed blood clots within two weeks of receiving the shot, The New York Times reported . One died; one is currently hospitalized.

The government is expected to pause administration of the vaccine at all of its federally run vaccination sites, The New York Times also reported , and expects that state health officials will follow its cue and do the same.

But up to this week, nearly 7 million people in the U.S. have received the Johnson & Johnson vaccine, which has been hailed as a game changer in the race to get millions of shots into arms as COVID-19 cases once again creep up and variants continue to circulate.

So, should you be concerned about this? What should you do if you already received the Johnson & Johnson vaccine? Heres what to know right now:

These blood clots really are very rare.

Again, so far there have been six reported cases in the U.S. of a type of blood clot called cerebral venous sinus thrombosis , or CVST, seen in combination with low levels of blood platelets (thrombocytopenia). That is out of more than 6.8 million doses of the vaccine that have been administered to date.

Whats being investigated is something thats extremely rare. ... Its less than 1 per million, Amesh Adalja , a senior scholar at the Johns Hopkins University Center for Health Security and an infectious disease expert, told HuffPost.

The six reported cases have all occurred in women between the ages of 18 and 48 who developed symptoms six to 13 days after vaccination.

One reason why the CDC and FDA have so publicly sounded the alarm over these really rare clots aside from the fact that they are extremely serious and can be deadly is to make sure health care providers around the country are aware that they must be treated differently than other types of blood clots.

As the two groups have explained : Usually, an anticoagulant drug called heparin is used to treat blood clots. In this setting, administration of heparin may be dangerous, and alternative treatments need to be given.

Its not yet clear whats causing them.

The CDC has said it will convene a meeting of the Advisory Committee on Immunization Practices on Wednesday to take a closer look at these cases and to determine what to make of them.

As of now, however, there is no evidence of a causal relationship between the Johnson & Johnson vaccine and these blood clots, as the company was quick to emphasize in an official statement last week .

What needs to be established is whether this is something that was caused by the vaccine, Adalja said. Thats going to take some time to understand.

The best current hypothesis although it is only a hypothesis is that the Johnson & Johnson vaccine may trigger an immune response that effectively interferes with blood clotting.

Also, federal regulators and health care providers must keep in mind the broader context, he urged. Just remember that COVID-19 causes blood clots. And COVID-19 causes blood clots at a higher rate than the vaccine does, Adalja said.

The pause is to mainly prepare health care providers on what to look out for.

In a media briefing on Tuesday, FDA and CDC officials emphasized that the pause was largely to educate health care providers around the country on signs of these rare blood clots, as well as provide them with information about how to treat and report them.

Because again, if health care providers administer the standard treatment for blood clots to patients, the outcome of the treatments themselves could be fatal. Health officials want to ensure patients who present with low blood platelets or blood clots are asked about their recent vaccine history.

The pause itself is not expected to be long. Janet Woodcock, the acting commissioner of the FDA, said in the media briefing that the time frame of the pause will depend on what we learn in the next few days. However, we expect it to be a matter of days for this pause.

Anne Schuchat, principal deputy director of the CDC, said the agencies were committed to an expeditious review of the available information, and to an aggressive outreach to clinicians so that they know how to diagnose, treat and report.

Overall, the Johnson & Johnson shot is very effective at preventing COVID-19 infection and death.

Since the Johnson & Johnson vaccine became the third COVID-19 vaccine available to Americans, there have been questions and concerns about its efficacy. The shot is 66% effective at preventing symptomatic disease, which is not as high as either the Pfizer or Moderna vaccines.

However, it is also 85% effective at preventing severe disease , and 100% effective against hospitalization and death, which are the primary goals of our collective COVID-19 vaccination efforts.

Experts have also cautioned against spending too much time comparing the vaccines, and have urged Americans to get whatever is available to them. As the chief health and science officer of the American Medical Association has said : These vaccines have not been tested head to head, so its impossible to do a really accurate comparison. What matters most is they are all effective at preventing the most severe COVID outcomes, including hospitalization and death.

People who have had the shot already should watch their symptoms but not freak out.

Experts with the CDC and FDA emphasized that anyone who received the Johnson & Johnson vaccine more than a month ago should rest assured their individual risk of developing blood clots is very low.

But no health experts are saying these are risks that should be ignored altogether.

Individuals who received their shot within the last several weeks should be on the lookout for severe headache, abdominal pain, leg pain or shortness of breath within three weeks after vaccination, according to the CDC and FDA. If you develop any of these symptoms, reach out to your health care provider right away.

So what do experts think about all of this?

On Tuesday, the Biden administration downplayed the impact of the FDA and CDCs decision to pause the Johnson & Johnson vaccine. This announcement will not have a significant impact on our vaccination plan: Johnson & Johnson vaccine makes up less than 5% of the recorded shots in arms in the United States to date, Jeff Zients, the White House COVID-19 response coordinator said in a statement.

Clinicians are a bit more concerned about the aftermath of the announcement. Ive gotten two calls already from people who were scheduled to get the Johnson & Johnson vaccine asking if they should cancel, Adalja said. I said no.

And he worries about the impact that kind of doubt will have on this next stretch of the pandemic, and on our ability to get more vaccines in peoples arms and slow down transmission of the virus.

I think whats going to happen is youre going to get a litany of Johnson & Johnson cancellations. And this is going to slow down the vaccine rollout. Then in a week or so, I think theyll say something like: Yes, theres a very small risk here. But the risk-benefit ratio favors the vaccine, he continued. What will have been done is well have lost time in vaccination, and you already have a vaccine-hesitant population and this is going to add to that vaccine hesitancy.

The CDC and the FDA still stress that vaccination against COVID-19 is critical, especially as more cases continue to appear.

Experts are still learning about COVID-19. The information in this story is what was known or available as of publication, but guidance can change as scientists discover more about the virus. Please check the Centers for Disease Control and Prevention for the most updated recommendations.