White Oak, Maryland - US health authorities on Tuesday recommended halting the use of Johnson & Johnson's Covid-19 vaccine after several cases of blot clots emerged.

The Food and Drug Administration (FDA) and the Centers for Disease Control (CDC) called for a stop to inoculations with the shot as a precaution after six women developed rare cerebral thrombosis after receiving it, according to a joint statement.

The women aged 18 to 48 developed the disease between six and 13 days after inoculation. In three cases they also developed thrombocytopenia, a lack of blood platelets.

The cases are now under investigation, but the FDA and CDC recommended not to use the vaccine until the results are available.

Until now, more than 6.8 million people have received Johnson & Johnson's single-shot vaccine against Covid-19, which was authorized in February.

"CDC will convene a meeting of the Advisory Committee on Immunization Practices (ACIP) on Wednesday to further review these cases and assess their potential significance," the statement said.

"FDA will review that analysis as it also investigates these cases. Until that process is complete, we are recommending a pause in the use of this vaccine out of an abundance of caution.

As a result of the recommendations, all federal vaccination centers across the country will stop administering the Johnson & Johnson jab for the time being, according to CNN.

The agencies also urged anyone who has experienced strong headaches, leg and abdomen pain, or shortness of breath within three weeks of a vaccination to contact their health provider.

CNN spoke to Dr. Carlos del Rio, executive associate dean of the Emory University School of Medicine at Grady Health System, who put the number of blood clots into perspective.

"It's a very rare event. You're talking about one per million, and when you give millions of doses of vaccines, you will see events like this that you couldn't see in the clinical trial just because you didn't have millions of people enrolled," he said.

Rollout of the AstraZeneca Covid-19 vaccine, which has not yet been approved for use in the US, hit a similar snag last month. Several European countries suspended its use due to fear of blood clotting.

Since the rare blood clots in the brain only mostly occurred in people under 60, the two-shot jab has since been administered only to older people.