US federal health agencies on Tuesday recommended pausing the use of Johnson & Johnson's COVID-19 vaccine for at least a few days after six women under 50 given the shot developed rare blood clots, dealing a fresh setback to efforts to tackle the pandemic.

Following the news, Johnson & Johnson (J&J) said it was delaying the rollout of the vaccine to Europe, a week after regulators there said they were reviewing rare blood clots in four recipients of the shot in the United States.

Janet Woodcock, acting commissioner of the US Food and Drug Administration (FDA) said the agency expected the pause to be a matter of days, and it was aimed at providing information to healthcare providers so they can diagnose, treat and report such blood clots.

The moves come after European regulators said earlier this month they had found a possible link between AstraZeneca's COVID-19 vaccine and a similar rare blood clotting problem that led to a small number of deaths.

J&J's single dose vaccine - most COVID-19 shots are delivered over two doses - and AstraZeneca's low-cost vaccine are seen as vital weapons in the fight against a pandemic that has claimed more than three million lives.

Immunology experts echoed US officials in underscoring that the risk posed by the J&J vaccine appeared extremely low, and it remained a valuable tool against the risks of COVID-19. But they acknowledged the need for health officials to proceed with caution to understand the best ways to mitigate any risk.

"Even if causally linked to the vaccine: 6 cases with about 7 million doses (lower than the risk of clots with oral contraceptives) is not something to panic about," Dr. Amesh Adalja, an infectious disease expert at the Johns Hopkins Center for Health Security in Baltimore, said in an email.

"People are asking me if they should cancel their J&J vaccine appointments and I have told them not to but I know many will and this will stall progress in controlling the pandemic."

The FDA told a briefing there had been one reported death from the rare blood clotting condition among recipients of the J&J vaccine, while another person was in a critical condition.

FDA official Peter Marks said it was "plainly obvious" the J&J cases were "very similar" to the AstraZeneca ones.

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However, officials said there had been no similar blood clot cases reported among recipients of the Moderna and Pfizer/BioNTech vaccines, which have accounted for most US vaccinations so far.

The White House said the pause in the J&J vaccine would not have a "significant" impact on its plan to administer about three million shots per day and a total of 200 million shots before President Joe Biden's 100th day in office.

Balance of Risks

Most of the available J&J vaccine has been used in the United States due to production issues that have limited the company's supply. As of April 12, more than 6.8 million doses of the J&J vaccine had been administered in the United States, representing about one reported case per million people.

An advisory committee to the US Centers for Disease Control and Prevention (CDC) will meet on Wednesday to review the cases, and the FDA will review the analysis, the agencies said in a statement.

All six cases involved women between the ages of 18 and 48, and the symptoms occurred six to 13 days after vaccination.

In these cases, a type of blood clot called cerebral venous sinus thrombosis (CVST) was seen in combination with low levels of blood platelets (thrombocytopenia).

J&J said it was working closely with regulators and noted no clear causal relationship had been established between the cases and the COVID-19 vaccine made by its Janssen unit.

"To put this into perspective, it's similar to the chance of being struck by lightning in any given year in the UK. On the other hand, the risks from COVID-19 are substantial," said Ian Douglas, Professor of Pharmacoepidemiology at the London School of Hygiene & Tropical Medicine.

"If all 6.8 million people who've received the J&J vaccine in the U.S. were infected with the virus, several thousand would likely die and many more, including younger adults, would experience serious and long-lasting after effects."

J&J's shares were down 2.6 percent in early New York trade.

Problems

The J&J and AstraZeneca vaccines both use an adenovirus vector - a harmless cold virus that instructs human cells to produce a protein found on the surface of the coronavirus, thereby spurring the immune system to prepare an arsenal against the COVID-19-causing virus.

Among leading global COVID-19 vaccine developers, China's CanSino Biological and Russia's Gamaleya Institute with its Sputnik V vaccine are also relying on this approach. The Pfizer/BioNtech and Moderna vaccines use mRNA technology.

The European Medicines Agency (EMA) continues to recommend the use of AstraZeneca's COVID-19 vaccine, saying the benefits outweigh the risks. Several EU countries, however, have limited its use to certain age groups.

J&J only began delivering its COVID-19 vaccine to European Union countries this week.