US federal health agencies on Tuesday recommended pausing the use of Johnson & Johnson's Covid-19 vaccine 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.
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 tools in the fight against a pandemic that has claimed more than three million lives.
The White House said the pause 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.
The US Food and Drug Administration (FDA) said one person had died from the rare blood clotting condition after taking the J&J vaccine and another was in a critical condition.
Immunology experts stressed the risk posed by the J&J vaccine appeared extremely low, and that the shot remained a valuable tool against the risks of Covid-19 . However 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."
FDA acting commissioner Janet Woodcock said it expected the pause to be a matter of days, and it was aimed at providing information to healthcare providers so that they can diagnose, treat and report such blood clots.
FDA official Peter Marks said that part of the reason for the pause was to warn doctors that administering the standard treatments for clots can cause tremendous harm, or be fatal.
'Less than one in a million'
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, compared with more than 180 million shots combined of the Moderna and Pfizer/BioNTech shots.
US health officials said during a press briefing there had been no similar blood clot cases reported among recipients of the Pfizer/BioNTech of Moderna vaccines.
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 joint statement.
All six cases involved women between the ages of 18 and 48, and the symptoms occurred six to 13 days after vaccination.
In the 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.
"Even if all of the cases were caused by the vaccine, the risk of less than one in a million would have to be set against the benefits of protection from Covid-19 disease; a disease which, in itself, causes clotting in many cases," said Peter English, a retired consultant in communicable disease control and a vaccine expert.
'Abundance of caution'
The FDA recommendation to pause the administration of the Johnson and Johnson vaccine out of an abundance of caution makes sense in terms of the nature of the unusual and serious side effect not seen with the other vaccines," said Dr. Robert Klugman at the UMass Memorial Medical Center in Massachusetts in an email.
J&J's shares were down 2.6% in early New York trade.
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 rollout of J&J's vaccine has been slowed by issues at production plants.
The company has supplied the United States with at least 20 million doses but is behind schedule on its deliveries. Problems at a Baltimore, Maryland plant run by outside contractor Emergent BioSolutions forced the companies to trash a batch of vaccine substance.
J&J only began delivering its Covid-19 vaccine to European Union countries this week. It has committed to delivering 55 million doses to the bloc by the end of June and another 120 million in the third quarter.
J&J has been doing vaccinations in South Africa to study the effects on health workers, and had vaccinated just under 300,000 people there as of Monday, according to data from the country's health department. J&J recently struck a deal with the African Union for up to 400 million doses.
Europe's drugs regulator 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.