US health agencies have recommended states pause the administration of the Johnson & Johnson coronavirus vaccine, after reports of rare and severe blood clots emerged in six women. More than 6.8m doses have been administered nationally. The concerns mirror those of drugs agencies in Europe and Australia over the AstraZeneca vaccine, which has not been authorized in the US. There have been no significant safety concerns raised about the two other vaccines that make up the majority of US supply, from Pfizer-BioNTech and Moderna. The acting FDA chief, Janet Woodcock, said: “We’re recommending this pause while we work together to full understand these events.” The decision was taken in coordination with the CDC. Woodcock said: “Right now, I’d like to stress these events appear to be extremely rare. However, Covid-19 vaccine safety is a top priority for the federal government. We take all reports of adverse events related to the vaccine very seriously.” The FDA and CDC said in a joint statement: “People who have received the J&J vaccine who develop severe headache, abdominal pain, leg pain, or shortness of breath within three weeks after vaccination should contact their healthcare provider.” Officials with both agencies said the events occurred between one and two weeks after the vaccine was administered, past the stage when many people might experience common, flu-like symptoms associated with the vaccine. Officials said people who received the vaccine more than one month ago should consider their risk “very low”. Anne Schuchat, the deputy principal director of the CDC, said she understood many Americans who had already received the vaccine were “probably very concerned” but emphasized the syndrome associated with the vaccine was exceedingly rare. Nevertheless, “it was clear to us we needed to alert the public,” Schuchat said. Officials are also expecting to discover more cases in the coming days, as clinicians realize the potential link to recent vaccination. Officials also warned clinicians to be on the lookout for the rare syndrome, as using a common treatment for blood clots – the blood thinner heparin – could be “dangerous” and worsen the syndrome. All six cases of the clotting disorder were women were between the ages of 18 and 48. One woman died, and a woman in Nebraska is hospitalized in critical care, according to the New York Times. Johnson & Johnson’s vaccine is also under scrutiny by the European Medicines Agency, which is investigating four cases of clotting. Health agencies said the blood clotting concern is “extremely rare”. Authorities are still investigating whether the clotting is indeed caused by the vaccine. The background rate of similar clotting disorders is between two and 14 people per million. However, the presence of low platelet count in conjunction with the clotting makes the syndrome remarkably similar to clotting seen with the AstraZeneca vaccine. As of 4 April, there have been 222 cases of blood clots among the more than 34 million globally who have received the AstraZeneca vaccine. The European Medicines Agency has examined 86 cases of blood clots, 18 of which have been fatal. US officials said they are “in constant contact with regulators worldwide”. J&J said in a statement: “The safety and wellbeing of the people who use our products is our No 1 priority. At present, no clear causal relationship has been established between these rare events and the Janssen Covid-19 vaccine.” Janssen is the division of Johnson & Johnson that developed the vaccine. Johnson & Johnson also said it will “proactively” delay the rollout of the vaccine in Europe. US officials also said they are looking closely at whether the technology shared by the AstraZeneca and Johnson & Johnson vaccine could have a link to the syndrome. J&J and AstraZeneca use a vaccine technology called an adenovirus platform to prompt immunity. The technology uses a second, weakened cold virus to deliver the genetic payload of the coronavirus to the immune system, and spur the body to make antibodies. AstraZeneca’s vaccine uses a virus derived from chimpanzees, where J&J’s uses a human virus. Dr Peter Marks, director of the center for biologics evaluation and research at the FDA, said: “It’s plainly obvious to us already that what we’re seeing with the Janssen vaccines looks very similar to what was being seen with the AstraZeneca vaccine.” However, a mechanism that may cause the potential connection between clotting and the adenovirus platform is not clear. Officials said they currently hypothesize the reaction may relate to an immune response involving platelets. Moderna and Pfizer use a different vaccine platform called messenger RNA, to provoke the body to build antibodies. Independent experts have said the “pause” in administration shows federal monitoring systems for potential adverse side effects of the vaccine are working. Scott Gottlieb, a former FDA commissioner, said on CNBC’s Squawk Box: “For most consumers, I wouldn’t be concerned about this. Really this is an alert to doctors. That’s how FDA framed it. It’s advice to doctors to be monitoring more closely.” The pause in administration of the Johnson & Johnson vaccine may also cause a hit to the American immunization campaign, which was set to administer hundreds of thousands of easy-to-administer doses provided by the company. The White House denied the pause would have a significant impact, and said vaccination appointments could be rescheduled. The J&J pause comes at a perilous time in the pandemic. Despite the strides made in immunizing all adult Americans, with 74 million people and 22% of the public fully immunized, Covid-19 cases are rising in several states, such as Minnesota, and remain high in others, such as Michigan. With a weary public and the return of warm weather in much of the country, politicians have had little appetite to renew restrictions on social life and many instead say the vaccination campaign can curb cases. The new concerns are also likely to inflame partisan tensions and the conspiracy theorists. Republicans, especially men, remain the most hesitant to receive a coronavirus vaccine after national politicians and Donald Trump downplayed the seriousness of the virus. Vaccine hesitancy could play an increasingly significant role in whether the US is able to curb the pandemic, or if the viral spread is allowed to continue and circulate regionally. Although the exact level of immunity needed to squash viral spread is not known, experts believe between 70-90% of the public will need to be vaccinated. Because children make up about a quarter of the US population, that level of immunity would require nearly every US adult to be vaccinated to stop spread.