The CDC will hold a meeting Wednesday of its Advisory Committee on Immunization Practices to review the cases and assess their potential significance, the statement said. The FDA will continue to investigate the cases.
“Until that process is complete, we are recommending a pause in the use of this vaccine out of an abundance of caution,” the statement said. “This is important, in part, to ensure that the health care provider community is aware of the potential for these adverse events and can plan for proper recognition and management due to the unique treatment required with this type of blood clot.”
The type of clot, called cerebral venous sinus thrombosis, was seen in combination with low levels of blood platelets, the agencies said. Treatment is different from what might typically be administered; while usually an anticoagulant called heparin is used to treat blood clots, it is dangerous to give heparin in this situation. Alternative treatments need to be given, they said.
The officials said the clots “appear to be extremely rare.” They said people who have received the Johnson & Johnson vaccine who develop severe headache, abdominal pain, leg pain or shortness of breath within three weeks after vaccination should contact their doctor.
The rare blood clots, paired with low levels of platelets, were first detected in people in Europe who had received the vaccine developed by AstraZeneca and the University of Oxford. European regulators said it was “plausible” that the clots were linked to that vaccination and have also been reviewing four similar clotting cases after vaccination with the Johnson & Johnson vaccine, which uses a similar technology.
In a statement, Johnson & Johnson spokesman Jake Sargent said the company shares all adverse event reports about individuals who receive the vaccine with health authorities.
“At present, no clear causal relationship has been established between these rare events and the Janssen COVID-19 vaccine,” Sargent said. “We continue to work closely with experts and regulators to assess the data and support the open communication of this information to healthcare professionals and the public.”
One of the cases happened in a Nebraska woman in her late 40s, who experienced a blood clot two weeks after receiving the Johnson & Johnson vaccine and received treatment at the Nebraska Medical Center in Omaha, according to state and federal health officials. The woman remains hospitalized, state officials said Monday.
European regulators in their investigation of the AstraZeneca vaccine said that out of 34 million people vaccinated, 222 cases of clots had been reported. There were at least 18 deaths.
Dan Barouch, a vaccine expert at Beth Israel Deaconess Medical Center in Boston who has been developing the technology used in the Johnson & Johnson vaccine for years, said in an email last week that there were a number of biological differences between the viral vectors, called adenoviruses, that are being used in the two vaccines.
The AstraZeneca vaccine uses an adenovirus that normally infects chimpanzees, while the Johnson & Johnson adenovirus normally infects humans.
“As such, issues that arise with one [adenovirus] vector do not necessarily apply to a biologically very different vector,” Barouch said.
State officials said they were not given any heads up about the decision to pause.
“We were not informed prior to the announcement,” said Michael Fraser, executive director of the Association of State and Territorial Health Officials. “We are assessing the impact in the states. We’re supportive of an abundance of caution, but we also need to balance getting everyone vaccinated as soon as possible.”
Nearly 7 million doses of Johnson & Johnson’s single-shot vaccine had been administered throughout the country as of Monday, according to CDC data. More than twice that amount, about 16 million doses, have been delivered to states and territories, and through federal channels, since the beginning of March.
Despite a recent dip in shipments of Johnson & Johnson’s vaccine, clinics were scheduled for use of the shot throughout the country, from a Walmart drive-through in Bentonville, Ark., to a clinic at Canisius College in Buffalo. The one-shot option has proved popular for many people who have gravitated to its convenience, and states have deployed it in a variety of settings, from mass sites to targeted efforts to reach transient populations, such as the homeless.
Sites in a handful of states temporarily halted use of Johnson & Johnson’s vaccine last week after adverse reactions were reported, though these were immediate events consistent with common side effects of some vaccines, such as dizziness. The adverse reactions were reported in Colorado, Georgia, Iowa and North Carolina. CDC said it investigated and found no reason for concern.
Isaac Stanley-Becker contributed to this report.