By Jeff Overley

Law360 (April 13, 2021, 10:40 PM EDT) -- The federal government's recommended pause of Johnson & Johnson coronavirus vaccinations involves safety issues that have been called exceedingly rare, but unmistakably comparable to risks that U.K. and European Union regulators recently linked to AstraZeneca's similar vaccine.

Tuesday's recommendation came from the U.S. Food and Drug Administration and the Centers for Disease Control and Prevention. It stemmed from reports of six American women who developed brain blood clots and low levels of platelets within two weeks of getting the J&J vaccine, leading to one woman's death and another woman's hospitalization in critical condition.

FDA and CDC leaders stressed Tuesday that the simultaneous sets of symptoms appear to be rare. J&J's single-shot vaccine has gone into the arms of 7 million Americans, meaning that about 0.00008% of doses have led to reports of cerebral clotting in conjunction with plunging platelets.

But when urging a pause, the agencies were also able to look at a larger group of patients in Europe, where some countries temporarily suspended the use of AstraZeneca's vaccine in recent weeks amid dozens of reports of blood clots alongside low platelet levels.

"It's plainly obvious to us already that what we're seeing with the [J&J] vaccines looks very similar to what was being seen with the AstraZeneca vaccine," Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, told reporters on Tuesday.

J&J and AstraZeneca are both using so-called adenoviruses — which in their normal forms typically cause mild illnesses — that have been neutralized and engineered into "viral vector vaccines" that teach the immune system to attack the novel coronavirus.

"I can't make some broad statement yet, but obviously they are from the same general class of viral vectors," Marks said Tuesday.

On April 7, the U.K.'s Medicines and Healthcare products Regulatory Agency unveiled new advice that found a "possible link" between AstraZeneca's inoculation and concurrent cases of blood clots and lowered platelets.

"The evidence of a link with COVID-19 Vaccine AstraZeneca is stronger" than once believed, "but more work is still needed," the U.K. agency said.

The U.K had reported 79 cases of clotting with reduced platelets in AstraZeneca vaccine recipients as of March 31, and 19 of those reported cases involved fatalities.

In a statement, Dr. June Raine, chief executive of the MHRA, said that 37 million injections of AstraZeneca's two-dose vaccine had been administered, thereby "saving thousands of lives."

"No effective medicine or vaccine is without risk," Raine said.

The European Medicines Agency announced the same conclusion in conjunction with the MHRA, saying in a statement that "unusual blood clots with low blood platelets should be listed as very rare side effects" of AstraZeneca's vaccine.

"EMA is reminding health care professionals and people receiving the vaccine to remain aware of the possibility of blood clots combined with low levels of blood platelets occurring very rarely within two weeks of vaccination," the European Union agency said.

U.K.-based AstraZeneca responded by observing that neither agency had identified "a definite cause for these extremely rare events."

During the FDA's news briefing on Tuesday, Marks also said that American regulators "don't have a definitive cause." But he added that the FDA thinks it's likely that J&J's and AstraZeneca's vaccines are both "very, very rarely" triggering a harmful immune system response.

"The probable cause that we believe may be involved here, that we can speculate, is a similar mechanism that may be going on with the other adenoviral vector vaccine," Marks said.

AstraZeneca's vaccine hasn't won emergency authorization in the U.S. J&J's vaccine won emergency clearance in February, and the company at the time identified various types of blood clots — including deep vein thrombosis, pulmonary embolism and transverse sinus thrombosis — as "important potential risks."

"FDA will recommend surveillance for further evaluation of thromboembolic events with deployment of the vaccine into larger populations," the agency told an advisory committee in February.

The combination of clots and low platelets is particularly dangerous because traditional treatment of clotting with the blood thinner heparin can make things worse, FDA officials said Tuesday.

In a Tuesday statement, J&J acknowledged the U.S. recommendation and announced that it would "proactively delay the rollout of our vaccine in Europe," where the European Commission last month granted the company a conditional authorization.

The vast majority of U.S. inoculations for COVID-19 have utilized two vaccines — one from Moderna Inc., and another from Pfizer Inc. and its German partner BioNTech — that use mRNA, not adenoviruses. Roughly 185 million doses of the Moderna and Pfizer-BioNTech vaccines have been given to Americans, and there haven't been any reports of simultaneous blood clots and platelet deficiency, according to the FDA.

There was widespread debate Tuesday about whether the FDA-CDC recommendation would damage or fortify vaccine confidence among the American public. But government officials insisted that the supplies of the Moderna and Pfizer-BioNTech vaccines will be more than sufficient for every adult who wants to get vaccinated.

Pfizer CEO Albert Bourla on Tuesday announced that the company expects to deliver 220 million doses by the end of May — exceeding its target by 10% — and to deliver its full commitment of 300 million doses by mid-July, two weeks ahead of schedule.

Anne Schuchat, the CDC's principal deputy director, emphasized Tuesday that J&J is supplying the "great minority" of doses, while adding that possible long-term supply impacts can't yet be forecast with precision.

The CDC has scheduled an emergency meeting for Wednesday afternoon, where its Advisory Committee on Immunization Practices will discuss whether the J&J vaccine label should be updated with new risk information.

Janet Woodcock, the FDA's acting commissioner, told reporters on Tuesday that regulators "expect it to be a matter of days" before J&J's vaccination pause is lifted.

--Editing by Regan Estes.

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