The FDA and U.S. Centers for Disease Control and Prevention (CDC) are recommending that U.S. states pause their administration of Johnson & Johnson’s Janssen COVID-19 Vaccine Candidate while they review six reported cases of patients developing what they termed rare and severe blood clots after receiving the vaccine.

The CDC’s Advisory Committee on Immunization Practices (ACIP) will meet Wednesday to review the cases and assess their potential significance. The FDA also plans to investigate those cases, and will review the CDC’s analysis, the agencies said this morning in a joint statement.

“Until that process is complete, we are recommending a pause in the use of this vaccine out of an abundance of caution,” the agencies said, in a statement by Peter Marks, MD, PhD, director of the FDA’s Center for Biologics Evaluation and Research and Anne Schuchat, MD, principal deputy director of the CDC and a retired rear admiral with the U.S. Public Health Service.

“Right now, these adverse events appear to be extremely rare,” Marks and Schuchat added. “COVID-19 vaccine safety is a top priority for the federal government, and we take all reports of health problems following COVID-19 vaccination very seriously.”

J&J issued its own statement this morning acknowledging it was “aware that thromboembolic events including those with thrombocytopenia have been reported with COVID-19 vaccines.”

“At present, no clear causal relationship has been established between these rare events and the Janssen COVID-19 vaccine,” J&J stated. “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.”

Shares of J&J dipped 2.5% in early trading to $157.55 as of 9:45 a.m., from Monday’s closing price of $161.64.

Janssen COVID-19 Vaccine Candidate is an adenovirus vector vaccine based on constructs created and tested by J&J with Beth Israel Deaconess Medical Center (BIDMC), part of Harvard Medical School, using Janssen’s AdVac® and PER.C6® vaccine technology platforms. AdVac can be used with Janssen’s PER.C6® technology to develop recombinant vaccines against infectious diseases.

CVST, low levels of platelets

In the six cases being reviewed by the FDA and CDC, a cerebral venous sinus thrombosis (CVST) was seen in combination with low levels of blood platelets. All six cases occurred among women between ages 18–48, with the symptoms occurring 6–13 days after vaccination.

Treatment of CVST differs from typical treatment for blood clots, which would usually involve administration of heparin—an option that may be dangerous in these cases, requiring the use of alternative treatments, the FDA and CDC cautioned.

The FDA and CDC urged J&J vaccine recipients who develop severe headache, abdominal pain, leg pain, or shortness of breath within three weeks after vaccination to contact their healthcare provider—who in turn were asked to report adverse events to the Vaccine Adverse Event Reporting System.

It was not known if the six cases include any of the cases that last week led to a pause in the “Vaccines for All” mass vaccination event at Dick’s Sporting Goods Park in Commerce City, CO, hosted with the state of Colorado. Eleven patients experienced adverse reactions to the Janssen COVID-19 vaccine candidate. Two of the eleven were transported to nearby hospitals “out of an abundance of caution” after medical staffers concluded they required additional observation, Centura said in a statement.

The J&J vaccine received emergency use authorization from the FDA for protection from COVID-19 on February 27—a day after all 22 members of the agency’s Vaccines and Related Biologic Products Advisory Committee (VRBPAC) unanimously recommended emergency authorization of the J&J vaccine. On February 24, the FDA’s staff recommended agency approval of the Janssen COVID-19 Vaccine Candidate in a briefing document released two days in advance of VRBPAC’s meeting.

The blood clot type seen in the six cases is similar to that seen in relatively small numbers of patients treated with another COVID-19 vaccine, AstraZeneca’s AZD1222. Last week, the European Medicines Agency (EMA) and the U.K. Medicines and Healthcare products Regulatory Agency both requested updates to the prescribing labels of COVID-19 Vaccine AstraZeneca, known in the EU as Vaxzevria, that include blood clots as an extremely rare potential side effect, following separate reviews.

The EMA received reports of 169 cases of CVST and 53 cases of splanchnic vein thrombosis as of April 4. An EMA safety committee reviewed in depth 62 cases of CVST and 24 cases of splanchnic vein thrombosis reported in the EU drug safety database (EudraVigilance) as of March 22, 18 of which were fatal.

The MHRA said it reviewed 79 cases (51 women, 28 men) of blood clots in the U.K. as of March 31—44 of the 79 cases were of CVST with thrombocytopenia; the other 35 were of thrombosis in other major veins with thrombocytopenia. The 79 cases resulted in 19 deaths (13 women, six men)—14 from CVST with thrombocytopenia; five from thrombosis with thrombocytopenia.

“Despite issues with J&J and AZN, we continue to believe a majority of Americans will/can be vaccinated by this summer through exclusive use of mRNA vaccines from MRNA [Moderna] and PFE [Pfizer and BioNTech], which are slated to deliver 200M doses in Q2 and another 200M doses by Q3, for a total of 600M doses,” Jefferies Equity Analyst Michael Yee wrote in a research note. ” The math clearly says there are more vaccines in USA than number of people who are amenable and necessary to take them. So the overall recovery in USA is still on track based on the math.”

The J&J vaccine candidate is among 21 “Front Runner” leading candidates among the more than 300 COVID-19 therapeutics under study in GEN’s updated and just-launched “COVID-19 Drug & Vaccine Candidate Tracker.”

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