(CNN) -- The US Centers for Disease Control and Prevention changed its recommendations for Covid-19 vaccines Thursday to make clear that shots made by Moderna and Pfizer/BioNTech are preferred over Johnson & Johnson's vaccine.
The new recommendation: "mRNA vaccines are preferred over the Janssen Covid-19 vaccine for the prevention of Covid-19 for those 18 years of age and over."
Earlier Thursday, the CDC's Advisory Committee on Immunization Practices endorsed the updated recommendation after hearing new data indicating that a rare blood clotting syndrome is more common among people who recently got a J&J vaccine than previously believed. CDC Director Dr. Rochelle Walensky accepted the updated recommendation within hours.
The CDC has logged 54 cases in the US of thrombosis with thrombocytopenia syndrome or TTS in the US since the vaccine became available. Nine people have died -- seven women and two men.
Rates are higher than previously estimated among both men and women, the CDC's Dr. Isaac See told the meeting. While only a few cases have been seen for every million people vaccinated by age group, they are higher than what was believed when vaccine advisers were last briefed.
"We've been struck on reviewing these cases by how rapidly a patient deteriorates ... to death," See said. Symptoms always begin within two weeks of getting the vaccine and, on average, within nine days.
"The US TTS case reporting rate following Janssen Covid-19 vaccination is higher than what was previously presented to ACIP and the case reporting rate for men 40-49 years and for women 50-64 is similar to that for women 18-29 years," See told the meeting.
Thirty-nine of the 54 reported cases occurred before the CDC and the US Food and Drug Administration paused administration of the vaccine in April to investigate the link. The agencies resumed permission for the vaccine's use later in April. Since then, CDC has learned of more cases, See said.
"The rates are now slightly higher for all age groups," he said. The highest rate of TTS is among women -- 10 per million vaccinations given in women ages 30 to 39 and 9 per million in women ages 40 to 49.
Committee weighed risks and benefits
The overall risk of developing TTS is 3.8 cases per million doses given. The death rate is less than one per million doses given: 0.57 per million. Risks are higher for women than for men, but they're higher among men than previously known, the CDC's Dr. Sara Oliver said.
ACIP members considered the new data and weighed it against numerous studies showing J&J's vaccine is less effective than the Moderna and Pfizer/BioNTech vaccines in preventing infection.
"The U.S. supply of mRNA vaccines is abundant -- with nearly 100 million doses in the field for immediate use," the CDC said later Thursday in accepting the vote.
"This updated CDC recommendation follows similar recommendations from other countries, including Canada and the United Kingdom. Given the current state of the pandemic both here and around the world, the ACIP reaffirmed that receiving any vaccine is better than being unvaccinated," it added.
"Individuals who are unable or unwilling to receive an mRNA vaccine will continue to have access to Johnson & Johnson's COVID-19 vaccine."
ACIP also discussed the risks of completely removing J&J's vaccine as an option, as well as the potential confusion and mistrust that might be caused by changing recommendations about the vaccine. "We will absolutely emphasize how important education around the risk of these events is," Oliver told the meeting.
"The VE (vaccine effectiveness) is lower for the Janssen vaccine compared to mRNA vaccines," Oliver said.
The risk of myocarditis -- a rare heart inflammatory condition -- is lower for the Moderna and Pfizer/BioNTech vaccines than the risk of blood clots from the J&J vaccine. But younger men may be advised to avoid mRNA vaccines, so Oliver said that complete removal of recommendations for the J&J vaccine would cause problems. "It would remove the option of the vaccine for younger men, who are at higher risk for myocarditis," she said.
Plus, the Janssen vaccine is often the only shot available for prison populations, the homeless and other high-risk groups.
Worries about undermining confidence in vaccines
There could be other fallout from a decision to remove support from the J&J vaccine, Oliver cautioned. "Diminished access could undermine confidence in the COVID-19 vaccine program," she said. "Lack of vaccine choices could contribute to mistrust and misinformation about COVID-19 vaccines available," she added.
Earlier this week, the FDA strengthened language in the fact sheet that goes along with the Janssen vaccine, saying it should not be given to anyone with a history of TTS. There is, however, no clear risk factor for predicting who will develop the condition after getting the vaccine, the CDC said.
The changes to the recommendation should reassure Americans that public health experts are watching over the safety of the vaccines, ACIP members said.
"I want to remind everybody that this is in part a reflection of the strength of our vaccine safety surveillance system," Dr. Matthew Daley of the Institute for Health Research at Kaiser Permanente Colorado told the meeting.
"This whole process should increase confidence in our vaccine safety surveillance system," Dr. Sandra Fryhofer of Emory University and a non-voting liaison to the committee for the American Medical Association, said.
"I do think this vaccine needs to be made available," she added. "We need a variety of vaccine platforms."
The Pfizer/BioNTech and Moderna vaccines use a technology known as messenger RNA or mRNA to elicit an immune response, while the Johnson & Johnson vaccine uses a so-called vector -- a non-replicating common cold virus called an adenovirus -- to carry the active vaccine material into he body.
Different vaccine types stimulate different immune responses. That could be important, Fryhofer said. "We don't know what variant is next," she said.
About 16 million Americans have been given the Janssen vaccine.
Dr. Penny Heaton, global therapeutic head for vaccines at Janssen, told the meeting that the J&J vaccine is unique.
"We are confident in the positive benefit/risk profile of our vaccine. It is saving lives in the US here today and on every continent around the globe. Our vaccine is different. It's long-lasting. It offers high levels of protection, and it offers breadth of protection," Heaton told the meeting. "It's easy to store and transport."
Heaton said the J&J vaccine provides a different type of immunity that grows more slowly but lasts longer. It's a one-dose vaccine, but the CDC now recommends that everyone who gets it receive a booster dose of any available vaccine.
"Even in the US, given its durable protection, it may be the preferred choice for people who can't or won't return for multiple vaccinations," Heaton argued. She said the company has several studies underway to understand risk factors for TTS.
"While TTS continues to be a rare event, unfortunately, cases of Covid-19 are not," she said, noting that Covid-19 carries a much higher risk of blood clots than vaccination does
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