It could also stiffen resistance by some people in the United States and abroad to get any COVID vaccine as cases caused by the Omicron variant surge.
“What I fear is that people who were hesitant to get a vaccine will see this as all the more reason why I shouldn’t get one, even though this problem is not a problem of mRNA vaccines,” said Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia.
The Pfizer and Moderna vaccines rely on technology called messenger RNA to stimulate an immune response in the body. That has not been associated with the rare blood-clotting condition, known as thrombosis with thrombocytopenia syndrome. The J&J vaccine spurs immunity by using a harmless cold virus to deliver part of the spike protein on the surface of the coronavirus into human cells, triggering the production of disease-fighting antibodies and T cells.
It has been linked to 54 confirmed cases of the clotting disorder and at least nine US deaths in the past year. The risk was greatest among women 30 to 49, estimated at 1 in 100,000 for those who received the shot.
J&J said in a statement that it “remains confident in the overall positive benefit-risk profile of its COVID-19 vaccine. Studies have shown that the Johnson & Johnson COVID-19 vaccine generates strong antibody and cellular immune responses and long-lasting immune memory and breadth of protection across variants.”
The company said the vaccine is an important option for people who “can’t or won’t return for multiple vaccinations, or who would remain unvaccinated without an alternative to the mRNA vaccines.” J&J also noted its ease of usage and transport ― unlike the mRNA vaccines, it doesn’t have to be kept frozen when stored for prolonged periods.
Those qualities, said J&J, “make it a vital tool” in low- and middle-income countries where it is sometimes the only option.
The mRNA vaccines have also been linked to rare side effects, but those have caused less alarm. In June, the Food and Drug Administration added a warning label for the Pfizer and Moderna shots about increased risk of myocarditis, or heart inflammation, but stressed that the coronavirus itself poses a much greater threat of such complications than vaccine-related complications.
Dr. Dan Barouch, an immunologist who runs Beth Israel’s Center for Virology and Vaccine Research, which helped develop the J&J shot and licensed technology to the health care giant, declined to comment.
Several vaccine experts said they still believe the benefits of the J&J vaccine far outweigh its risks and noted that the CDC didn’t withdraw its authorization for emergency use. Nonetheless, they said they understood why a CDC advisory panel voted 13-0 on Thursday to recommend the Pfizer and Moderna over J&J, guidance that the director of the agency, Dr. Rochelle Walensky, endorsed several hours later. The United States has a surplus of mRNA vaccines, the experts said, and the J&J shot has played a marginal role in the country’s vaccination campaign.
“I think the actual impact will be small, in part because our availability of vaccines is so good,” said Dr. Eric Rubin, an immunologist at the Harvard T.H. Chan School of Public Health and editor-in-chief of the New England Journal of Medicine.
Rubin, who sits on an FDA advisory panel that recommended the three vaccines be cleared for emergency use, said the J&J vaccine works even though studies have indicated it is relatively less effective than the mRNA shots.
“In this country we have the luxury of having other vaccines that aren’t associated with that side effect, and they seem to work well and are relatively safer,” he said. “And so it makes sense to turn to them first.”
The J&J vaccine relies on a design Barouch pioneered about 20 years ago for two experimental vaccines that have shown promise against HIV and Zika and a third that won approval from the European Union last year to prevent Ebola. The COVID-19 vaccine uses a virus that causes colds, adenovirus serotype 26 ― or Ad26 ― as a Trojan horse to deliver part of the spike protein on the surface of the virus into human cells to stimulate immunity.
Several health experts said the CDC’s recommendation is likely to discourage countries from using other adenovirus vector vaccines such as the one-dose vaccine from AstraZeneca, which has also been linked to the rare clotting condition. The AstraZeneca vaccine is not authorized in the United States.
“It could have a chilling effect on all the adenovirus vaccines globally,” said Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine. He helped develop a COVID-19 vaccine that uses different technology, called a recombinant protein, that he hopes will be cleared for emergency use in India next week.
Several other COVID-19 vaccines developed in other countries also use adenovirus vector technology, including Russia’s Sputnik V and China’s CanSino Biologics.
The FDA authorized the J&J vaccine on Feb. 27, more than two months after the Pfizer and Moderna vaccines, and it quickly encountered problems. In April, federal officials imposed a 10-day pause on its use when concerns first arose about clotting problems.
Two months later, federal regulators told J&J that about 60 million doses of the vaccine at a troubled Baltimore factory operated by Emergent BioSolutions could not be used because of possible contamination. Meanwhile, results showing the vaccine’s lower effectiveness compared with the protection generated by the Pfizer and Moderna vaccines has dampened interest in it.
While the risk of the J&J shot causing blood clots remains low, several experts said the CDC’s recommendation will almost certainly cause other countries to say that if the vaccine isn’t good enough for Americans, it’s not good enough for their citizens.
“Public perception is everything when it comes to vaccines,” said Hotez. “It doesn’t take much for a good vaccine to be voted off the island even if it’s a good vaccine.”
Jonathan Saltzman can be reached at jonathan.saltzman@globe.com.