• More Americans will likely refuse to get J&J's Covid-19 vaccine after U.S. health regulators said six women developed a rare blood-clotting disorder, experts on public health and vaccines told CNBC on Tuesday.
• People were already skeptical about vaccines before the coronavirus emerged in December 2019.
• Some Americans were also hesitant to get the J&J vaccine in particular because they perceived it as inferior to Pfizer's and Moderna's.
• The warning from U.S. officials to states is likely to fuel more hesitancy in taking J&J's shot and other vaccines, threatening to stall the pandemic recovery.

More Americans will likely refuse to get Johnson & Johnson's Covid-19 vaccine after U.S. health regulators said six women developed a rare blood-clotting disorder that left one dead and another in critical condition, experts on public health and vaccines told CNBC on Tuesday.

The Food and Drug Administration asked states early Tuesday to temporarily halt using J&J's single-shot vaccine "out of an abundance of caution" after six women, ages 18 to 48, out of the roughly 6.9 million people who received the shot developed a blood-clotting disorder known as cerebral venous sinus thrombosis, or CVST.

All of the women developed the condition, which occurs when a blood clot forms in the brain's venous sinuses, preventing blood from draining out of the brain back to the heart, within about two weeks of receiving the shot, health officials told reporters on a call.

"For people who recently got the vaccine within the last couple weeks, they should be aware to look for any symptoms," Dr. Anne Schuchat, the principal deputy director of the Centers for Disease Control and Prevention, said during a press briefing Tuesday. "If you've received the vaccine and developed severe headaches, abdominal pain, leg pain or shortness of breath, you should contact your health-care provider and seek medical treatment."

Shortly after the FDA issued the warning, more than a dozen states as well as some pharmacies took steps to halt inoculations with J&J's vaccine, some replacing scheduled appointments with either the Pfizer or Moderna vaccine. Some doctors say they're already fielding calls from worried patients.

People were already skeptical about vaccines before the coronavirus emerged as a new pathogen in China in December 2019, infecting more than 31.2 million Americans and killing at least 562,718 in a little over a year. The warning from U.S. health officials to states is likely to fuel even more hesitancy in taking J&J's shot and the other vaccines, threatening to stall the nation's recovery from the pandemic, health experts told CNBC.

"Sadly, it will likely exacerbate those who have some degree of hesitancy towards receiving a vaccine," said Isaac Bogoch, an infectious disease specialist who has sat on several drug data and safety monitoring boards. "Senior public health officials have to continue to be open, honest, transparent and most importantly contextualize that this is low risk."

The goal, according to President Joe Biden's chief medical advisor, Dr. Anthony Fauci, is to vaccinate between 70% and 85% of the U.S. population — or roughly 232 million to 281 million people — to achieve herd immunity and suppress the pandemic.

So far, more than 120 million Americans, or 36% of the total U.S. population, have received at least one dose of a Covid-19 vaccine, according to data compiled by the CDC. Roughly 74 million Americans, or 22% of the total U.S. population, are fully vaccinated, according to the CDC. Kids under age 16 are not yet authorized in the U.S. to take the shots, and some adults will likely refuse to get any vaccine.

"This throws a wrench into the plans. It will slow down the rollout," said Dr. Jeffrey Kahn, director of the Berman Institute of Bioethics at Johns Hopkins University. "People will say, 'I don't want that one, I want one of the others that doesn't have that problem,' even if it's an extraordinarily rare event."

Some Americans, especially in Black, Hispanic and rural communities, were already hesitant to get the J&J vaccine in particular because they perceived it as inferior to Pfizer's and Moderna's. The J&J shot, which is highly effective, especially against severe disease, demonstrated 72% effectiveness in the U.S. in protecting against Covid about a month after the inoculation. That compares with the roughly 95% efficacy of Pfizer's and Moderna's two-dose vaccines.

Single-dose vaccines such as J&J's were crucial to "getting to communities where a two-dose regimen wasn't practical or really even possible," Kahn said. U.S. health officials were largely using J&J's vaccine to reach poorer urban and rural areas where residents couldn't easily get to a vaccine clinic or didn't have reliable internet access.

"Those communities also are the ones most severely affected by Covid," Kahn said. "Pausing the use of J&J [is] a blow to doing that effectively and quickly."

Dr. Stephen Schrantz, who was part of the team that led a J&J vaccine trial at the University of Chicago Medicine, said he already had patients who did not want the J&J vaccine and said the news will give them further evidence to say, "See, I told you."

"I suspect vaccine rollout and uptake will slow down, there will be a migration away from the J&J vaccine, even if the CDC and FDA conclude there is not a causal relationship," he said. "And as mask-wearing wanes we may begin to see more cases, such as we have in Michigan, show up elsewhere."

Dr. Scott Gottlieb, who sits on Pfizer's board, predicted the move will "fuel the hesitancy" of some people to get a Covid vaccine.

"Even if there isn't a causal relationship, even if this is exceedingly rare, I think we're going to see that whole conversation now get ignited on social media," he told CNBC in an interview.

Dr. Purvi Parikh, a specialist in infectious disease allergy and immunology at NYU Langone Health, on Tuesday called the FDA warning a "double-edged sword," saying it will likely add concerns to already hesitant Americans. She also said she has already gotten "panicked calls" from her own patients about the J&J vaccine.

"But if anything, again, I want to reiterate: This only gives me more faith in our system because those safety checks and balances are working. So hopefully it gives some people peace of mind," she added on "Squawk on the Street." "Again, just to look at the big picture, the benefits still far outweigh the risks of this vaccination."

Dr. Archana Chatterjee, pediatric infectious diseases specialist and member of the FDA's Vaccines and Related Biological Products Advisory Committee, echoed Parikh's remark. She added that there was nothing "unusual" in the way U.S. health regulators addressed the issue.

"This is a normal procedure that occurs," she said.

"But obviously, anytime that serious adverse events are reported about any vaccine that raises concerns in the public's mind," she added. "When you talk about vaccine confidence or vaccine hesitancy, could that have an impact? It's certainly possible."

Dr. Paul Offit, another member of the Vaccines and Related Biological Products Advisory Committee, said he hopes Americans think about the issue "rationally," adding that the cases of blood clots appear to be extremely rare. He noted persuading folks in hard-to-reach communities could be a challenge.

"It should be reassuring to people that officials are continuing to look [at the vaccine], even for rare side effects," he said.

— CNBC's Kevin Stankiewicz contributed to this article.