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By Talal Ansari and Jim Carlton 

Vaccination sites across the country canceled thousands of appointments after U.S. health authorities paused the use of Johnson & Johnson's Covid-19 shots Tuesday over reports of rare but severe blood clots, while other sites scrambled to switch to one of the two other authorized vaccines.

In New York City, 4,000 Johnson & Johnson vaccination appointments are being rescheduled, Mayor Bill de Blasio said Tuesday. In Alaska, officials said they were holding 24,322 doses of the Johnson & Johnson vaccine that had not yet been administered pending further review. And in Washington state, hundreds of patients scheduled to receive the single-dose vaccine at a fairground were told they would instead get the first of two doses of the vaccine developed by Pfizer Inc. and BioNTech SE.

"It's a less than one-in-a-million chance of this side effect," said Umair Shah, an emergency physician who was appointed Washington's secretary of health last year, at a news conference Tuesday. "This action is being taken out of extreme caution."

Numerous states announced that they would follow the new guidelines from the Food and Drug Administration and Centers for Disease Control and Prevention.

In California, the Johnson & Johnson vaccine accounts for 4% of the overall supply, Gov. Gavin Newsom said Tuesday. "We do not anticipate this pause to impact our supply or vaccine distribution efforts," the governor's press office said in a statement.

Nationwide, federal officials expected to allocate about 26.5 million doses of the Pfizer-BioNTech and Moderna Inc. vaccines combined this week, compared with 1.5 million of J&J's.

Federal authorities recommended a temporary halt to use of the J&J vaccine after six women between the ages of 18 and 48 years who got the vaccine developed blood clots, and one died. Over seven million doses of the vaccine have been administered in the U.S., according to the CDC.

Johnson & Johnson's vaccine was the third coronavirus inoculation to be approved after those of Pfizer-BioNTech and Moderna. Unlike the other two, it requires only one dose.

"I was a little disappointed my daughter couldn't get a time slot, but given her blood clotting disorder I am relieved," Joanne Hunter of Snohomish, Wash., said in a parent Facebook group for Southern California's Chapman University, where her 18-year-old daughter, Cara, is a freshman.

Ms. Hunter said in an interview that her daughter would wait until she returns home for summer break before attempting to get a dose again.

Some health researchers said the pause could harden beliefs among people who believe that Covid vaccinations aren't safe, despite their approval by federal health agencies. A February poll by the Pew Research Center found that 30% of Americans said they do not plan on getting vaccinated, with the majority citing side effects as their main concern.

"People who were already in the group who said 'this was too rushed and we don't know how safe it is' -- it gives them one more reason to decline the vaccine," said William Curry, a professor of medicine at the University of Alabama at Birmingham.

Johnson & Johnson's single dose has emerged as the preferred vaccine for some underserved communities, including the homeless and certain isolated communities where patients find it harder to return for a second injection. The J&J shot also doesn't require the same degree of cold storage as the Pfizer and Moderna vaccines, making it easier to transport and store.

"With hard-to-reach populations, you really want to be able to give a vaccine shot just once," said Keith Mueller, a specialist in rural health policy at the University of Iowa's College of Public Health.

Some remote rural counties in the U.S. were already contending with a shortage of qualified medical personnel to administer vaccines, or not enough chain pharmacies with the proper infrastructure to distribute shots. The pause "slows down some of the momentum that was building to use Johnson & Johnson to get to those populations," Mr. Mueller said.

In Sacramento County, Calif., officials canceled 800 appointments for the Johnson & Johnson shot scheduled for Tuesday and Wednesday at the cavernous Cal Expo building and supplemented two clinics that were supposed to administer those shots with extra doses of the Pfizer vaccine.

Janna Haynes, spokeswoman for Sacramento County Public Health, said the pause wasn't too disruptive because the county had been allocated only 1,700 doses of the J&J vaccine for this week compared with 11,700 doses of Pfizer and 6,800 of Moderna.

At the Luther Burbank High School in Sacramento, where J&J was to be given out, she said a line wrapped around the building with people waiting to receive injections of either Pfizer or Moderna despite the county's offer to reschedule appointments.

"The atmosphere was celebratory," Ms. Haynes said of the school vaccination area, where upbeat pop music played Tuesday.

--Robbie Whelan contributed to this article.

Write to Talal Ansari at Talal.Ansari@wsj.com and Jim Carlton at jim.carlton@wsj.com

(END) Dow Jones Newswires

April 14, 2021 11:34 ET (15:34 GMT)

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