Several states swiftly pause the use of Johnson & Johnson’s vaccine after a federal advisory.

US and European public health experts have emphasized that for most people, the benefits of Covid-19 vaccines far outweigh the risks. Less than one in a million Johnson & Johnson vaccinations in the US are currently being studied. If there is indeed a risk of dangerous blood clots from the vaccine – which has yet to be determined – experts say the risk is extremely small. The risk of contracting Covid-19 in the United States is much higher.

Like many states, New York had already prepared for a sharp drop in supplies of the Johnson & Johnson vaccine after federal officials said supplies would be limited due to a production problem at a Baltimore manufacturing facility. On Friday, Governor Andrew M. Cuomo said New York would expect 34,900 Johnson & Johnson admissions, an 88 percent decrease from the previous week.

Dr. Sugar, New York’s health commissioner, said the state will meet appointments at state bulk vaccination sites for the Johnson & Johnson vaccine by giving people the Pfizer BioNTech vaccine instead. Speaking at a press conference, Mr. Cuomo said the state has enough Pfizer and Moderna vaccines to keep pace with its current vaccination rate.

New York City Mayor Bill de Blasio said the city will work with its vaccination centers to help people get appointments for the Pfizer or Moderna vaccine instead.

“Each site was instructed this morning not to take any more J&J shots,” he said at a press conference.

Both Mr Cuomo and Mr de Blasio received the Johnson & Johnson vaccine at various appearances last month to build confidence in the vaccine’s rate of effectiveness and to resolve vaccine hesitation.

The city’s health commissioner, Dr. Dave Chokshi said around 234,000 residents have received the Johnson & Johnson vaccine and have not yet reported any blood clots. The city had relied on the vaccine to vaccinate hard-to-reach New Yorkers, including people who live in their homeland.

Regulators in Europe and elsewhere are concerned about a similar issue with another coronavirus vaccine developed by researchers from AstraZeneca and Oxford University. This vaccine has not been approved for use in an emergency in the United States.

Rebecca Robbins, Jill Cowan, Denise Grady, Carl Zimmer and Sharon Otterman contributed to the coverage.