A health care worker in New York City suffered what officials are calling a "serious adverse event" after receiving Pfizer’s COVID-19 vaccine. The worker, who was not identified, is stable after being treated for a significant allergic reaction.

"With more than 30,000 COVID-19 vaccinations administered in New York City, we have received a single report of a serious adverse event in a health care worker," the New York City Health Department said in a statement on Wednesday.

It was not clear when or where the health care worker received the vaccine or how soon after the reaction occurred.

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"The City Health Department is closely tracking reports of more severe side effects in collaboration with the CDC, and this is the first serious adverse event we have encountered in New York City," the department said in a statement. "We will continue to move forward with the coronavirus vaccine distribution to ensure that health care workers and nursing home staff and residents are protected against COVID-19."

While the instance is the first to be reported in New York, there have been several other reports of adverse reactions across the U.S. A health care worker in Alaska who suffered an allergic reaction within 10 minutes of receiving the first dose of the vaccine last week is believed to have been the first reported in the U.S.

Since then, at least five more reactions have occurred, prompting the CDC to issue new guidance.

"If you have ever had a severe allergic reaction to any ingredient in a COVID-19 vaccine, CDC recommends that you should not get that specific vaccine," the health agency stated. "If you have had a severe allergic reaction to other vaccines or injectable therapies, you should ask your doctor if you should get a COVID-19 vaccine. Your doctor will help you decide if it is safe for you to get vaccinated."

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However, the aforementioned guidance only applies to those who have had severe reactions to vaccines, and not others who may suffer from serious reactions to other elements such as food.

"CDC recommends that people with a history of severe allergic reactions not related to vaccines or injectable medications – such as allergies to food, pet, venom, environmental, or latex – may still get vaccinated," the CDC said.

"People with a history of allergies to oral medications or family history of severe allergic reactions, or who might have had an milder [sic] allergy to vaccines (no anaphylaxis) – may also still get vaccinated."