The US Centres for Disease Control and Prevention (CDC) on Saturday issued new guidelines to those who have been reported severe allergic reactions to the COVID-19 vaccine.

CDC has "learned of reports" of some people experiencing severe allergic reactions after getting inoculated. The agency defined a "severe reaction" as one where a person needs to be treated with epinephrine or requires hospitalisation, The Hill reported.

People who have a severe allergic reaction after the first dose should not get the second shot and those who have had severe allergic reactions to components in a COVID-19 vaccine should not get that specific vaccine, The Hill quoted CDC as saying.

The agency to consult doctors before getting inoculated. However, people with a history of severe allergic reactions not related to vaccines or injectable medicines may still get vaccinated, CDC added.

The guidelines come after the Food and Drug Administration (FDA) said it was looking into five severe reactions to Pfizer's vaccine reported this week, in Alaska, and the others have been reported in other states.

The agency wasn't sure what caused the reaction, but said a chemical called polyethylene glycol, which is present in Pfizer and Moderna's coronavirus vaccine, "could be the culprit," Peter Marks, who leads the agency's Center for Biologics Evaluation and Research said.

However, CDC said on Friday that people with a history of severe allergic reactions to any component of the vaccine should avoid getting inoculated.

Earlier this month, the UK's Medicines and Healthcare Products Regulatory Agency warned that people with "significant" histories of allergic reactions should avoid getting Pfizer's vaccine after two people reported adverse reactions.