WASHINGTON (REUTERS) - The US Centres for Disease Control and Prevention (CDC) said on Wednesday (Jan 6) it is carefully monitoring allergic reactions to the coronavirus vaccines from Pfizer and Moderna and urged individuals who had a serious reaction not to get the second dose.

In a conference call with reporters, the US public health agency said allergic reactions are occurring at a rate of 11.1 per one million vaccinations.

That compared with flu vaccines, in which such reactions occur at a rate of 1.3 per one million shots.

The severe reactions are still "exceedingly rare," they said, stressing the need for people to get vaccinated when the shots become available to them, given the threat of death and serious disease from the coronavirus that has already claimed more than 357,000 lives in the United States alone.

The CDC said it is monitoring allergic reaction incidents closely and plans to post weekly updates on its website.

The agency is also urging that venues that deliver the vaccine be prepared not only to recognise serious allergic reactions, known as anaphylaxis, but be trained on how to treat them and recognise when individuals need to be referred to hospitals for additional care.

CDC officials said 28 people who received the coronavirus vaccine developed by Pfizer and BioNTech had severe allergic reactions.

They also noted one case of anaphylaxis, which can cause throat swelling and breathing difficulty, after an individual received Moderna's vaccine.

Officials attributed the difference largely to the fact that the Pfizer/BioNTech vaccine was authorised earlier than the Moderna shot, and said the precautions apply to both.

A study published on Wednesday in the CDC's weekly report on death and disease looking at cases between Dec 14 and Dec 23, identified 21 cases of anaphylaxis after the administration of 1,893,360 doses of the Pfizer/BioNTech vaccine.

Of these, 71 per cent occurred within the first 15 minutes after vaccine administration.

Britain's medical regulator has said that anyone with a history of anaphylaxis, or severe allergic reactions to a medicine or food, should not be given the Pfizer/BioNTech vaccine.