Britain has advised people who suffer from significant allergic reactions not to take the Pfizer/BioNTech vaccine.
The warning comes a day after two health workers had allergic reactions after taking the vaccine on Tuesday.
Should we be worried? What does it mean for the roll-out? All the key questions are answered here:
Britain’s National Health Service said on Wednesday that people who have a history of severe allergic reactions should not take the vaccine.
The warning was issued after two staff members received the jab on Tuesday and had anaphylactoid reactions shortly afterwards.
An anaphylactoid reaction generally involves a skin rash, breathlessness and sometimes a drop in blood pressure.
This is not the same as anaphylaxis, which can be fatal.
The two workers, who carry adrenalin pens, are recovering well.
How does it change the Pfizer rollout?
The roll-out of the vaccine is unaffected.
But those administering the shot will now ask patients if they suffer any severe allergic reactions.
Anyone who has had a significant allergic reaction to a vaccine, medicine or food should not take the dose.
That includes anyone who has been told to carry an adrenalin pen or others who are at risk of potentially fatal allergic reactions.
Stephen Powis, medical director for NHS England, said this was common with new vaccines.
The Medicines and Healthcare products Regulatory Agency will carry out an investigation into whether the reactions were directly linked to the vaccine and why they occurred.
Dr June Raine, head of the MHRA, told a Parliamentary committee allergic reactions were not a feature of the clinical trials.
“We know from the very extensive clinical trials that this wasn’t a feature,” she said.
“But if we need to strengthen our advice, now that we have had this experience in the vulnerable populations, the groups who have been selected as a priority, we get that advice to the field immediately.”
Pfizer and BioNTech said they were supporting the MHRA in its investigation.
Is the Pfizer vaccine safe?
The vaccine was found to be safe and effective by the MHRA. But it is not considered to be entirely risk-free.
That is the same for any vaccine, including the flu shot.
Even in non-emergency situations, health authorities must closely monitor new vaccines and medications because studies in tens of thousands of people can’t detect a rare risk that would affect one in one million.
Dr Peter Openshaw, a professor of experimental medicine at Imperial College London, said there was a very small chance of an allergic reaction to any vaccine.
“The fact that we know so soon about these two allergic reactions and that the regulator has acted on this to issue precautionary advice shows that this monitoring system is working well,” he said.
Prof Stephen Evans, from the London School of Hygiene and Tropical Medicine, said the benefits far outweighed the risks.
“If you mean absolutely no adverse effect, then no vaccine is ‘safe’ and no drug is ‘safe’. Every effective medicine has unwanted effects,” he told the BBC.
“What I mean by safe is the balance of unwanted effects compared with the benefit is very clearly in favour of the benefit.”
What side effects can happen with the Pfizer vaccine?
The most common side effect experienced by people who received the Pfizer vaccine was pain, redness or swelling at the injection site, generally the arm.
That was followed by short-term fatigue, headache and muscle pain.
But beyond these mild effects, there was no notable difference in health conditions between the vaccinated and control groups during the study period.
Is only the Pfizer vaccine affected?
Russia’s Sputnik V vaccine, which was authorised under an accelerated process before clinical trials ended, is also subject to a safety warning.
Health officials said on Wednesday anyone getting vaccinated against Covid with the Sputnik V vaccine should give up alcohol for almost two months.
Anna Popova, head of the consumer health watchdog, said people should stop drinking alcohol at least two weeks before getting the first of two injections.
They should continue to abstain for a further 42 days, she advised.
She said alcohol would reduce the body’s ability to build up immunity to Covid-19.
“It’s a strain on the body. If we want to be healthy and have a strong immune response, don’t drink alcohol,” she said.
The Pfizer vaccine is the only coronavirus shot approved for use in the western world.
What ingredients are in the Pfizer vaccine?
The active ingredient in the Pfizer vaccine is the messenger RNA (mRNA), which carries genetic instructions for making the virus’s spike protein.
It is synthetic and does not contain a virus.
The RNA-bearing particles are suspended in saline solution and injected into muscle tissue in the upper arm.
How does the Pfizer vaccine work?
Pfizer’s 95 per cent effective shot is an mRNA vaccine. Traditional vaccines inject people with a dead or weakened part of a virus so the body produces antibodies to fight it, as it would in a natural infection. But mRNA vaccines differ in that they encourage the body to become its own miniature vaccine factory.
The vaccine, an abbreviation of messenger RNA, delivers genetic instructions that prompt the body to produce virus proteins, without exposing the body to any threat.
Once this happens, the immune system begins to build up protective antibodies to guard against infection.