Questions have arisen following reports of three cases of anaphylaxis, or the rapid onset of severe allergic reactions, after vaccinations. The Straits Times looks into the severity of these reactions and whether they should be a cause for concern.

Q Why is it that the Pfizer-BioNTech vaccine is able to cause anaphylaxis in people, even if they do not have any history of it?

A Professor Dale Fisher, a senior infectious diseases consultant at the National University Hospital, said a possibility could be that the vaccine contains polyethylene glycol (PEG), which is an adjuvant, or an ingredient used to create a stronger immune response. The chemical is used in some medications, and has caused anaphylaxis in the past, though the cases have been rare.

Infectious diseases expert Leong Hoe Nam from the Mount Elizabeth Novena Hospital said that another possibility could be allergic reactions to the lipid nanoparticles, which are used to stabilise the mRNA in the vaccine so the body does not destroy it too quickly.

Dr Asok Kurup, who chairs the Academy of Medicine's Chapter of Infectious Disease Physicians, said there is a possibility that such reactions are unpredictable. He added that all medical professionals have been advised to ensure that all post-vaccine observation sites should have the "relevant support and supplies to treat anaphylaxis and other serious reactions". Some patients who have had previous serious reactions that are not tantamount to anaphylaxis can also opt to get their vaccines at hospital sites instead.

Q What are the general causes of anaphylaxis?

A Dr Tseng Hsien-Cho, physician lead at the Raffles Medical Group who is also involved in the vaccine centre operations, said anaphylaxis is the result of one's natural immune system overreacting to a trigger. This could be something one is allergic to, but it might not always be the case, he noted.

Some common triggers include foods such as nuts, milk, fish, shellfish or eggs, as well as medicine such as antibiotics or non-steroidal anti-inflammatory drugs, and insect stings.

However, there might be instances where there is no obvious trigger, which is known as idiopathic anaphylaxis, said Dr Tseng. This explains why those who have received the vaccine are observed for 30 minutes afterwards as a precautionary measure.

Symptoms of anaphylaxis are often severe and almost immediate, said Prof Fisher. These include swelling of the face and airways, wheezing, and the falling of blood pressure.

Q How can anaphylaxis be treated?

A An adrenaline auto-injector, or an EpiPen, can be used to quickly reverse allergic reactions, for example improving breathing and reducing swelling, said Dr Tseng. A second injection can be administered after five to 15 minutes if the symptoms do not improve.

Other treatment options include an oxygen mask to help with breathing, fluids to be directly injected into a vein to increase blood pressure, as well as other medications such as antihistamines and steroids to relieve symptoms.

Q Is anaphylaxis also a common reaction to other types of vaccines?

A Allergic reactions are a common risk for all vaccines, and allergies may or may not be known to the patient and doctor, said Dr Tseng.

Prof Fisher said anaphylaxis is a rare side effect which affects around one or two people per million vaccinations. The effect has also been described in flu vaccines, pneumococcal vaccine, hepatitis A vaccine, and the MMR vaccine (which prevents measles, mumps and rubella).

Q Should I still be taking the vaccine if I have any known allergies?

A Yes, though it is recommended that you discuss with your doctor if you are worried.

Prior to taking the vaccination, people will be asked to share their medical history and complete a health assessment form to declare any known allergies, said Dr Tseng. "Physicians will then have to weigh the risk and benefit per vaccination. For instance, if an individual has known allergic reactions, such as small patches of rashes that can be treated with antihistamines, the benefits of getting a vaccine would outweigh the risk," he said.

However, Dr Leong Choon Kit, a family physician at Mission Medical Clinic, whose clinic is one of the 21 Public Health Preparedness Clinics which have begun administering the vaccines, said those with prior allergies, such as allergic rhinitis, have the same chance of developing anaphylaxis as others, and are not facing significantly higher risk.

Q When do I need to see a doctor for vaccine symptoms or side effects?

A According to the Ministry of Health's website, most side effects will usually resolve on their own within three days. To relieve these symptoms, take one to two tablets of paracetamol every six hours where necessary.

Though uncommon, it is also possible to experience swelling of the lymph nodes after taking the vaccine. This usually takes seven to 10 days to resolve. However, you should see a doctor if your symptoms persist or worsen, or if your fever persists for more than two days.

If you experience a severe allergic reaction such as difficulty breathing, swelling of your face, throat, eyes or lips, dizziness and weakness, or a bad rash all over your body, seek medical attention immediately.