Having an anaphylactic reaction after vaccination is very rare. It has been estimated to occur in around one case per million vaccine recipients

However, the prevalence varies between different vaccines. Prepare for an anaphylactic reaction in all vaccination situations.

Anaphylaxis typically occurs within 15 minutes of vaccination, rarely more than an hour later. It is usually based on an IgE-mediated allergy for one of the ingredients of the vaccine. 

Symptoms of anaphylaxis

The early stages of an anaphylactic reaction include tachycardia (high heart rate) and the skin, mucous membrane and respiratory tract symptoms mentioned below.

Symptoms on the skin or mucous membranes

  • itching, tingling or redness (nearly) all over the body
  • swelling in areas such as eyelids, face or neck
  • generalised hives

Respiratory tract symptoms

  • coarseness
  • swelling of lips or tongue
  • feeling of swelling in the back of the throat
  • coughing spells
  • asthma-like symptoms.

Breathing difficulties can occur when breathing in (inhaling) and out (exhaling).

Nausea, severe abdominal pain, vomiting and diarrhoea may also occur.

The sooner the symptoms begin and progress, the more severe the reaction. The most serious cases include signs of circulatory collapse.

If the symptoms of anaphylaxis are not treated, they will progress and cause a life-threatening condition. Signs of an impending shock include

  • cold sweat
  • pallor
  • sudden drop in blood pressure and increasing heart rate  
  • loss of consciousness.

Treatment of anaphylaxis

Treatment must be promptly started. However, if the symptoms disappear before starting treatment, the person is not having an anaphylactic reaction.

Follow-up vaccinations

People who have had an anaphylactic reaction will primarily no longer be given the vaccine that caused the reaction.

Any suspected anaphylaxis cases must always be carefully assessed to make sure that the grounds for omitting a series of vaccinations or a booster vaccine are not too light. 

It is worth remembering that anaphylaxis may also have been reported in patient records in situations that ended up not actually involving anaphylaxis. Symptoms occurring after vaccination are often caused by a reason other than an actual anaphylaxis triggered by the vaccination.

Differential diagnostic situations

Most of the symptoms occurring immediately after vaccination are not anaphylaxis. For example, the following reactions should be distinguished from anaphylaxis:

  • nausea, hyperventilation related to fear after the injection situation
  • fainting or vasovagal symptoms
  • a condition similar to a young child fainting, hypotonic-hyporesponsive episode (HHE), which can be easily mistaken for anaphylaxis
  • So-called discoloured leg reaction, which can be easily mistaken for an impending anaphylaxis as it starts quickly.

Fainting in connection with vaccinations
Hypotonic hyporesponsive episode (HHE)
Swelling in a lower limb and changes in skin colour after a vaccination i.e. a discoloured leg reaction

The vaccine recipient will quickly regain consciousness after fainting or a vasovagal reaction, and is quick to react to his or her surroundings normally. The person's vital functions, or heart rate, respiratory rate and blood pressure, will soon return to normal.

Young children also recover from the symptoms of HHE or discoloured leg reactions without treatment.