Vaccine reactions

Classification of AEFIs

A vaccine reaction can be an individual’s response to the inherent properties of the vaccine, even when the vaccine has been prepared, handled and administered correctly (vaccine product-related reactions). It can also be due to a defect in a vaccine (or its administration device) that occurred during the manufacturing process (quality defect-related reactions).

Vaccine reactions can be classified according to their severity into two groups:

Minor reactions

Severe reactions is a term including serious reactions but also including other severe reactions.

Severe reactions

Key point

There is low public tolerance of vaccine adverse reactions. Vaccines are therefore only licensed when the frequency of severe reactions is very rare and when only minor, self-limiting reactions are reported.

Minor vaccine reactions

Local reaction: Swelling/redness at the site of injection

Source: Barbara Pahud (Children’s Mercy Hospitals and Clinics, UMKC)

Ideally vaccines will cause no, or only minor (i.e. non-severe), adverse reactions.

Vaccination induces immunity by causing the recipient's immune system to react to antigens contained in the vaccine. Local and systemic reactions such as pain or fever can occur as part of the immune response. In addition, other vaccine components (e.g. adjuvantsAdjuvantA pharmacological agent (e.g., aluminum salt, oil-in-water emulsions) that modifies the effect of other agents, such as a drug or vaccine, while having few if any direct effects when given by itself. Adjuvants are often included in vaccines to enhance the recipient's immune response to a supplied antigen, while keeping the injected foreign material to a minimum., stabilizersStabilizersCompounds that are used to help vaccine maintain its effectiveness during storage. Vaccine stability is essential, particularly where the cold chain is unreliable. Factors affecting stability are temperature and pH., and preservativesPreservativesCompounds that are added to multi-dose vaccine vials to prevent bacterial and fungal growth. The most commonly used product is called thiomersal, a mercury-containing compound.) can trigger reactions. A successful vaccine keeps even minor reactions to a minimum while producing the best possible immune response.

The frequency of vaccine reactions likely to be observed with some of the most commonly used vaccines, and their treatments, are listed below. These reactions typically occur within a day or two of immunization (except for rash reactions after measles vaccine, which can arise up to 6 to 12 days after immunization) and persist from one to a few days.26



Common, minor vaccine reactions and treatment

Vaccine Local reactions Systemic reactions
(pain, swelling, redness) Fever > 38°C Irritability, malaise and systemic symptoms
  • Cold cloth at injection site
  • Paracetamol6
  • Give extra oral fluids
  • Wear cool clothing
  • Tepid sponge or bath
  • Paracetamol6
  • Give extra oral fluids
BCG1 90 – 95%
Hepatitis B Adults up to 15%
Children up to 5%
1 – 6%
Hib 5 – 15% 2 – 10%  
~ 10% 5 – 15% 5% (Rash)
OPV None Less than 1% Less than 1%2
Pertussis (DTwP)3 up to 50% up to 50% up to 55%
Pneumococcal conjugate5 ~ 20% ~ 20% ~ 20%
~ 10%4 ~ 10% ~ 25%
  1. Local reactogenicity varies from one vaccine brand to another, depending on the strain and the number of viable antigen in the vaccine.
  2. Diarrhoea, headache and/or muscle pains.
  3. When compared with whole cell pertussis (DTwP) vaccine, acellular pertussis (DTaP) vaccine rates are lower.
  4. Rate of local reactions are likely to increase with booster doses, up to 50 – 85%.
  5. Source:
  6. Paracetamol dose: up to 15mg/kg every 6–8 hours, maximum of 4 doses in 24 hours.

Severe vaccine reactions

Severe vaccine reactionsSevere vaccine reactionThis is not a regulatory term. It refers to vaccine reactions that usually do not result in long-term problems, but can be disabling and, rarely, life threatening. Severe reactions include serious reactions but also include other severe reactions. include, among others, seizuresSeizureUncontrolled electrical activity in the brain, resulting in convulsion, physical signs, thought disturbances, or a combination of symptoms., thrombocytopeniaThrombocytopeniaA severe decrease in the number of blood platelets, the cells involved in clotting. Thrombocytopenia may stem from failure of platelet production, splenic sequestration of platelets, increased platelet destruction, increased platelet utilization, or dilution of platelets., hypotonic hypo­res­pon­si­ve episodes (HHE)Hypotonic hyporesponsive episode (HHE)A recognized serious reaction to immunization, especially pertussis-containing vaccine. It is defined as an acute loss in sensory awareness or loss of consciousness accompanied by pallor and muscle hypotonicity. No long-term sequelae have been identified in the small number of children who have had long term follow-up. HHE is not a contraindication for further doses of pertussis vaccine. and prolonged crying, they should be reported within 24 hours to allow immediate action. Most severe vaccine reactions do not lead to long-term problems. Anaphylaxis, while potentially fatal, is treatable without leaving any long-term effects.

Key point

Severe allergic reactions (e.g. anaphylaxis) can be life threatening. Health workers who give vaccinations should know the signs of allergic reactions and be prepared to take immediate action.

Severe vaccine reactions, onset interval, and rates associated with selected childhood vaccines

Vaccine Reaction* Onset interval26 Frequency per
doses given
BCG28 Fatal dissemination of BCG infection 1 – 12 months 0.19 – 1.56/1,000,000
OPV29 Vaccine associated paralytic poliomyelitis (VAPP)** 4 – 30 days 2 – 4/1,000,000
DTwP30 Prolonged crying and seizures*** 0 – 24 hours < 1/100
HHE 0 – 24 hours < 1/1,000 – 2/1,000
Measles31 Febrile seizures 6 – 12 days 1/3,000
Thrombocytopenia 15 – 35 days 1/30,000
Anaphylaxis 1 hour 1/100,000

* Reactions (except anaphylaxis) do not occur if already immune (90% of those receiving a second dose); children >6 years unlikely to have febrile seizures.

 ** VAPP risk higher for first dose (1 in 750 000 compared with 1 in 5.1 million for subsequent doses), and for adults and immunocompromised patients.

*** Seizures are mostly febrile. The risk of having a seizure depends on the persons age. The risk is much lower in infants <4 months of age.

The difference between serious and severe adverse events

Serious’ and ‘severe’ are often used as interchangeable terms but they are not. Severe is used to describe the intensity of a specific event (as in mild, moderate or severe); the event itself, however, may be of relatively minor medical significance. Seriousness is based on patient/event outcome or action criteria and defines regulatory reporting obligations. An AEFI will be considered serious if it results in death, is life-threatening, requires in-patient hospitalization or prolongation of existing hospitalization, results in persistent or significant disability/incapacity, or is a congenital anomaly/birth defect, and any other important medical events that may jeopardize the patient or may require intervention to prevent one of the outcomes above.105