Varicella Vaccine: Active Ingredient, Mechanism of Action, Indications, Dosage, Side Effects, Warnings and Interactions

We are talking about a childhood viral disease that can be prevented with vaccination and is highly contagious.

The vaccine triggers an immune response that protects individuals from contagion if exposed to the chickenpox virus.

People vaccinated against the varicella-zoster virus are susceptible to contracting the disease, but symptoms are usually milder.

The duration of protection against chickenpox infection after vaccination is unknown.

Vaccination with the chickenpox vaccine may not protect all healthy and susceptible children, adolescents, and adults.

Symptoms are worse in adults and can sometimes lead to hospitalization.

Groups at risk for chickenpox

Chickenpox is a common childhood infection. In most cases, the symptoms are mild, and complications are rare. The disease can be more severe in adults.


Almost all children develop immunity to chickenpox after infection, so it is only contagious once.

Certain groups of people are at increased risk of severe complications from chickenpox.

This includes weakened immune systems through diseases, such as HIV, or treatments, such as chemotherapy.

Chickenpox can be severe for an unborn baby when a pregnant woman catches the infection. It can cause severe congenital disabilities and severe illnesses in the baby when born.

Vulnerable population

Health workers

The chickenpox vaccine is recommended for healthcare workers who do not have a history of chickenpox.

If you are not sure you have had chickenpox in the past, a blood test should be done to see if the person is immune to the disease.

Healthcare workers include anyone who may come into contact with a patient, including medical and nursing staff and other workers, such as:

  • Hospital cleaners.
  • Hospitality staff.
  • Ambulance staff.
  • Hospital receptionists.
People in close contact with vulnerable people

The chickenpox vaccine is also recommended for anyone who does not have a history of chickenpox and is likely to contact someone who has a weakened immune system.

Children with severe disabilities

The chickenpox vaccine is recommended for severely disabled children who have never had chickenpox if they live in special residential units.

Women of childbearing age

Vaccination is recommended for women of childbearing age who have never had chickenpox, not immune to this age.

The active ingredient in the chickenpox vaccine

The chickenpox vaccine contains a part of the virus that has been modified to reduce its virulence (attenuated virus) while offering protection against the disease.

The chickenpox vaccine is now combined with portions of the measles, mumps, and rubella viruses offered to children at 18 months of age.

The vaccine does not contain thiomersal (mercury).

Mechanism of action of the chickenpox vaccine

The chickenpox vaccine contains a small amount of the weakened varicella zoster virus.

The vaccine causes the immune system to produce antibodies that will help protect against chickenpox.

The vaccine is recommended for people who are likely to come into contact with people in “risk” groups. This reduces the risk of people passing the infection to people at risk.

For example, if chemotherapy is being received, it is recommended that non-immune children receive the chickenpox vaccine.

If you are about to begin work in a radiation therapy department and do not have a history of chickenpox, the vaccine is recommended.


The vaccine is indicated for active immunization in preventing chickenpox in individuals 12 months of age or older.


The varicella vaccine is given as a first dose of approximately 0.5 ml by subcutaneous injection in the outer part of the upper arm (deltoid region).

It can also be administered to the anterolateral thigh. This product should not be administered intravascularly or intramuscularly.

The second dose is given after a minimum of 3 months between doses in children (12 months to 12 years of age).

In adolescents over 13 years of age and adults, the two doses should be administered with a minimum of 4 weeks between doses.

To minimize loss of potency, the vaccine should be administered immediately after reconstitution and discarded if the reconstituted vaccine is not used within 30 minutes.

A sterile syringe free of preservatives, antiseptics, and detergents should be used for each reconstitution and injection, as these substances can inactivate the vaccine virus.

To reconstitute the vaccine, first, withdraw the total volume of the provided sterile diluent into a syringe. All the diluent withdrawn from the lyophilized vaccine vial is injected and gently shaken to mix well.

The entire contents are withdrawn into the syringe, and the total volume of the reconstituted vaccine is injected.

Chickenpox Vaccine Effectiveness

It has been shown that 9 out of 10 children vaccinated with a single dose will develop immunity against chickenpox.

A two-dose schedule is now recommended for everyone as it provides a better immune response.

Three-quarters of adolescents and adults vaccinated with two doses will develop immunity to chickenpox.

If varicella vaccination is required, two doses are given with four to eight weeks between doses.

Side effects

The most common side effect of the chickenpox vaccine is pain, swelling, redness around the injection site, and a small lump that appears at the injection site.

This side effect develops in about one in five children and four adolescents and adults.

A mild rash can occur in 1 in 10 children and 1 in 20 adults.

The rash lasts up to 5 to 26 days after vaccination; if this occurs, cover the rash and avoid contact with people who have impaired immunity for the duration of the rash.

Serious side effects, such as anaphylaxis (a severe allergic reaction), are rare. They occur in less than 1 in 100,000 vaccination cases.

Millions of doses of the vaccine have been administered, and there is no evidence that there is an increased risk of developing a long-term health condition due to vaccination.

Other common side effects may include fever, usually mild and well-tolerated, and muscle aches.

Warnings and Contraindications

People who have a weakened immune system such as:

  • Immunosuppressed or immunocompromised individuals, including those with a history of primary or acquired immunodeficiency states.
  • Leukemia, lymphoma, or other malignancies affecting the bone marrow or lymphatic system.
  • AIDS or receiving immunosuppressive therapy.

All patients with conditions such as those listed inside should consult a doctor about whether or not they should receive the vaccine.

The chickenpox vaccine should not be given to people who have experienced an anaphylactic reaction (severe allergic reaction) to a previous dose of the vaccine or any of the ingredients used in the vaccine.

The chickenpox vaccine should not be administered to pregnant women, as it is contraindicated for use in pregnant women because the vaccine contains live, attenuated varicella virus.

If acquired during pregnancy, the chickenpox virus is known to cause congenital chickenpox.

Avoid getting pregnant for three months after the last dose of the chickenpox vaccine.

No research reports whether the attenuated virus from the varicella vaccine is excreted in human milk.

Vaccination is not recommended for people who are seriously ill. It should be delayed until they recover.

People vaccinated against chickenpox should avoid contact with high-risk people susceptible to chickenpox

because of a possible risk of transmission.

After receiving blood or plasma transfusions or administering immunoglobulins, you should wait a period greater than or equal to 6 months to receive immunization with the varicella vaccine.

There are insufficient data to evaluate the rate of protection of the varicella vaccine against severe complications of chickenpox in adults in encephalitis, hepatitis, pneumonia, and congenital varicella syndrome during pregnancy.

Chickenpox vaccine interactions

The use of salicylates should be avoided for six weeks after the chickenpox vaccine is given to children and adolescents.