Varicella Vaccine

Since February 2016, the varicella vaccine has been available again in Spanish pharmacies after having been withdrawn for almost three years. In addition, it has been introduced into the official vaccination schedule for children aged 12 and 15 months, at-risk groups and those over 12 years old who have not had the disease. Many families lack information or have doubts about vaccination. Others do not know if their child is properly immunized because they were unable to complete the second dose due to lack of access to the vaccine.

What is chickenpox?

Chickenpox is a very common infectious disease, especially in childhood, characterized by a very typical vesicular rash that allows clinical diagnosis. It occurs after primary infection by the varicella-zoster virus. Humans are the only reservoir. Transmission is mainly direct, through Pflügge droplets emitted by patients or by contact with skin lesions. Contagiousness is very high and the contagious period extends from 1-2 days before the appearance of the rash to 5-7 days after it. Its clinical course is usually benign, but in 2-6% of cases it can cause major complications. If the disease occurs during pregnancy, it can affect the fetus causing congenital malformations or chickenpox in the newborn, sometimes with a very severe course. Etiological treatment, when indicated, is not always effective. Hence the importance of vaccination for primary prevention of the disease.

Varicella vaccine

The varicella vaccine is an attenuated virus vaccine. The usual route of administration is subcutaneous, in the external anterolateral region of the thigh in young children and in the deltoid region in older children, adolescents and adults. It is generally very safe and well tolerated. Adverse reactions are generally mild and occur with a frequency ranging between 5 – 35% of those vaccinated. The most frequent are local reactions in the form of pain, redness or swelling. Systemic effects are fever and mild rashes appearing 5 to 30 days after vaccination. Between 3 – 5% of vaccinated children have localized rash with few maculo-papular rather than vesicular elements in the vicinity of the injection site. Another similar percentage may present a generalized rash. In adolescents and adults the frequency of exanthematous reactions may be somewhat higher, especially after the first dose. For practical purposes, it is considered to be a natural pre-vaccine infection when varicella rash appears within the first 15 days after vaccination; varicella vaccinalis when it appears between 16 and 42 days; and moderate varicella in vaccinated individuals, if it appears after 42 days after vaccination. It is very rare for vaccinated healthy children to transmit the virus to susceptible contacts. Transmission has occurred only when the vaccinated person develops rash. The incidence of herpes zoster is lower with the vaccine virus than with the wild-type virus. Overall, it is estimated that the incidence of herpes zoster in vaccinated children is 4 to 12 times lower than in unvaccinated children, which shows that the vaccine virus has a lower reactivation capacity.

Protective efficacy and number of doses

In pediatrics, administration of a first dose induces seroprotection six weeks later in 85% of children between 1 and 12 years of age and rises to 99.5% after the second dose. In those over 12 years of age and adults, seroprotection after the first dose is 80% and 98% after the second dose. The effectiveness of one dose of vaccine in children between 1 and 12 years of age is estimated to be 80-85% for any form of the disease and 95-98% for severe varicella. However, protection against the disease with a single dose of vaccine decreases over time and especially after 5 years of age. Therefore, two doses of vaccine are necessary for maximum protective efficacy, the first at 12 to 15 months and the second at 2 to 4 years of age.

Indications and contraindications

Varicella vaccine is indicated routinely in susceptible persons aged 12 months or older. Also in susceptible individuals who have been exposed to varicella, since vaccination within 3 and 5 days after exposure may prevent clinically apparent infection or modify the course of infection. In these cases the recommended schedule is two doses separated by an interval of at least one month. The vaccine can also be used in certain immunodeficiencies in which, after an individualized evaluation, the benefit is assessed as outweighing the risk.

Regarding contraindications, in addition to the general contraindications of vaccines (anaphylactic reaction to previous dose, severe hypersensitivity to any component of the vaccine, severe acute disease) should be noted: congenital immunodeficiencies (cellular or mixed) and acquired immunodeficiencies (leukemias, lymphomas and malignant tumors) in active phase of the disease; immunosuppressive treatment (up to three months after its completion); treatment with high doses of corticosteroids (up to one month after its completion); HIV infection with CD4+ lymphocyte percentage

Vaccination in particular situations

1. Children who have had chickenpox before their 1st birthday. Vaccination, regardless of the history of having had chickenpox in the first year of life, reinforces the protection of those who may have developed incomplete immunity after natural infection in that period of time, due to the interference of maternal antibodies received through the placenta. However, cases can be assessed on an individual basis, primarily on the basis of age and certainty of diagnosis of varicella. Infants over 6 months of age with clinically clear varicella will probably develop complete and long-lasting immunity. They can be considered immune and, therefore, omit vaccination. Infants with varicella under 6 months of age and those under 1 year of age in whom the disease is mild or very mild, such that its diagnosis is doubtful, should be vaccinated at the age indicated in general, regardless of the above-mentioned history.
2. Children with a history of clinically very mild varicella and with little diagnostic certainty at the age of standard vaccination: they should be vaccinated according to the normal two-dose schedule.
3. Children who have had chickenpox (or shingles) after the administration of the first dose of the vaccine: they do not need to receive the second dose.
4. Children who, upon reaching the age of vaccination of the susceptible adolescent (12 years of age) report having received only one dose previously: they should receive the second dose.
5. Pregnancy. After surveillance of women inadvertently exposed to the vaccine previously or during pregnancy, no case of congenital varicella has been described, so it is deduced that its teratogenic capacity is very low. However, it is not recommended to vaccinate during pregnancy, nor for a woman to become pregnant within one month after administration of the vaccine.