Key messages

• Vaccination against chickenpox decreases the risk of developing the disease in the long term.
• Vaccination against chickenpox probably decreases the risk of developing the disease in the short term.
• Vaccination against chickenpox increases the occurrence of local reactions 48 hours after its administration and probably increases the presence of fever and chickenpox-like rash.

What is the evidence.
See evidence matrix  in Epistemonikos later

We identified two systematic reviews [3], [4], which including 16 primary studies reported in 17 references [5], [6], [7], [8], [9], [10], [11], [12], [ 13], [14], [15], [16], [17], [18], [19], [20], [21], of which two correspond to randomized trials reported in three references [19], [20], [21]. This table and the summary in general are based on the latter, since observational studies did not increase the certainty of existing evidence, nor did they provide additional relevant information.

What types of patients were included*

All trials included healthy patients, with no history of chickenpox or vaccination against the disease.

One trial included patients between one and 14 years old [20], while the other trial included patients between 10 to 30 months old [21].

What types of interventions were included*

All trials evaluated one dose of Oka strain varicella vaccine [20], [21].

Both trials compared vaccination against an unvaccinated control group [20], [21].

In addition, one of the trials included a comparison with no follow-up of the control group [20], so in this summary the authors decided to evaluate it assuming the worst case scenario, meaning that no one in the unvaccinated control group develop chicken pox in the follow-up years.

What types of outcomes 
were measured

• Development of chickenpox disease.
• Percentage of vaccinated patients.
• Disease rate among unvaccinated participants.
• Time between vaccination and disease occurrence.
• Adverse effects: fever, local reactions, chickenpox-like rash.
• Transmission of chickenpox from vaccinated individuals to others.
• Risk of herpes zoster following vaccination.
• Variations in age presentation.
• Cost-effectiveness for varicella vaccine.

To whom this evidence does and does not apply

• The evidence presented applies to healthy children and adolescents not previously exposed to chickenpox disease.
• It does not apply to exposed or immunosuppressed patients, with direct exclusion of these in clinical trials. These patients could present different clinical outcomes than those described, which would be determined by variations in their immune response to the vaccine.

• The outcomes selected are those considered critical for decision-making, according to the opinion of the authors of this summary and generally coincide with those evaluated by the systematic reviews identified.
• However, the reviews do not report the outcome of severe or complicated disease, which was considered a critical outcome as it implies greater morbidity and mortality, as well as an increase in costs for the community [1], [2].

• The evidence shows benefits associated with vaccination in preventing chickenpox disease development both in the long and short term. On the other hand, the application of the vaccine is associated with the occurrence of adverse effects, but these are mild and have little clinical relevance.
• Considering that the certainty of evidence of the results presented in this summary is moderate or high, the balance between benefits and risks favors the administration of the vaccine.

• Considering that chickenpox is a highly prevalent disease, which causes school absenteeism and significant expenses, it seems appropriate to invest resources in the vaccination of healthy patients prior to exposure, since it decreases disease development.
• A systematic review about the economic analysis of vaccines in Spain from 1990 to 2012 concluded that the administration of the varicella vaccine seems to be justified in children aged 15 to 24 months if the indirect cost of suffering the disease is considered [22]. Another Italian systematic review concluded that the implementation of universal vaccination in all regions of Italy by 2015 would be cost-effective from a social perspective, and would imply a favorable cost-effectiveness profile from the NHS perspective [23].

• With the information provided in this summary, most clinicians should recommend vaccination against chickenpox, since there is evidence of a significant decrease in the incidence of the disease, without severe adverse events associated. 
• However, there could be variability regarding this intervention in clinical decisions made by patients, especially those with personal ideologies regarding vaccination or those who insist on the usually benign and self-limited course of disease.

Differences between this summary and other sources

• These conclusions agree, in general, with systematic reviews identified regarding the outcome of incidence of chickenpox disease after the administration of the vaccine and its adverse effects.
• The key messages of our summary are consistent with the recommendations of the Advisory Committee on Immunization Practices, which suggests the application of the varicella vaccine in patients who have not had the disease [24].

• It is unlikely that future evidence will change the conclusions of this summary, because of the high certainty of the evidence related to the development of the disease in the long term. The conclusions regarding adverse effects could change, since they present moderate certainty evidence, excluding the appearance of local reactions whose certainty of the evidence is high.
• We did not identify ongoing trials answering this question in the International Clinical Trials Registry Platform of the World Health Organization.
• We did not identify systematic reviews in progress in the PROSPERO register (International prospective register of systematic reviews).