Zoster vaccine safety and varicella vaccine safety in immunocompromised populations
Zoster vaccine safety
In a follow-up to the December 2012 GACVS meeting at which a general summary of varicella vaccine safety was presented, experts from the US Food and Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC) presented systematic post-licensure safety reviews of the zoster vaccine (Zostavax®) and safety of varicella vaccine in immunocompromised populations. The FDA completed a 7-year safety update of Zostavax® by summarizing key post-licensure observational studies conducted by CDC and Merck, a literature review from the date of licensure (May 2006) through February 2013, and analysis of reports from the Vaccine Adverse Event Reporting System (VAERS) from May 2006 through February 2013. The CDC Vaccine Safety Datalink study on Zostavax®, together with 3 post-licensure studies conducted by Merck as FDA regulatory commitments, included a total of >190 000 vaccinated study subjects. No new safety signals were identified in these studies. More than 12 000 reports were submitted globally for Zostavax® to VAERS from May 2006 through February 2013, of which 1057 were considered serious. The 3 most frequent terms for serious adverse events were herpes zoster, pain, and rash. FDA data mining using disproportionate analysis revealed adverse events predominantly associated with vaccine failure (i.e. herpes zoster despite vaccination), as well as accidental exposure and inappropriate vaccine administration (i.e. use of Zostavax® in subjects younger than the FDA approved age of ≥50 years). In summary, although safety data on the subpopulation of individuals aged ≥80 remains limited, no new safety risks have been identified or confirmed since initial licensure.
Safety of varicella vaccine in immunocompromised populations
Because diseases caused by wild type VZV are more severe and fatal in persons with defects in cell-mediated immunity, varicella vaccine has been studied for safety and efficacy in select immunocompromised populations. Studies of the safety and effectiveness of varicella vaccines were conducted in children with cancer, HIV, and post-organ transplant. All but one of the studies was conducted in developed countries. Compared to healthy children, varicella vaccine is associated with a higher risk of adverse reactions, some severe, in selected subpopulations of children with deficiencies in cell-mediated immunity. Varicella vaccine is contraindicated or should be used with caution, under strict protocol, in persons with leukaemia. Two doses of varicella vaccine are effective and safe in preventing varicella in children with HIV with CD4 T-cell count ≥15%. Case reports were identified describing other immunocompromised children primarily due to natural killer T-cell deficiency discovered after vaccination. In countries with routine childhood varicella vaccination programmes, children are likely to be vaccinated without knowledge of their immune deficiency states. The size of this group will depend largely on the prevalence of undetected and untreated HIV infection. This fact is an important consideration in introducing varicella vaccination, but should be balanced against the benefits of reducing more severe wild-type varicella disease in this subpopulation.