Varicella immunity in vaccinated healthcare workers

Background: Nosocomial spread of varicella-zoster virus (VZV) infection can cause severe disease among vulnerable patient-populations and healthcare personnel (HCP). Limited data are available on duration of varicella vaccine-induced protection among adults and to what extent cell-mediated immunity (CMI) and antibody avidity contribute to protection. Objective: Evaluate humoral and cell-mediated immune responses of HCP who received a 2-dose regimen of varicella vaccine, and observe the responses to a 3rd vaccine dose among HCP who were seronegative after vaccination. Study design: A convenience sample of HCP with documented 2 doses of varicella vaccine was used to assess acquired VZV immune parameters (cytokine production, IgG avidity). HCP seronegative after 2 doses of vaccine were offered a third dose and evaluated further. Vaccine recipients' immune responses were compared with responses from persons with history of wild-type VZV infection. Results: The convenience sample consisted of 101 HCP with documented 2 doses of varicella vaccine; 12 (11.9%) were seronegative post-vaccination. 11.5% of 61 seropositive 2-dose recipients produced low avidity antibody, suggesting suboptimal response to vaccine. Seven 2-dose vaccinees who were VZV seronegative seroconverted after a third dose; however, 3/7 (42.9%) produced low avidity IgG. 142 persons with a history of varicella were all VZV IgG seropositive, and all had moderate to high avidity IgG. Conclusions: Measurements of serum IgG titers alone may not accurately reflect vaccine protection. Varicella vaccination of HCP remains important but further studies are needed to evaluate CMI and antibody avidity responses in HCP vaccinated with two doses of varicella vaccine.