BACKGROUND There have been occasional reports on varicella infection among healthcare workers (HCWs) despite varicella-zoster virus (VZV) seropositivity. We compared the levels of humoral and cellular immunity to VZV in seropositive HCWs who had acquired immunity by natural infection or vaccination.

METHODS Seropositive healthy HCWs with an apparent history of varicella or VZV vaccination once or twice were recruited. Their samples were assessed for anti-VZV IgG levels, the relative avidity index (RAI), and the frequencies of VZV-specific cytokine-producing or polyfunctional CD4⁺ or CD8⁺ T cells.

RESULTS A total of 75 seropositive HCWs (29 with a history of varicella, 25 vaccinated once, and 21 vaccinated twice) were assessed for humoral immunity. Cellular responses could be analyzed in 59 (28, 21, and 10 in the respective groups). The anti-VZV IgG level, RAI, and memory CD4⁺ T cell responses were significantly higher in the past infection group than in the vaccinated once group. The RAI levels were significantly higher in the past infection group than in the vaccinated twice group.

CONCLUSION Seropositive HCWs without a varicella history, especially those who received the vaccine only once, had significantly lower levels of immune responses to VZV. Such HCWs might need to comply with airborne precautions.