Parents who returned the questionnaire were sent a consent form and a kit to collect oral fluid, with clear instructions on how to obtain a sample

from their child, which they were asked to return to the Health Protection Agency (HPA). Approximately 7000 introductory letters were distributed by schools; 550 questionnaires were returned with a positive history of chickenpox, 84 with a negative history, and 56 with an uncertain history, and 1 was incomplete. We posted 268 oral fluid kits, including 128 to respondents with a positive history of chickenpox and all those with negative or uncertain histories. Families were informed at the outset in the initial study information pack that, as a token of appreciation, a voucher for £10 would be sent to them once a sample was received in the laboratory. Children found to be susceptible to varicella were offered two doses of varicella vaccine ABT-888 cost without charge. Oral fluid samples and consent forms were received by the HPA Virus Reference Department, MS-Colindale, and processed to extract VZV-IgG using standard methods and diluents. Oral fluid samples were stored at −30 °C prior to batch testing. For semi-quantitative determination of IgG antibodies to VZV, the in-house VZV-IgG time resolved fluorescence immunoassay, (TRFIA), [12] was modified for testing oral fluid. Testing of paired serum and oral fluid samples, had previously established that measurements above a cut-off of 0.35 mIU/mL should

be considered positive, below a cut-off of 0.25 mIU/mL as negative, with an equivocal range between 0.25 and 0.35 mIU/mL. [HPA unpublished data] Selleck ZD6474 Samples testing negative or equivocal were also tested for total IgG to determine whether the sample had been taken appropriately and contained sufficient total IgG, using a cut-off of greater than 2.5 mg/L. Data were analysed using Stata v12 (Statcorp, TX, US). For each chickenpox history group, we aimed for a sample size of 100, to estimate with reasonable precision

the proportion with VZV-IgG (95% confidence interval within ±10%). The study was not designed or powered to detect differences by ethnicity. Exact 95% confidence intervals for proportions were calculated and proportions compared according to history using two-sided Unoprostone Fisher’s exact tests. We also undertook a sensitivity analysis to investigate the impact of using the oral fluid assay in populations with different VZV-IgG prevalence by modelling the effect of different values for the negative predictive value (NPV) of the assay. 120 oral fluid samples were received from respondents with a positive history of chickenpox, 77 with a negative history and 50 with an uncertain history. The average age of respondents was 13 years, and 85% were white, 6% mixed ethnicity, 6% Asian, 3% Black, and 1% Chinese. The groups with different history responses were not significantly different with respect to age or ethnicity (data not shown). Overall, 109 (90.8% [95% CI 85.