Identification of potential candidates for varicella vaccination by history: questionnaire and seroprevalence study

Introduction

Guidance from the Joint Committee on Vaccination and Immunisation regarding varicella vaccination of workers in healthcare settings is imminent.1 The new recommendations will advise that all those with a negative or uncertain history of chickenpox or shingles at pre-employment assessment should be tested for varicella zoster virus IgG. Vaccination with the newly licensed varicella vaccine will be advised for those with a seronegative result.

A history of chickenpox has a high positive predictive value for immunity among healthcare workers in Europe, where the seroprevalence is as high as 98.5%.2 The validity of a history of chickenpox is unknown in those from tropical countries, however, where the mean age of infection is in early adulthood.3 Guy's and St Thomas' Hospital NHS Trust in inner London has some 8000 staff and 4700 healthcare students of increasingly heterogeneous origins.4 5 We conducted a questionnaire and seroprevalence study at the hospital to ascertain the relations between history of chickenpox, countries of birth and residence, and serological status.

Participants, methods, and results

From September 2001 to July 2002 a nurse administered questionnaire was completed for 747 staff and students consecutively attending pre-employment screening. Country of birth or of first residence was noted, together with age range during subsequent residence in other countries. Recruits were asked about prior chickenpox or shingles, with details recorded. Serology was requested in line with usual practice. Immune status was determined by using the Diamedix enzyme immunoassay kit for varicella zoster virus IgG (Miami, FL, USA).

Of 629 (84%) recruits tested, six yielded equivocal results and were excluded from further analysis. Those who denied or were unsure of past infection were considered as one group. We assigned participants to “temperate” or “tropical” subgroups according to country of birth or first residence. We compared differences in seroprevalence and proportions of groups with a positive history by using the χ² test; we also analysed other variables (unadjusted and adjusted) using logistic regression.

The table shows the results. Additional information about prior chickenpox (nature and site of rash, residual scar) did not increase the positive predictive value. Shingles had a positive predictive value of 100%, however, with all 22 participants testing IgG positive. Twenty nine participants in the temperate group had not lived solely in temperate zones during their first 12 years and had a reduced risk of testing seropositive (odds ratio 0.35 compared with those born and brought up in a temperate climate)—similar to that of those born and brought up in a tropical climate (odds ratio 0.36).

Participants from Sub-Saharan Africa and the Caribbean were disproportionately represented among the 52 seronegative participants (29% (15) and 17% (9) respectively, compared with just 18% (113) and 5% (31) respectively among all new recruits). Among participants with a history of prior infection, the false positive rate was 9.3% in the tropical group versus 3.7% in the temperate group (P = 0.019) (table).

Comment

Candidates for varicella vaccination (seronegative staff) will inevitably be missed if a blanket policy of screening by history alone is implemented. As shown here, their number will increase considerably if groups with a significantly higher false positive rate account for a substantial proportion of the workforce.

The overall seroprevalence and positive predictive value are lower than figures usually quoted for healthcare workers in Europe, but the distribution of geographical origins largely explains the discrepancy. It is a weakness of this study that some of the countries defined as temperate in the questionnaire—for example, India—also contain regions with tropical climates. Serological testing of recruits from these areas should also be considered. Individuals born or raised in tropical climates should have serological screening regardless of a history of chickenpox.