Introduction

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The age distribution of people with chickenpox and mortality from the disease in developed countries are changing.¹ In England and Wales deaths in adults have increased in number.² In the United States chickenpox is the leading cause of deaths that could be prevented by vaccine.³ As part of a larger study on the burden of chickenpox we reviewed deaths from chickenpox in England and Wales for 1995-7.

	Methods

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We obtained copies of death certificates that mentioned "chickenpox" or "varicella" (ICD-9 (international classification of diseases, 9th revision) code 052; ICD-10 (10th revision) code B01) for 1995-7 and data on the annual numbers of deaths for 1985-97 from the Office for National Statistics. We sent questionnaires to the physicians responsible for the patients to clarify the diagnosis. Using this information we classified deaths into those in which primary infection with varicella zoster virus was the definite or probable underlying cause of death and those in which it could be excluded. We also reviewed confidential inquiries into maternal deaths in the United Kingdom for 1985-96.⁴ The research unit of the Royal College of General Practitioners provided data on age and sex specific incidences of chickenpox.

	Results

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Response rates and varicella classification
We obtained 119 death certificates. Of these, we classified 71 (60%) deaths as definitely or probably due to chickenpox (for example, chickenpox pneumonia, generalised chickenpox, etc). Of these, 58 replies from physicians confirmed 56 (97%) as definite, so we estimated 97% (12.6) of the 13 remaining cases for which we had no confirmation as definite (table 1). Of the 48 (40%) that we classified as uncertain or not due to chickenpox, 19 of 36 physicians (53%) confirmed the diagnosis on the certificate. We therefore estimated 53% (6.3) of the 12 remaining as chickenpox. Thus, of the original 119 deaths, we estimated that 94 (79%) were really attributable to chickenpox (table 1). This is a lower bound value. We had no further information for 25 cases (no reply from physician in charge in 14 cases and notes lost in 11 cases). The overall response rate therefore was 88% (105/119).

Estimating case fatality
We used general practitioners' consultation rates to estimate the number of cases of clinical chickenpox in the population. The case fatality was 9.22/100 000 for 1985-97. In each year, the number of deaths certified generally closely paralleled the consultation rate for chickenpox.

	Discussion

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Mortality from chickenpox is not negligible. During 1995-7, 81 deaths were recorded by the Office for National Statistics. However, we received 119 certificates that mentioned chickenpox or varicella. After detailed inquiries, we estimated that at least 75 were genuine cases of chickenpox. This suggests at least 25 deaths from chickenpox annually. In 1996-7 there were seven certified deaths from whooping cough, mumps, measles, and Haemophilus influenzae type b (Hib) meningitis in England and Wales compared with 67 from chickenpox.

Specificity of diagnosis
We asked the clinician concerned to review our presumptive diagnosis taken from the certificate. There was a good response and excellent consistency between our assessment and theirs, especially in those we classified as true chickenpox (consistency 97%). This suggests high specificity. Sensitivity may have been lower because, conversely, some deaths from chickenpox may not be correctly coded or certificated,⁵ and we may have lost some. Thus our figure of 25 deaths a year from chickenpox is probably an underestimate.

Case fatality and age
The imbalance in the number of deaths from chickenpox between children and adults in England and Wales has been consistent since 1967. Deaths in adults have increased in number and proportion: they accounted for 48% (88) of all deaths from chickenpox in 1967-77, 64% (120) in 1978-85,⁶ and 81% (269) in 1986-97. Adults accounted for over 81% of the confirmed deaths in our study but for only 19% of the consultations. Chickenpox is also an important cause of death in children aged 0-4 years because more children in that age group catch chickenpox and because a higher proportion of them die from the illness.

Role of vaccine
A chickenpox vaccine is available, though not yet licensed in the United Kingdom. Our results do not on their own provide sufficient evidence for mass vaccination. We need information not only on the burden of disease at primary and secondary care levels, but also on the effect of the vaccine on herpes zoster. We also need to ensure a high enough uptake so that the disease does not shift towards the older population and a higher mortality.