Laboratory surveillance and serosurveys
National serosurveys
A collaborative national network
of more than 50 laboratories has been set up, and more than 13 000 sera have been collected. These
sera have been collected from children and adults. They provide a cross-sectional national collection,
which can be studied to measure levels of protection against different diseases.
Measles antibody assays on 6000
sera collected from subjects aged 1-18 years before (3000 sera) and after (3000 sera) the Measles Control
Campaign showed statistically significant increases in the level of protection against measles among
preschool and primary school age children, to 89% and 94% respectively. Among older children, 91% were
protected. Low levels of protection (66%) in children aged 1-2 years suggested that young children were
not being vaccinated at the recommended age of 12 months (see Measles Control Campaign).
There was good correlation between the post-Campaign serological data and a set of sera collected by cluster sampling in Victoria.
These 6000 sera have now been tested
for rubella antibodies and the results show a marked increase in seropositivity following the Measles
Control Campaign. However, there is a significant gap in immunity in teenaged males and older males compared
with females; this reflects the results of the rubella vaccination program which initially targeted only
teenage females.
Hepatitis A immunity increased progressively
with age, with about 50% of subjects immune at 50 years of age, and 70% immune by 60 years or more. Evidence
of past hepatitis B infection was detected in only 1.1% of subjects: Of 1716 subjects tested, 28%had
been vaccinated for hepatitis B, including 39% of those aged 1-5 years.
Diphtheria and tetanus tests are
completed and the results are being analysed. Mumps antibody titres will also be assayed on these sera.
Melanie Wong and Mark Hanlon in the immunology lab
Pertussis
Sensitivity and specificity of serological
diagnosis
This study used the clinical case definition as the gold standard and, following a series of telephone interviews with subjects, determined that the routine laboratory diagnostic test (the pertussis-specific whole-cell IgA test) was 98% specific for the diagnosis of pertussis, though only 24% sensitive. It was concluded that cases notified on the basis of a positive IgA test are highly likely to be pertussis (Reference 63).
Epidemiology
In collaboration with the South Western Sydney Public Health Unit serum is being collected for pertussis antibody assays from a group of adolescents involved in a hepatitis B vaccine trial. History of coughing illness is also being collected at entry into the study and prospectively over a 12-month period. An attempt is being made to serologically validate the use of reported cough as a marker for clinical pertussis in this age group.
Genotyping of B. pertussis
In The Netherlands, it was thought that an increase in the incidence of pertussis was associated with the appearance of a new strain of B. pertussis. Two years ago, NCIRS coordinated the dispatch of isolates from a number of Australian centres to The Netherlands. Data relating to 62 of the 140 isolates sent (18 collected before 1989 and 44 after 1989) show that there was no mutant pertactin strain in Australian specimens before 1989, but the mutant strain appeared in Australian specimens associated with increased pertussis activity in the 1990s. Though these results are interesting, the samples were neither random nor representative. Results from typing of further isolates are awaited.
Varicella (chicken pox)
Immunity in antenatal clinic patients
in Darwin
The seroprevalence of varicella-zoster virus (VZV) antibodies in women in Darwin was found to be higher than that reported from other tropical regions. This suggests that factors other than climate may be associated with the incidence of childhood chickenpox in the Australian tropics. It is possible that childhood infection in tropical parts of Australia may often be subclinical or atypical. This study was completed in collaboration with the Northern Territory Department of Health (Reference 78).