This article {extract} was published in Communicable Diseases Intelligence Volume 23 Number 11 - 28 October 1999 and may be downloaded as a full version PDF from the Table of contents page.

Results continued

Vaccine preventable diseases

This section summarises the national notification data for diseases targeted by the current standard childhood vaccination schedule. The only change to the schedule since 1994 (when the fifth dose of DTP for children aged 4-5 years was introduced) occurred in the final quarter of 1998. At this time the second dose of MMR was moved from 10-16 years of age to 4-5 years of age as part of the Measles Control Campaign. Other notifiable diseases for which vaccines are available but which are not incorporated in the standard childhood schedule (hepatitis A, hepatitis B, and some serotypes of meningococcal disease) are not described here. The 1998 influenza surveillance data, and investigations for polio and acute flaccid paralysis have been published in earlier editions of CDI.^26,27,28 Congenital rubella notifications have not been included in this report. Identified cases for 1998 are reported in the Sixth Annual Report of the Australian Paediatric Surveillance Unit.²⁹

Diphtheria

There were no cases of diphtheria notified in 1998. The last known case occurred in 1992 and was notified in 1993.

Haemophilus influenzae type b infection

There were 35 notifications of Haemophilus influenzae type b (Hib) infection in 1998. This is the lowest annual number of notifications recorded since national surveillance began in 1991 (Figure 20). As in previous years, most notified cases were aged less than 5 years (71.4%) (Figure 21). The notification rate for this age group was 1.9 per 100,000 compared to a rate of 0.2 per 100,000 overall. The highest notification rates were for children aged less than 2 years. The male to female ratio for all ages was 1:1.3.

Figure 20. Notifications of Haemophilus influenzae type b, 1991-1998, by month of onset and less than 5 years and all ages

Figure 21. Notification rate of Haemophilus influenzae type b, 1998, by age group and sex


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Measles

There were 306 measles notifications in 1998, a rate of 1.6 per 100,000 (Tables 1 and 2). This is the lowest annual rate since national surveillance began in 1991 (Figure 22). As in recent years, the highest notification rate was in the 0-4 years age group (15.6 per 100,000) (Figure 23). Within this age group, notification rates were highest for infants aged less than 1 year (32.1 per 100,000). There were slightly more notifications for males than females (male to female ratio 1.1:1). The notification rate for Tasmania (7.6 per 100,000) was more than double the rate for any other State/Territory.

Figure 22. Notifications of measles, 1991-1998, by month of onset



Figure 23. Notification rate of measles, 1998, by age group and sex

Mumps

In 1998 there were 183 notifications of mumps, a rate of 1.0 per 100,000. Annual numbers of notifications have remained relatively constant since mumps became notifiable in all States and Territories (July 1996) (Figure 24). Notifications were spread across most age groups, but as in previous years, most (41.0%) were aged less than 10 years. The highest notification rates were in the 5-9 years age group (3.1 per 100,000) followed by the 0-4 years age group (2.6 per 100,000). Overall, numbers of notifications were evenly divided between males and females (male to female ratio 1:1.0) (Figure 25). However, there were more notifications for males than females in the 5-9 years age group (male to female ratio 2.2:1).

Figure 24. Notifications of mumps, 1992-1998, by month of onset

Figure 25. Notification rate of mumps, 1998, by age group and sex

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Pertussis

There were 40% fewer pertussis notifications in 1998 than in 1997 (Figure 26). In 1998, there were 6,432 notifications, a notification rate of 34.3 per 100,000. As expected, numbers of notifications peaked in the spring and summer months. There were slightly more notifications for females than males (male to female ratio 1:1.1). As in recent years, the highest notification rate was for children aged less than 1 year (113.5 per 100,000). Rates were also high in the 5-9 years (91.2 per 100,000) and 10-14 years age groups (89.2 per 100,000) (Figure 27).

Notification rates varied by geographic location (Map 8). At the State/Territory level, rates were highest for South Australia (47.1 per 100,000), Queensland (42.4 per 100,000), and New South Wales (40.5 per 100,000). The Statistical Division of Far West in New South Wales had by far the highest notification rate (268.3 per 100,000). Rates of over 100 per 100,000 were also recorded in the Statistical Divisions of Kimberly in Western Australia (119.1   per 100,000), East Gippsland (107.2 per 100,000) in Victoria, and Mackay (104.6 per 100,000) in Queensland.

Figure 26. Notifications of pertussis, 1991-1998, by month of onset

Figure 27. Notification rate of pertussis, 1998, by age group and sex

Map 8. Notification rate of pertussis, 1998, by Statistical Division of residence

Polio

No cases of polio were reported in 1998. Indigenous transmission of wild type polio virus is estimated to have ceased in the early to mid 1960's and the last imported case was over 20 years ago.³⁰ Laboratory investigations are currently being performed to identify the last known case of wild type polio in Australia.³¹

Rubella

Since 1995, annual numbers of rubella notifications have been declining (Figure 28). In 1998, there were 772 notifications, a notification rate of 4.1 per 100,000. The highest number of notified cases occurred in August, slightly earlier than the expected peak in spring months. Males aged 15-19 years continued to have the highest notification rate (15.8 per 100,000) (Figure 29). However, rates for this group have declined markedly since 1994/5, the time when a second dose of MMR vaccine for both sexes aged 10-16 years replaced the school girl rubella vaccination program.³² In 1998, rates for males aged 15-19 years were comparable to those for males aged 20-24 years (13.1 per 100,000) and males in the 0-4   years age group (14.4 per 100,000). All States and Territories had lower notification rates than in 1997, with the greatest reduction occurring in South Australia.
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Figure 28. Notifications of rubella, 1991-1998, by month of onset

Figure 29. Notification rate of rubella, 1998, by age group and sex

Tetanus

There were seven notifications of tetanus in 1998, a similar number to that reported in 1997. As in recent years, there were more notifications for females than males (male to female ratio 1:2.5) and most (6/7, 85.7%) were at least 55 years of age. For notified cases aged at least 55 years, the notification rate was 0.2 per 100,000.

Childhood immunisation coverage reports

Estimates of immunisation coverage both overall and for individual vaccines for children at 12 months of age continued to improve in 1998 (Table 4). This trend was also evident in each State and Territory. A number of factors have probably effected coverage estimates, including the linking of immunisation status as recorded on the ACIR to childcare payments and the General Practice Immunisation Incentives (GPII) scheme, both of which commenced during 1998. Intensive efforts to improve data transfer and handling in the Northern Territory have resulted in the data more accurately reflecting immunisation coverage in that jurisdiction.

Immunisation coverage at 2 years of age was first reported in 1998. Coverage estimates for vaccines recommended at 12 months and 18 months of age improved during 1998 (Table 5). 'Fully immunised' coverage levels were reported to be lower than estimates for individual vaccines. Reasons for this discrepancy are being investigated. One likely factor is poor identification of children on records of immunisation encounters, which leads to difficulties matching new and existing vaccination records on the ACIR. It is important to note that in other countries such as the United Kingdom, 3 doses of DTP and Hib vaccine constitute full immunisation with these vaccines at 2 years of age compared to 4 doses of DTP and 3 or 4 doses of Hib vaccines.

Table 4. Percentage of Australian children born in 1997 immunised at 1 year of age according to data available on the Australian Childhood Immunisation Register

Vaccine group	Per cent for each birth cohort (%)
	1/1/97-31/3/97	1/4/97-30/6/97	1/7/97-30/9/97	1/10/97-31/12/97
DTP	82.4	86.5	86.1	86.3
OPV	82.5	86.4	85.9	85.9
Hib	82.1	86.0	86.0	86.4
Fully immunised	80.2	84.3	84.5	84.9

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Table 5. Percentage of Australian children born in 1996 immunised at two years of age according to data available on the Australian Childhood Immunisation Register

Vaccine	Per cent for each birth cohort (%)
	1/1/96-31/3/96	1/4/96-30/6/96	1/7/96-30/9/96	1/10/96-31/12/96
DTP	76.0	78.0	80.2	80.9
OPV	82.7	83.8	85.0	85.6
Hib	76.5	78.4	80.4	81.0
MMR	82.5	83.2	85.0	86.4
Fully immunised	63.8	66.1	68.8	70.3

Discussion

In 1998, notification rates for most vaccine preventable diseases were lower than in recent years. Rates for measles and Hib infection were the lowest recorded since the establishment of the current notification system in 1991. The epidemic of pertussis that occurred in 1997 receded in 1998 and numbers of rubella cases have been declining since 1995. These are promising findings that are likely to reflect a true reduction in the incidence of disease.

During 1998, substantial initiatives were undertaken to eliminate measles in Australia. A major initiative was the Measles Control Campaign (MCC). The Campaign involved a one-off school based vaccination program for children aged 5-12 years. This was necessary because of the decision to change the recommended age for the second dose of MMR vaccine from 10-16 years to 4-5 years. In addition, parents of preschool children who had no record of a first MMR vaccination according to the ACIR were sent a reminder letter. The Australian Measles Control Campaign 1998 Evaluation Report estimated that 96% of the 1.8 million school children aged 5-12 years were vaccinated during the MCC.³³ In the preschool group, the report estimated that 97.5% of those aged 12 months to 3.5 years had received a first dose of MMR vaccine. The successful implementation of the MCC means that Australia is on track to fulfil the national measles vaccination targets set for the year 2000.³⁴ Results from the campaign would not be expected to be reflected in the NNDSS figures for 1998.

To accurately monitor progress towards the elimination of measles following the MCC, high quality surveillance data are required. In 1998, the National Measles Surveillance Strategy was formulated to provide guidelines for improving surveillance data quality.³⁵ These guidelines recommend that all States/Territories implement a uniform method for recording information about measles notifications and that laboratory confirmation be sought for all sporadic cases. Laboratory confirmation is increasingly important, as the positive predictive value of a clinical diagnosis becomes poor when the number of measles cases declines. A recent review of methods of diagnosis of measles cases notified in 1992-1997 indicated less than one third had been confirmed.³³ Information reported from the Victorian enhanced surveillance system for measles emphasises the need to improve levels of confirmation, as the majority of suspected cases who had laboratory testing performed did not have measles.^33,36 This may explain high rates in some States.

Trends in the incidence of vaccine preventable disease are shown from notification data even though NNDSS data are incomplete and underestimate the incidence of disease. Additional data about the method of diagnosis and vaccination status of cases would enhance the usefulness of notification data. Vaccination records could be used to identify vaccine failures, while information about the method of diagnosis may help to explain changes in notification rates. Efforts to incorporate this information onto the NNDSS database are currently underway. The vaccination status of Hib cases for 1998 is available in the Sixth Annual Report of the Australian Paediatric Surveillance Unit.²⁹