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Vaccine preventable diseases

Introduction

This section summarises the national notification data for diseases targeted by the standard childhood vaccination schedule in 1999. The only change to the schedule in 1999 was the recommendation that DTPa (Diphtheria-Tetanus-acellular Pertussis) be used for all 5 infant doses. Previously this vaccine was only funded nationally for the 2 booster doses. Other diseases for which vaccines are licensed in Australia but which were not incorporated into the standard childhood schedule in 1999 (hepatitis A, hepatitis B, invasive pneumococcal disease, influenza, some serotypes of meningococcal disease, varicella and Q fever) are not described in this section. The 1999 influenza surveillance data, and investigations for polio and acute flaccid paralysis have been published in earlier editions of CDI.^39,40,41 Congenital rubella notifications for 1999 (5 notifications, one definite congenital rubella infection late in pregnancy) are described in the Seventh Annual Report of the Australian Paediatric Surveillance Unit.⁴²

The third annual report of vaccination coverage estimates for children aged 12 months and the second annual report for children aged 24 months (using data extracted from the Australian Childhood Immunisation Register - ACIR) are also included in this section. A full description of the methodology used for calculating these estimates have been described previously.⁴³

Diphtheria

There were no cases of diphtheria notified in 1999. Prior to this, the last known case occurred in 1992 and was notified in 1993.

Haemophilus influenzae type b

There were 40 notifications of Haemophilus influenzae type b (Hib) disease in 1999, five more than in 1998 but considerably less than in the pre-vaccine era (Figure 25). As in previous years, most notified cases (52.5%) were less than 5 years of age and the highest notification rates were in children less than 2 years of age. The notification rate for children 0 to 11 months of age was 3.6 per 100,000 population and for one-year-olds was 4.0 per 100,000 population. This compares with an overall notification rate of 0.2 per 100,000 population. There were slightly more females than males (male:female ratio 1:1.2). The Northern Territory had the highest notification rate (1.6 per 100,000 population, 3 cases) although most cases (25/40) were from New South Wales and Queensland.

Figure 25. Notifications of Hib, Australia, 1991 to 1999, by month of onset

Measles

There were 230 measles notifications in 1999, a rate of 1.2 per 100,000 population. This is the lowest annual rate for Australia since national surveillance began in 1991 (Figure 26).

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Geographical distribution

All States and Territories except Victoria and the Northern Territory had their lowest ever annual notification rates of measles. In 1999, the Northern Territory reported 10 cases, nine more than in 1998, leading to a notification rate for 1999 which was at least double that of any other jurisdiction (Table 2). Although the Northern Territory had the highest rate, Victoria had by far the greatest proportion of cases (48%). Most of the Victorian cases (62/111, 56%) occurred during an outbreak in February/March (Figure 26). This outbreak had as its index case a returned traveller from Bali, and all except 10 of the 62 cases were young adults aged 18-30 years.⁴⁴ Two other smaller clusters of measles cases occurred in Victoria between August and October. The first was traced to a returned traveller arriving from London via Malaysia, while the second was associated with evacuees from East Timor.³⁵ Queensland experienced increased numbers of notifications in June and July while other jurisdictions recorded their lowest numbers for the year at that time.

Figure 26. Notifications of measles, Australia, 1991 to 1999, by month of onset (and State/Territory of residence)

Age and sex distribution

As in recent years, age-specific notification rates for measles were highest for 0-4 year-olds (6.3 per 100,000 population) especially those aged less than one year (15.4 per 100,000 population) and one year of age (7.6 per 100,000 population). However, rates for these age groups were considerably lower than in the past (Figure 27). Rates for 5-9 year-olds were also the lowest on record. In contrast, rates for older ages increased compared with those for the previous year - a reflection of the outbreak amongst young adults in Victoria in 1999. The most apparent rise was in the 20-24 year age group (rate: 4.0 per 100,000 population) which accounted for 20 per cent of the cases in 1999 (compared with less than 10 per cent in the previous 6 years). This age group had the second highest age-specific rate. As in past years there were similar numbers of male and female cases, with slightly more females than males in 1999 (male:female ratio 1:1.2).

Figure 27. Notification rate for measles, Australia, 1996 to 1999, by age group and year of onset

Mumps

In 1999 there were 184 notifications of mumps, a rate of 1.0 per 100,000 population. This is similar to the number of notifications in the past 2 years. There were notifications from most age groups (Figure 28) with 50.8 per cent from people aged at least 15 years. As in previous years the highest notification rates were in the 5-9 year age group (2.7 per 100,000 population) and the 0-4 year age group (2.6 per 100,000 population, Figure 29). However rates in these age groups have been lower since 1995 while rates in people aged at least 15 years have been steadily increasing since 1993. This pattern was apparent even in New South Wales where only laboratory-confirmed cases are notifiable. In 1999, there was a secondary peak in notifications in the 25-29 year age group (1.4 per 100,000 population).

Figure 28. Notification rate for mumps, Australia, 1999, by age and sex



Figure 29. Notification rate for mumps, Australia, 1993 to 1999, by age group and year of onset



Overall, there were similar numbers of mumps notifications from males and females (male:female ratio 1.1:1), however, there were more notifications for males than females in the age groups most frequently reported. The rates were highest in the Australian Capital Territory and Western Australia (Table 1) while Victoria provided most of the notifications (39.7%) (Table 1). Notified cases occurred throughout the year, but peaked in May largely due to increased reports from Victoria at this time.

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Pertussis

There were 4,396 notified cases of pertussis in 1999, 1,430 fewer than in 1998. The annual notification rate of 23.2 per 100,000 population was the lowest recorded since 1992. The majority (63%) of the notifications occurred in the second half of the year when there were outbreaks in Tasmania, Western Australia, Queensland and Victoria. Notifications peaked in November 1999, when 572 cases were notified (Figure 30).

Figure 30. Notifications of pertussis, Australia, 1991 to 1999, by month of onset



For the first time since the establishment of the current notification system, the 10-14 year age group had the highest notification rate of pertussis instead of infants aged less than one year (Figure 31). Children aged 1-4 years had the lowest rate, which is also in contrast to past years when rates were lowest in adults. The notification rate in 5-9 year-olds continued to decline, both overall and relative to all other age groups except those aged less than one year. In 1999, the rate for 5-9 year-olds was only marginally higher than the rate in adults and 1-4 year-olds.

Figure 31. Notification rate for pertussis, Australia, 1993 to 1999, by age group and year of onset



Notification rates of pertussis varied considerably by geographic location (Map 8). At the State/Territory level, rates were highest in Tasmania (129.9 per 100,000 population) and lowest in the Northern Territory (only 2 cases notified, giving a rate of 1 per 100,000 population).

Map 8. Pertussis notification rates by Statistical Division of residence, 1999



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Poliomyelitis

No cases of poliomyelitis were reported in 1999.

It is difficult to determine exactly when the last case of locally acquired poliomyelitis occurred in Australia. However, the last laboratory confirmed case was in 1967 and there were three clinically compatible cases notified in 1972 with no additional information currently available.⁴⁵ All cases notified since 1972 have been investigated further and this has led them to be re-classified as cases of vaccine-associated paralytic poliomyelitis (VAPP). The last known imported case of poliomyelitis was due to wild poliovirus type 1 in 1977.

Rubella

Secular and geographic distributions

Since 1995, annual numbers of rubella notifications have been declining (Figure 32). In 1999, there were 376 notifications, a notification rate of 2.0 per 100,000 population. This is half the number/rate of 1998, and the lowest on record both nationally and in each State and Territory. As in 1998, the highest number of notified cases occurred in August, which is slightly earlier than the expected seasonal increase in spring months. This peak was predominantly due to increased notifications from Victoria and Queensland who also contributed most (73.9%) of the notifications for the year. Despite most notifications coming from these 2 States, the highest notification rate was from the Australian Capital Territory (Table 2).

Figure 32. Notifications of rubella, Australia, 1991 to 1999, by month of onset

Age and sex distribution

In 1999, notification rates were highest for both males and females aged 0-4 years (rate 8.0 per 100,000 population, Figure 33), which is in contrast to the previous 6 years when males aged 15-24 years had the highest rate.²⁴ This altered distribution reflects a continued decline in rates for 15-24 year-old males, while rates for 0-4 year-olds remained constant between 1998 and 1999. Rates for young adult males have been decreasing in recent years due to the replacement of the schoolgirl rubella program with adolescent vaccination of both males and females between 1994 and 1998. Despite the lower rates for males aged 15-24 years, this group continued to contribute a significant proportion of the notifications (25.8%) and overall there were still more males than females notified with rubella (male:female ratio 1.4:1). Within the 0-4 year age group, the majority of cases (80.4%) were aged less than 2 years. Those aged less than one year had the highest rate (21.9 per 100,000 population) and were the only age group to show a marked increase in rates in 1999.

Figure 33. Notifications of rubella, Australia, 1999, by age sex



Notification rates in 1999 for women of childbearing age were the lowest on record; there were 82 cases, a rate of 1.9 per 100,000 population. The rate reductions were seen in each of the 5-year age groups between 15 and 44 years.

Tetanus

In 1999 there were 2 cases of tetanus (one male and one female). Both cases were aged at least 60 years with one reported from Queensland and the other from New South Wales. This is the lowest number of cases reported for a year since the establishment of the current notification database in 1991.

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Childhood vaccination coverage reports

Estimates of vaccination coverage both overall and for individual vaccines for children at 12 months of age continued to improve in 1999 (Table 7). This trend was also evident in each State and Territory.

Table 7. Percentage of Australian children born in 1998 vaccinated according to data available on the Australian Childhood Immunisation Register. Estimate at one year of age

Birth date	1 January to 31 March 1998	1 April to 30 June 1998	1 July to 30 September 1998	1 October to 31 December 1998
Vaccine group	% vaccinated	% vaccinated	% vaccinated	% vaccinated
DTP (3)	87.6	88.0	88.3	89.5
OPV (3)	87.3	87.9	88.3	89.5
Hib (2 or 3*)	87.4	87.7	87.9	88.9
Fully vaccinated	86.1	86.5	87.0	88.1

* Number of doses depends on the vaccine used

Vaccination coverage at 2 years of age was first reported in 1998. Coverage estimates for vaccines recommended at 12 months and 18 months of age were higher in 1999 compared with the previous year, as were the estimates for being fully vaccinated at 2 years of age (Table 8). However, only MMR coverage nationally and DTP coverage for the Northern Territory showed any trend upwards during 1999. 'Fully vaccinated' coverage levels were reported to be lower than estimates for individual vaccines. One likely factor is poor identification of children on immunisation encounter forms, 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 vaccination with these vaccines at 2 years of age.

Table 8. Percentage of Australian children born in 1997 vaccinated according to data available on the Australian Childhood Immunisation Register. Estimate at 2 years of age

Birth date	1 January to 31   March 1997	1 April to 30 June 1997	1 July to 30   September 1997	1 October to 31   December 1997
Vaccine	% vaccinated	% vaccinated	% vaccinated	% vaccinated
DTP (4)	82.8	83.8	82.8	83.6
OPV(4)	87.7	89.8	82.8	83.7
Hib(3 or 4*)	82.8	83.8	82.4	83.4
MMR (1)	87.8	88.7	89.0	89.7
Fully vaccinated	73.5	75.9	74.9	76.7

* Number of doses depends on the vaccine used

In 1999, notification rates for measles, rubella and tetanus were the lowest on record. Rates for Hib infection also remained low, while pertussis rates were the lowest since before the epidemic of 1997. Although overall pertussis notification rates for Australia were the lowest since 1992, many temporally and geographically distinct outbreaks occurred in 1999, with adolescents aged 10-14 years emerging as the age group most at risk. Improved vaccination coverage for the first 4 doses of DTP vaccine and the inclusion of a fifth dose at 4 years of age (in 1994) have been associated with a more rapid decline in rates for ages less than 10 years old compared with those for 10-14 year-olds. The implications of this trend, now that a pertussis vaccine is available for ages 9 years and over, will be considered by a working party of the Australian Technical Advisory Committee on Immunisation (ATAGI) in 2001.

The record low notification rates for measles and rubella highlight the success of the Measles Control Campaign (MCC),⁴⁶ and the current vaccination program. The MCC actively targeted pre-school and primary school-aged children and significantly improved their immunity to both measles and rubella.⁴⁷ As a result, in 1999 Australia recorded the lowest ever notification rates for measles and rubella for children in these age groups. Importantly, improved rubella control has also led to the lowest rate of rubella amongst women of childbearing age with only one definite congenital rubella infection reported in 1999.⁴²

With record low rates of measles, most clinically compatible cases are now likely to be due to other viral infections.⁴⁸ Hence, it is imperative that the recommendations for measles surveillance proposed in the National Measles Surveillance Strategy are introduced; laboratory confirmation should be sought on all sporadic clinical notifications and at least 2 cases during an outbreak.⁴⁹

Clusters of measles cases continued to occur in 1999, mostly amongst young adults. Serosurveys have shown that some young adults may have low levels of measles immunity,⁵⁰ as they are too old to have been part of the two-dose MMR vaccination program (introduced in 1994) but have grown up in a period when exposure to wild measles virus was declining. A vaccination initiative to improve MMR coverage in this age group is currently under way.⁵¹ In addition to reducing the incidence of measles, it is hoped that the initiative will impact on the burden of mumps in adults, which has also been increasing in recent years.

Vaccination coverage estimates from the ACIR continued to increase in 1999. During 1999, the impact of the General Practice Immunisation Incentives (GPII) Program, which began in July 1998, would have been expected to improve both reporting of vaccinations to the ACIR and vaccination delivery in general practice. Given the role of general practitioners as the largest single group of immunisation providers nationally, the GPII Program together with linking maternity and child-care allowance payments to vaccination uptake are expected to lead to continued improvement in vaccination coverage at 12 and 24 months. Improvements in the accuracy and timeliness of ACIR data compared to earlier years⁵² should enable their use in documenting the vaccination status of notified cases of vaccine preventable diseases. This, together with continued efforts to improve the quality of NNDSS surveillance data will be important components of enhanced surveillance in the future.


This article was published in Communicable Diseases Intelligence Volume 25, No 4, November 2001.