Vaccine preventable diseases

The childhood immunisation schedule remained unchanged from August 1994, (when a fifth dose of pertussis-containing vaccine was substituted for childhood diphtheria tetanus (CDT) vaccine at 4 to 5 years of age) to the final quarter of 1998 when the second dose of MMR vaccine moved from 10 to 16 years of age to school entry (4 to 5 years of age), following the 1998 measles control campaign. This section summarises the national notification data for diseases targeted by the current routine childhood immunisation schedule. Other diseases for which vaccines are available but which are not incorporated in the immunisation schedule (hepatitis A, hepatitis B, pneumococcal disease, influenza, some serotypes of meningococcal disease) and potentially vaccine preventable diseases (varicella, rotavirus) are not discussed here. The National Influenza Surveillance Scheme published a report of the 1997 influenza data in an earlier edition of CDI.²⁷ Coverage estimates at 12 months of age for the vaccines given in the first year of life from the Australian Childhood Immunisation Register (ACIR) are included for the first time.

Diphtheria

There were no cases of diphtheria reported in 1997. The last diphtheria notification was in 1993.

Haemophilus influenzae type b infection

Notifications of Haemophilus influenzae type b (Hib) infection have remained low since 1995. There were 53 cases of Hib infection notified in 1997, of whom 33 (62%) were under 5 years of age. The overall notification rate per 100,000 population was 0.3, compared with 2.6 for the 0 to 4 year age group. The highest notification rate was in children less than 2 years of age, declining to very low rates in children over 4 years of age (Figure 16). These rates are similar to those in 1996, suggesting that the notification rate is now stabilising following the introduction of conjugate Hib vaccines in 1992 (Figure 17).

Figure 16. Notification rate of Haemophilus influenzae type b infection, 1997, by age

Age in years

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Figure 17. Notifications of Haemophilus influenzae type b infection, 1993-1997, by month of onset

Measles

Notifications of measles increased in the second half of 1997 but remained well below the epidemic years of 1993 and 1994. There were 852 cases reported to the NNDSS in 1997, an annual notification rate of 4.6 per 100,000 population. The highest notification rate was in the Australian Capital Territory (25.5 per 100,000) followed by Tasmania (8.0) and Queensland (7.7). Notifications were highest in the months from September to November when outbreaks occurred in Queensland (Far North, Fitzroy and Wide Bay-Burnett), northern New South Wales and the Australian Capital Territory.²⁸ These outbreaks are reflected in the map of measles incidence shown by statistical division (Map 8).

Map 8. Notification rate of measles, 1997, by Statistical Division of residence

As in previous years, the highest notification rate (27.2 per 100,000 population) was for children 0 to 4 years of age. Within this age group, the highest rates of notification were for children less than 2 years of age. However, cases 5 to 29 years of age accounted for 50 per cent of total measles notifications. The male to female ratio was 1:1.1.

Mumps

In 1997 there were 191 cases of mumps reported to the NNDSS, an annual notification rate of 1.0 per 100,000 population. The highest notification rates were in the Northern Territory (5.3 per 100,000), the Australian Capital Territory (2.3 per 100,000) followed by Western Australia (2.0) and South Australia (1.8).

Children less than 10 years of age accounted for 44 per cent of all cases, a similar pattern to 1996. Children 0 to 4 year age group had a notification rate of 2.9 per 100,000 and children 5 to 9 year age group a rate of 3.5 per 100,000. The number of cases was evenly divided between males and females.

Pertussis

1997 was an epidemic year for pertussis, with a record number of 10,668 cases notified, compared with 4,031 in 1996. The national notification rate was 57.6 per 100,000 persons, but with substantial regional variation. The notification rate in South Australia was 114.1 and rates of over 100 per 100,000 were also seen in the south west of Western Australia, the Hunter region of New South Wales and southern Queensland (Map 9). As in previous years the number of notifications peaked in the spring and summer months (Figure 18).

Notification rates more than doubled in all age groups compared with 1996, with the greatest increase in notifications and the highest rates found among school aged children (5 to 14 years) for whom notification rates trebled compared with 1996 (Figure 19). Children in the 5 to 14 year age group accounted for 45 per cent of all notifications. Although notification rates were relatively low in persons aged 20 years and over, this group accounted for 36 per cent of the total notifications. The notification rate for females was greater than males for all age groups, with an overall male to female ratio of 1:1.2.

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Map 9. Notification rate of pertussis, 1997, by Statistical Division of residence

Figure 18. Notifications of pertussis, 1993-1997, by month of onset

Figure 19. Notification rate of pertussis, 1997, by age group and sex

Polio

No cases of polio were reported in 1997. There have been no cases of polio due to the wild type virus since 1978, although vaccine associated cases were reported in 1986 and 1995.²⁹

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Rubella

Rubella notifications have continued to decline from the elevated levels of 1992 to 1995, with 1,446 cases notified in 1997 (Figure 20). Queensland and South Australia reported the highest notification rates of rubella at 16.9 and 13.4 per 100,000 respectively, compared with the overall notification rate of 7.8 per 100,000. Seasonal variation was apparent with a marked increase in cases with onset dates between September and October, consistent with previous years.

As in previous years, the highest notification rate (45.5 per 100,000) was for males in the 15 to 19 year age group, followed by those in the 20 to 24 year age group (24.4 per 100,000) (Figure 21). Young males in the 15 to 24 year age group accounted for 33 per cent of all rubella notifications, and the overall male:female ratio was 1.9:1.

Figure 20. Notifications of rubella, 1993-1997, by month of onset

Figure 21. Notification rate of rubella, 1997, by age group and sex

Tetanus

There were eight notifications of tetanus in 1997 (1 male, 7 females), a four-fold increase from 1996. Four of these notifications were from New South Wales, two were from Queensland, and one each was from Tasmania and Victoria. All cases were aged over 44 years and seven cases were older than 60 years.

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Childhood immunisation coverage reports

The Australian Childhood Immunisation Register commenced operation on 1 January 1996 and records details of vaccination of children up to 7 years of age. Immunisation coverage at 12 months of age is estimated by the cohort method and assumes that all doses have been given if the last in the series is recorded.³⁰ The proportion of Australian children fully immunised with each of the vaccines (DTP, OPV and Hib) listed in the Australian Standard Vaccination Schedule and the proportion immunised with all vaccines at 1 year of age are shown in Table 4.³¹ These figures represent the four 3-month birth cohorts of 1996 who were assessed at 1 year of age in 1997.

Table 4. Proportion of Australian children born in 1996 immunised at one year of age

Birth date
Vaccine group	1/1/96 - 30/3/96	1/4/96 - 30/6/96	1/7/96 - 30/9/96	1/10/96 - 31/12/96
	%	%	%	%
DTP	77.4	78.2	78.9	80.7
OPV	77.2	78.4	78.9	80.7
Hib	77.2	78.4	79.0	80.7
Fully immunised	74.9	75.8	76.7	78.6

Discussion

The striking feature of the 1997 NNDSS data is the very high notification rate for pertussis. This increased notification rate is probably related to both increases in the underlying incidence of pertussis over historical figures and improvements in detection and/or reporting of cases. The highest notification rate was recorded by South Australia where a recent, more detailed analysis suggested that increasing use of diagnostic tests, particularly serology in the older age groups, had contributed to increased detection and reporting of pertussis.³² However, increased testing alone cannot explain these high notification rates, especially amongst infants of less than 1 year of age in whom morbidity is most severe and serology is not diagnostic. This is highlighted by the nine deaths in Australia between October 1996 and November 1997, compared with a total of 21 deaths in the 20 years from 1976 to 1995.³³ There is evidence of the impact of immunisation, with the lowest notification rates in the most highly immunised group in the population (1 to 4 year olds) and a trend towards lower rates in the age group eligible for the fifth dose of DTP at 4 to 5 years of age (5 and 6 year olds).³⁴ Analysis of pertussis notification data to the end of 1998 is planned to confirm this trend, particularly in view of the high proportion of school aged children in the 1997 pertussis notifications.

Following the measles outbreaks in 1993 and 1994 (Figure 22) Australia was in an interepidemic measles trough until a number of outbreaks occurred in late 1997.²⁸ Data suggesting that further major outbreaks were likely in 1998-99 was confirmed by seroepidemiologic data from New South Wales, Victoria and South Australia, showing a high proportion of susceptibles.³⁵

Figure 22. Notifications of measles, 1993-1997, by month of onset

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The likelihood that a measles epidemic, similar to the one which occurred in New Zealand,³⁶ was imminent prompted the establishment of the Measles Elimination Advisory Committee. The Committee recommended a National Measles Control Program consisting of four elements: the Measles Control Campaign, moving the second dose of measles vaccine (MMR2) to age 4 years from age 12 years, increasing uptake of MMR1 and MMR2 to 95 per cent, and improved surveillance. The Measles Control Campaign was primary school-based (1.75 million children, aged 5 to 12 years in 7000 schools) and held between August and November 1998. In addition, the parents of the 160,000 children under the 5 years of age who were eligible for MMR1 vaccination but who, according to the Australian Childhood Immunisation Register, were unimmunised were sent a letter urging them to make sure that their child's MMR immunisation was up to date. More than 1.3 million of the 2 million eligible children had been vaccinated by the end of November 1998, and regular campaign updates have been included in CDI.³⁷

It is expected that this campaign will help interrupt measles transmission, lengthen the interepidemic cycle, and allow time to increase uptake of the 2-dose MMR schedule to at least 95 per cent. However, continuing disease activity may be expected in teenagers and young adults. The importance of enhanced surveillance continues to be emphasised, and each jurisdiction has agreed to the National Surveillance Strategy, which sets out the importance of laboratory confirmation of cases, uniform case definition, collection of minimal data set including fields for vaccination status and laboratory confirmation. It is only by such national cooperation that measles will eventually be eliminated.

Poliomyelitis has been eliminated from Australia and work is continuing toward fulfilling the criteria for the Global Commission for Certification of Poliomyelitis Eradication, which requires proof beyond doubt that polio has been eradicated in Australia.³⁸ No vaccine associated polio has been reported since 1995 and Australia is pursuing enhanced surveillance of acute flaccid paralysis cases to prove that such paralysis is not caused by polio virus.³⁸

Notifications for invasive Hib disease are likely to accurately reflect the true incidence as cases are all diagnosed in hospital laboratories and almost all jurisdictions have compulsory laboratory reporting. Notifications have also decreased in older persons with rates remaining highest in children less than 2 years of age. There is no evidence of an increase in invasive Hib disease in older children who have not been eligible for Hib immunisation.

Like other diseases, notification data for vaccine preventable diseases are sensitive to changes in diagnostic and reporting practices and must be interpreted with caution. Gaps in NNDSS data, including lack of information on the method of diagnosis and vaccination status, have been identified previously.^34,7 Date of birth of cases, rather than age in years, has been recorded nationally since the beginning of 1997, allowing calculation of disease specific notification rates for children under 1 year of age which will be included in the 1998 NNDSS annual report.

Estimates of immunisation coverage at 12 months of age have continued to increase, although the ACIR still under-reports true coverage. Initiatives have been implemented to improve reporting to the ACIR. These include the General Practice Immunisation Incentives and the requirement of ACIR documentation in order to receive childcare assistance, both of which should improve immunisation coverage and reporting to the ACIR. With improvements in both the reporting of immunisation status of cases of vaccine preventable diseases and immunisation status of all children, Australia will be much better placed to monitor the performance of its immunisation program.

This article {extract} was published in Communicable Diseases Intelligence Vol 23 Number , 21 January 1999 and may be downloaded as a full version PDF from the Table of contents page. Volume 23 1999.