Invasive pneumococcal disease in non-Indigenous people in north Queensland, 2001–2009

Although pneumococcal vaccines had been available for Indigenous people for several years beforehand, they became freely available for non-Indigenous Australian people in 2005. From the beginning of 2005, the 7-valent pneumococcal conjugate vaccine (7vPCV) became available for non-Indigenous children in a three-dose (at 2, 4 and 6 months of age) primary vaccination schedule, with catch-up vaccination (with fewer doses, depending on age) up to the child’s second birthday.1 7vPCV was already funded for children with certain underlying medical conditions up to 59 months of age, with a “booster” dose of the 23-valent pneumococcal polysaccharide vaccine (23vPPV) at 4–5 years of age.1

At the same time, 23vPPV was made freely available for all non-Indigenous adults ≥ 65 years of age. Although 23vPPV was also recommended for those aged < 65 years with certain underlying diseases and conditions (including tobacco smoking),1 23vPPV is not funded for these “at-risk” non-Indigenous people (unless they are < 5 years of age).

It is now well recognised that 7vPCV has the potential to protect unvaccinated individuals from the serotypes included in the vaccine via herd, or indirect, effects.2 However, the vaccine may also allow the emergence of invasive pneumococcal disease (IPD) caused by non-7vPCV serotypes — so-called replacement disease.2

Our objectives were to examine trends in the epidemiology of IPD in non-Indigenous people in north Queensland between the 4 years before the vaccines were made freely available (2001–2004) and the 4 years (2006–2009) after the transitional introductory year (2005), and to examine the proportion of cases that occurred in 2006–2009 that could be vaccine-preventable, considering current and forthcoming vaccines.

IPD is defined by the isolation of Streptococcus pneumoniae from a usually sterile body site. It is a notifiable disease in Queensland, and diagnostic laboratories are required to notify local public health units of each case. Data on all cases are collected using a standardised questionnaire that records demographic and clinical features, and this information is linked with the pneumococcal vaccination status of each patient and with the infecting pneumococcus serotype.

For IPD in children, the seven serotypes that are included in 7vPCV were defined as “7vPCV serotypes” and the remainder as “non-7vPCV serotypes”. In adults, the seven serotypes common to both vaccines were defined as “7vPCV and 23vPPV common-serotypes”, the other 16 serotypes included in 23vPPV as “23vPPV-only serotypes”, and the remainder as “non-vaccine serotypes”. All invasive serotype 6A pneumococci isolated before mid 2009 were re-examined to determine if any should be reclassified as the more recently recognised serotype 6C; from mid 2009, all serogroup 6 IPD isolates were routinely examined for serotype 6C.3

Average annual IPD incidence rates during the two 4-year periods were calculated using the 2006 experimental estimated resident populations (ERPs) derived by the Queensland Treasury’s Office of Economic and Statistical Research. These ERPs, based on 2006 national census data,4 have been specifically developed to define Queensland health service district populations. The non-Indigenous ERP in north Queensland in 2006 was about 581 850. Confidence intervals for the incidences were calculated from tabulated 95% significance factors.5

Ethics approval was not necessary for this study, as surveillance and notification of IPD are required by law.

There were 216 cases of IPD in non-Indigenous people in north Queensland in 2001–2004, and 142 in 2006–2009. For all ages, the average annual IPD incidence declined significantly after the introduction of the vaccines (P < 0.05), as did the incidence of 7vPCV serotype IPD (P < 0.05) (Box 1).

Australia’s funding of two large population-based pneumococcal vaccination programs simultaneously allows an assessment of the concurrent impacts of the two vaccines. The marked decline (34%) in overall IPD incidence in 2006–2009 can be attributed to pneumococcal vaccination, as it occurred so soon after the introduction of the vaccines.

Particularly impressive was the 77% decline, across all ages, of IPD caused by 7vPCV serotypes. The decline in children was clearly a direct effect of 7vPCV, whereas that in adults was probably an indirect effect of 7vPCV in young children rather than a direct effect of 23vPPV in adults. The latter conclusion is based on the absence of any decline in 23vPPV-only serotype IPD in adults and on the apparently low uptake of 23vPPV in at-risk adults.

This powerful indirect effect of 7vPCV has been documented elsewhere.2,6 Although two young infants developed 7vPCV serotype IPD in 2006–2009, it is likely that many more cases in infants too young to have been fully vaccinated were also prevented via this indirect effect.6,7

Many countries recommend a fourth (booster) dose of 7vPCV in the second year of life.2 As a three-dose primary vaccination schedule is used in Australia, it is reassuring that there was only one (serotype 6B) vaccine failure. Serotype 6B appears to be the most frequently identified serotype in children who represented vaccine failures in the United States.8

The lack of any discernible decline in 23vPPV-only serotype IPD in adults might suggest that the 23vPPV has not had any direct effect. However, although there was virtually no change in 23vPPV-only serotype IPD in adults ≥ 65 years of age, there was a marked increase in adults aged 15–64 years, for whom the vaccine is not freely available. Therefore it is plausible that 23vPPV has had a direct effect in adults aged ≥ 65 years, and without the vaccine this group may also have seen a marked increase in 23vPPV-only serotype IPD.

A marked increase in 23vPPV-only serotype IPD in adults has also been seen elsewhere, but has been documented in both adult age groups in the US.2,7 The most plausible explanation for our observations in north Queensland is increased nasopharyngeal carriage of these serotypes in children given 7vPCV, and increased exposure of adult contacts via these children. The many at-risk adults aged 15–64 years who have not been vaccinated with 23vPPV are susceptible to disease from these serotypes, whereas the vaccine has presumably provided some degree of protection to older adults.

The increase in replacement serotype 19A IPD (now the most common across all age groups) is striking. Although this serotype has emerged markedly elsewhere following the introduction of 7vPCV,2,6 it has not yet shown any apparent increase in IPD in Indigenous people in north Queensland.9 Molecular studies of serotype 19A isolates indicate that the emergence of this serotype in children in the US was a consequence not only of a pre-existing 19A clone becoming more prevalent following the introduction of 7vPCV, but also of the emergence of new 19A clones.10 These new clones have apparently emerged through capsular switching, whereby a non-19A clone acquired the genes expressing the 19A capsule.10 It has been suggested that switching a 7vPCV to a non-7vPCV capsule has enabled some pneumococci to “evade the immune pressures evoked by the vaccine”.11 If this is correct, serotype 19A could become even more dominant in north Queensland, and might eventually gain dominance in Indigenous people in the region.

The coverage of the pneumococcal vaccines in the age groups for whom they are freely provided is not yet known in north Queensland. However, as the coverage of three doses of 7vPCV was 91% by 12 months of age for all children in Queensland in 2007,12 it can be assumed that uptake of three doses in these infants is probably greater than 90%. Our observations of vaccine failure and lack of vaccination in adults aged ≥ 65 with 23vPPV serotype IPD suggest that coverage is not yet optimal and the vaccine has lesser effectiveness in this group, many of whom have significant comorbidities. A recent systematic review acknowledged there was still uncertainty about the effectiveness of the vaccine in adults with chronic illnesses.13 There is also considerable uncertainty about the duration of protection afforded by the vaccine,14 which creates uncertainty about revaccination policies. However, given that 23vPPV may have provided a direct protective effect, further efforts to improve vaccine uptake in adults aged ≥ 65 years is warranted.

The marked increase in 23vPPV-only serotypes in adults aged 15–64 years with IPD is of concern. The uptake of (unfunded) 23vPPV in this age group appears to be very low, even though 85% of those with 23vPPV serotype IPD had an underlying disease or condition for which the vaccine is recommended.1 The most common risk factor was tobacco smoking, which leads to adverse structural and immunological changes within the respiratory tract and also increases the adherence of pneumococci to epithelial surfaces.15 Given that the vaccine is likely to be more effective in many non-Indigenous adults aged 15–64 years (eg, smokers with no other risk conditions) than in older adults, this raises the question of whether 23vPPV should be funded for people in this age group with underlying diseases or conditions. One option could be to lower the age for funded 23vPPV from ≥ 65 to ≥ 50 years of age. However, of the adults aged 15–64 years with 23vPPV serotype IPD in 2006–2009, only 44% were aged 50–64 years (data not shown). Cost–benefit studies that take into account the changing profile of IPD in adults aged 15–64 years should be considered.

Most of the serotyped isolates in 2006–2009 were 23vPPV serotypes. Because there is no recognised indirect protective effect from 23vPPV,14 very high coverage with 23vPPV will be required if further substantial reductions in IPD are to be achieved. However, because 23vPPV is not funded for at-risk adults aged 15–64 years, and the vaccine uptake in adults aged ≥ 65 years seems suboptimal despite free availability, it seems unlikely that high coverage will be readily achieved.

The significant declines in incidence of 7vPCV and 23vPPV common-serotype IPD in both adult age groups are indicative of the powerful impact of the 7vPCV indirect protective effect. It has been estimated that in the US twice as many cases of IPD have been prevented by indirect than by direct 7vPCV effects.2,6 Capitalising on these indirect effects will be essential to gain further reductions in incidence. As more birth cohorts of children receive 7vPCV, the indirect effects may continue to increase. However, only 33 (23%) of the IPD cases that occurred in non-Indigenous people of all ages in 2006–2009 were 7vPCV serotypes, indicating that there will be limitations on how much more can be expected from this vaccine in the future.

A 10vPCV (containing the 7vPCV serotypes and serotypes 1, 5 and 7F) is now available in Australia and has been used in the Northern Territory childhood vaccination schedule since October 2009.16 This vaccine would have the potential to prevent, via both direct and indirect effects, more IPD than 7vPCV, as 46 (32%) of the IPD cases that occurred in all ages in 2006–2009 were serotypes included in this vaccine.

A 13vPCV (containing the 10vPCV serotypes and serotypes 3, 6A and 19A) has recently replaced 7vPCV in the childhood immunisation programs in the United Kingdom17 and the US.18 This vaccine would have the potential to prevent, via both direct and indirect effects, up to 64% of the IPD that occurred in non-Indigenous people in north Queensland in 2006–2009. Serotypes 19A and 3 account for most (88%) of the extra potential benefit of 13vPCV over 10vPCV in preventing IPD in the region.

Continued surveillance will be essential to monitor further trends in IPD in north Queensland and elsewhere. For example, serotype 22F, which is not included in any conjugate vaccine, has emerged as a dominant serotype in adults, both Indigenous9 and non-Indigenous, in the region. This serotype, or other non-vaccine serotypes, could exploit opportunities for serotype replacement should either 10vPCV or 13vPCV be introduced to replace 7vPCV in the childhood vaccination schedule in Queensland.