Authors: John D Mooney (corresponding author) [1,2]; Amanda Weir [1,3]; Jim McMenamin [1]; Lewis D Ritchie [4]; Tatania V Macfarlane [4]; Colin R Simpson [4]; Syed Ahmed [5]; Chris Robertson [1,3]; Stuart C Clarke [6,7]
Background
The current pneumococcal polysaccharide vaccine (23vPPV), which consists of 23 serotype antigens corresponding to over 90% of all invasive disease isolates, has been available since the early 1980's [5]. Its efficacy was first established against pneumococcal pneumonia in randomized controlled trials conducted amongst novice gold-miners in South Africa [6]. The balance of the evidence from a subsequent wealth of prospective and retrospective studies in immuno-competent older adults [7] together with cost effectiveness evaluations in the US [8] and Europe [9] tends to support the targeting of older age groups for vaccination as a worthwhile and cost saving intervention. The most recent Cochrane review also concluded that 23vPPV was effective against IPD although the evidence was not sufficient against pneumonia [10].
In winter 2003/2004, 23vPPV was recommended for all those aged 65 and over in Scotland and promoted in parallel with an influenza vaccination programme for the same age group [11]. This approach was at variance with the phased three year introduction programme for ten year age-bands (beginning with those aged 85 and over) that was adopted in England and Wales and completed in 2005/2006. The experience seen in Scotland may therefore serve as an early indication of the UK wide impact of the programme. Previous to the age targeted campaign, 23vPPV vaccine had only been recommended for all persons over the age of two years who were at increased risk of IPD due to any of the following underlying medical conditions: asplenia or splenic dysfunction; chronic renal disease or nephrotic syndrome; immunosuppression resulting from disease or treatment; chronic heart disease; chronic lung disease; chronic liver disease including cirrhosis and diabetes mellitus [12]. The two principal outcome measures by which the impact of the vaccination campaign was assessed in this evaluation were firstly the extent to which there was a reduction in the expected winter incidence of invasive disease in the target age-groups, since this was the major rationale behind the policy and secondly, the estimated vaccination effectiveness for those age 65 and over. During the time period of this evaluation, 23vPPV was the only population level age-targeted intervention against pneumococcal disease in Scotland since the 7-valent pneumococcal conjugate vaccine (Prevenar) was not introduced into the childhood vaccination programme until September 2006 [13].
Methods
Study design, population and time period
The impact of the pneumococcal vaccination campaign in winter 2003/2004 was evaluated using a retrospective cohort design which looked at vaccination effectiveness and the age-specific incidence of IPD. The principle outcome measures were observed changes in the 2003/2004 winter season incidence rates of IPD in the vaccine targeted population of those aged 65 and over (divided into males and females aged 65-74 and 75 and over). For comparison, the incidence rates of invasive disease in younger age bands (0-4, 5-34, 35-49 and 50-64) were also examined for the same winter season. Figure 1 gives an overview of the study design and data sources.
IPD incidence
Reports of all pneumococcal laboratory isolates from every diagnostic laboratory in Scotland are collated at Health Protection Scotland (HPS). In an ongoing collaboration with the Scottish Meningococcus and Pneumococcus Reference Laboratory (SMPRL), surveillance for invasive pneumococcal disease (IPD) involves obtaining laboratory confirmation, serotype identification and antibiotic resistance profiling for all blood and CSF isolates. There were no changes in culture procedures or in the criteria used by diagnostic laboratories to submit isolates over the period of the study. Laboratory reports of pneumococcal infection for the winter seasons (weeks 40 of preceding year to week 20 of the following year) were extracted from the national HPS database for 1999/2000; 2000/2001; 2001/2002; 2002/2003; 2003/2004. Total IPD isolates (i.e.: blood and CSF) for these time periods were used to derive winter season incidence rates by age-band (0-4, 5-34, 35-49, 50-64, 65-74 and 75+) and sex. For ease of comparison with other published studies, crude annualised incidence rates were estimated by assigning a two-thirds weighting to the winter season total (according to the mean proportion of annual cases which occur during weeks 40 to 20).
Population totals were obtained from the General Register Office for Scotland Statistics Library [14], the midyear estimates preceding each winter season being used as denominators. To verify any continuing trends, incidence rates for winter 2004/2005 were also determined. A Poisson regression model, using the log (population) as an offset variable was used to predict the expected number of cases for winter season 2003/2004, from which were derived standardised incidence ratios (SIR). The Poisson model used data on the incidence rates of the four preceding winter seasons and included the following terms: winter season (continuous variable), age group, sex and the interaction term of sex and age. The 95% confidence interval (CI) for the SIR was calculated using the error factor and a Chi-square test was used to compare numbers of observed and expected cases.
Vaccine uptake
Estimates of 23vPPV vaccine uptake in winter 2003/2004 across the whole population were assessed through a sentinel surveillance network called the continuous morbidity record (CMR), which covers a seven percent representative sample of the Scottish population [15]. In contrast, influenza vaccine uptake data is not …
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