The effectiveness of pneumococcal polysaccharide vaccines in adults: a systematic review of observational studies and comparison with results from randomised controlled trials. Conaty S, Watson L, Dinnes J and Waugh N. Vaccine 2004;22(23-24):3214-3224.
Explanatory note: Streptococcus pneumoniae is a group of 90 serogroups of bacteria—also known as pneumococci—which inhabit the respiratory tract and occasionally cause serious infections in all age groups; serious infections are more common in infants, toddlers, the immune compromised, and the elderly.
The pneumococcal polysaccharide (PS) vaccine used in older children and adults was first licensed in 1977, and now contains 23 types of pneumococcal PS (PPS-23), targeting about 85 to 90% of the types of the pneumococci that cause serious infections in the older age groups.
A different type of pneumococcal vaccine—containing 7 serogroups of pneumococcal PS (PCV7)—given to young children has proven effective at reducing invasive pneumococcal diseases due to those serogroups in the vaccine, including pneumonia, in children as well as in older individuals who have not received the vaccine. That vaccine is not currently licensed for use in older children, individuals with chronic health problems, or the elderly.
Estimating the effectiveness of the PPS-23 vaccine in elderly populations is difficult because the studies done before the vaccine was licensed focused on very vulnerable populations and identified specific endpoints—such as well defined pneumococcal pneumonia. In clinical practice, there are other causes of pneumonia in addition to S. pneumoniae which could mask a true effect of the vaccine on pneumococcal pneumonia. Invasive pneumococcal disease is easier to establish definitively, but is a much less common clinical entity than is pneumococcal pneumonia in the elderly.
What has been the effectiveness of PPS-23 vaccines in preventing invasive pneumococcal disease and pneumonia from all causes among adults more than 65 years of age or with chronic health problems?
Researchers reviewed 19 observational studies—that is, studies on some individual who had received the PPS-23 vaccine and some others who were selected because they had not—on the effectiveness of PPS-23 vaccine published in the refereed literature. They assessed the quality of the studies and extracted data on vaccine effectiveness. They then compared the results of this review with a previous review of 9 prospective randomized controlled trials that had used the same vaccine.
Of the 19 observational studies, 13 studies evaluated the vaccine’s efficacy for protection against invasive pneumococcal disease; 5 evaluated protection against all types of pneumonia. One of these latter studies, however, was excluded from the final analysis because the subjects were also immunized against influenza (which also causes pneumonia in this age group) along with the PPS-23.
This review estimated that the efficacy of the PPS-23 vaccine for protection against invasive pneumococcal disease in adults more than 65 years of age and/or with chronic health problems was 53-55%. Although these studies varied greatly in design and quality, the results were consistent and homogenous. The prior randomized clinical trials estimated that PPS-23 was about 38% efficacious against invasive pneumococcal disease.
Analyses of these observational studies were unable to consistently demonstrate an effectiveness of PPS-23 vaccine against pneumonia due to all causes.
Despite many methodological differences, this review found that these observational studies consistently demonstrated an efficacy of approximately 50% for protection of those more than 65 years of age and/or with chronic health problems for invasive pneumococcal disease. However, this analysis was unable to demonstrate an effect of PPS-27 on the prevention of all cause pneumonia.
These types of studies are subject to a number of limitations, including the fact that negative findings may not be published, selection bias because healthier people may select to take the vaccine, as well as other types of bias that might cause an over estimate of the effectiveness of the vaccine against invasive pneumococcal disease. Similarly, the evaluation of vaccine effectiveness against all causes of pneumonia might cause an actual effect against pneumococcal pneumonia to not be detected.
The fact that the observational studies reported here showed a consistent effect of PPS-23 against invasive pneumococcal disease—despite major differences in study design—suggests that this vaccine may be effective in this vulnerable group. The absence of a measurable effect on all cause pneumonia in these types of studies is not surprising.