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Feature Article

Pneumococcal disease and vaccination recommendations. The state of play

Sanjay Jayasinghe
OPEN ACCESS

Figures

© DR_MICROBE/ISTOCKPHOTO.COM
© DR_MICROBE/ISTOCKPHOTO.COM

Abstract

Overall large reductions in cases of the severe form of pneumococcal disease have been achieved with the pneumococcal vaccination program targeting all infants and older adults and individuals with risk conditions. However, uptake of vaccination recommendations targeting groups with risk conditions and Indigenous adults is suboptimal, and currently a disproportionate burden of pneumococcal disease is borne by these people. Ensuring these individuals receive the full schedule of recommended vaccine doses on time is crucial.

Key Points

  • Pneumococcal disease is a collection of clinical manifestations that includes life-threatening meningitis, pneumonia and septicaemia caused by Streptococcus pneumoniae.
  • 13-valent pneumococcal conjugate vaccine (13vPCV; Prevenar 13) and 23-valent pneumococcal polysaccharide vaccine (23vPPV; Pneumovax 23) are offered under the National Immunisation Program (NIP) and recommendations for their use differ by target population.
  • The recent change in the recommended schedule for 13vPCV for infants to include a 12-month age booster dose after primary doses at age 2 and 4 months is expected to prolong protection for vaccinated children as well as improve herd benefit for others.
  • For individuals with underlying medical conditions that increase their risk of pneumococcal infection there are targeted vaccine recommendations and vaccine providers need to ensure that all such people are identified and given the full course of the recommended pneumococcal vaccines on time.
  • A dose of 23vPPV is recommended and fully funded under the NIP for Indigenous adults at 50 years of age and non-Indigenous adults at 65 years of age.

Pneumococcal disease is a collection of clinical manifestations caused by Streptococcus pneumoniae (also called pneumococcus). In studies of the global disease burden of pneumococcal disease in children published in both 2009 and 2018, about 11% of all deaths among children under 5 years of age were reported to be attributable to pneumococcal infection.1,2 Invasive pneumococcal disease (IPD) is the severe end of the pneumococcal disease spectrum. In IPD, S. pneumoniae is detected in normally sterile sites such as blood and cerebrospinal, pleural, pericardial, peritoneal or joint fluid.3 IPD causes significant mortality and morbidity in children, particularly among young infants. In developed countries, IPD commonly (about 70% of cases) presents in children as bacteraemia with no identifiable specific focus of infection.4,5Among adults, the most common presentation of IPD is bacteraemic pneumonia.6,7 Noninvasive pneumococcal disease, which is localised mucosal infections of S. pneumoniae, is generally less serious and more common than IPD. Among pneumococcal disease manifestations that are noninvasive, acute otitis media is the most common in children.8 Also, most cases of community-acquired pneumonia (CAP) caused by pneumococci among adults are noninvasive.9

Bacteriology of pneumococcal disease

S. pneumoniae is an encapsulated Gram-positive coccus. The polysaccharide capsule is the important virulence factor.3,10,11 Currently, about 97 pneumococcal serotypes belonging to about 40 serogroups have been identified.12,13 Serotypes differ in the chemical composition of their polysaccharide capsules and are therefore immunologically distinct.14,15 In most cases, S. pneumoniae resides in the nasopharynx leading to stable asymptomatic colonisation (carriage), which is a precursor to disease and plays an important role in horizontal transmission between individuals.16 High pneumococcal carriage seen in young children acts as the main reservoir for disease in older adults.16,17 Pneumococcal serotypes vary in their tendency to cause asymptomatic carriage or disease, and a limited number of serotypes are responsible for pneumococcal disease.14,16,18-20 Vaccines target the serotypes that commonly cause disease.

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Pneumococcal vaccines available in Australia

Two types of pneumococcal vaccines  have been developed and used against pneumococcal disease: pneumococcal polysaccharide vaccines (PPVs) and pneumococcal conjugate vaccines (PCVs).21-23 PCVs comprise a selected number of pneumococcal polysaccharides conjugated to a protein carrier.24,25 PPVs generate protective antibodies against pneumococcal disease without involving T cells, which are required for long-term immune memory.13 As a result, immunity triggered by PPVs is relatively short lived, and they are less immunogenic in children under 2 years of age. The covalent coupling to the protein carrier in PCVs converts the pneumococcus polysaccharide to a T cell-dependent antigen, thereby inducing immune memory and enhancing the antibody response.21 PCVs are therefore able to elicit robust, high-quality immune responses sufficient to prevent pneumococcal disease even in very young infants.

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Figures

© DR_MICROBE/ISTOCKPHOTO.COM
© DR_MICROBE/ISTOCKPHOTO.COM
Dr Jayasinghe is a Medical Epidemiologist and Research Fellow at the National Centre for Immunisation Research and Surveillance, Sydney, NSW.