Pneumococcal Infections and Vaccines
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Direct answer
Pneumococcal infections are caused by Streptococcus pneumoniae, an encapsulated, optochin-sensitive diplococcus that causes pneumonia, otitis media, sinusitis and meningitis. Invasive disease is most likely in asplenia, HIV infection, chronic kidney and liver disease, and in older adults. Conjugate vaccines such as PCV13 and PCV20 are the main prevention, with PPSV23 added in selected adults for wider serotype coverage.
What you must remember
- Morphology: lancet-shaped gram-positive diplococci, alpha-haemolytic, optochin-sensitive and bile-soluble, with a polysaccharide capsule that defines more than 90 serotypes.
- Conjugate versus polysaccharide: PCV links capsular polysaccharide to a protein carrier, producing T-cell dependent memory, while PPSV23 is a polysaccharide vaccine with weaker and shorter immunity.
- Asplenia: functional or anatomical asplenia, including sickle cell disease, raises the risk of fulminant pneumococcal sepsis and calls for vaccination and penicillin prophylaxis in children.
- HIV: the risk of invasive pneumococcal disease rises as CD4 counts fall, which is why vaccination is part of routine HIV care.
- Adult sequencing: a conjugate vaccine is given first to immunocompromised adults; PCV20 can complete the series alone, whereas PCV13 or PCV15 is followed by PPSV23 after an interval set by current schedules.
- Meningitis: pneumococcus causes the highest mortality among bacterial meningitis, and dexamethasone is given with or before the first antibiotic dose.
- Indian programme: PCV is part of the Universal Immunisation Programme, given as a primary infant series with a booster, as set out in national guidance.
Conjugate and polysaccharide vaccines compared
The two vaccine families are not interchangeable, and the difference explains most schedule questions. Exam stems usually give an age or a risk condition, and that clue decides which product applies. The table compares them on the features that exam stems test.
| Feature | PCV (conjugate) | PPSV23 (polysaccharide) |
|---|---|---|
| Antigen | Capsular polysaccharide linked to a carrier protein | Purified capsular polysaccharides |
| Response in infants | Effective from early infancy | Poor under two years |
| Immune memory | Good, T-cell dependent | Weak, T-cell independent |
| Serotype breadth | Fewer serotypes, covering most invasive types | Wider, 23 serotypes |
| Role | Primary immunisation, including in children | Added to extend serotype coverage in adults |
Conjugate vaccines are the foundation of prevention because they produce memory and work in infants. PPSV23 widens serotype coverage in adults at risk, and the order of doses follows the schedule for the patient's risk group. Immunocompromised adults, such as those with HIV or asplenia, are the group where the sequence matters most.
Traps in pneumococcal stems
Most errors come from picking the wrong vaccine for the age group or missing the risk group.
- Trap: PPSV23 alone is given to a child under two years to prevent invasive disease. Correct: infants respond to conjugate vaccines, not to the polysaccharide vaccine, so the PCV schedule is used for young children.
- Trap: an asplenic patient receives antibiotics for pneumonia but no vaccines. Correct: asplenic patients need pneumococcal vaccination along with meningococcal and Haemophilus vaccines, and penicillin prophylaxis is standard in children.
- Trap: dexamethasone is withheld in pneumococcal meningitis for fear of infection spread. Correct: dexamethasone given with or before the first antibiotic dose reduces neurological sequelae, a classic exam answer.
Frequently asked questions
Which vaccine is given first to adults at risk of pneumococcal disease?
A conjugate vaccine such as PCV13 or PCV20 is given first to adults at risk of pneumococcal disease, because it produces stronger memory responses. PPSV23 follows PCV13 or PCV15 where extra serotype coverage is needed, whereas PCV20 can complete the series alone, following the current schedule.
Why is PPSV23 not given to children under two years?
PPSV23 produces weak and short-lived responses in children under two years, because their immune system responds poorly to polysaccharide antigens. Conjugate vaccines are the appropriate choice for infants.
Which patients with asplenia need pneumococcal vaccination?
All patients with functional or anatomical asplenia, including those with sickle cell disease, need pneumococcal vaccination. Penicillin prophylaxis is standard in children, and vaccines are given before planned splenectomy where timing allows.
Which antibiotics are used for pneumococcal meningitis?
Ceftriaxone is the usual empirical antibiotic for pneumococcal meningitis, with vancomycin added when penicillin resistance is possible. Dexamethasone is given with or before the first antibiotic dose to lower the risk of neurological sequelae.
What does the optochin test show in Streptococcus pneumoniae?
Optochin sensitivity is the classic laboratory test for S. pneumoniae, because the organism is inhibited by an optochin disc on blood agar. Bile solubility gives a confirmatory result, since pneumococci lyse in bile salts.