From April 3 to April 24, 2001, nine cases of pneumonia occurred
in elderly residents (median age of 86 years) living at a long-term
care facility in New Jersey. Seven of the nine patients had
Streptococcus pneumoniae isolated from blood cultures, with
capsular serotyping revealing that all isolates were serotype 14
and of the same clonal group. Seven of the nine patients also lived
in the same wing of the nursing home. The two patients that were
culture negative did contain gram-positive diplococci in their
sputum and had chest X-rays consistent with pneumonia.
Epidemiological studies of the patients and controls revealed that
all who developed pneumonia had no documented record of vaccination
with the pneumococcal polysaccharide vaccine (PPV). In contrast,
about 50% of the controls were vaccinated with PPV. Even though
other risk factors were assessed, the lack of vaccination with PPV
was the only one strongly associated with illness. Unfortunately,
despite treatment, four of the nine patients with pneumonia
died.

Once the outbreak was recognized, PPV was offered to those 55
residents who had not yet been vaccinated: 37 of these were
vaccinated, whereas the other 18 were either ineligible or refused
the vaccine. Other control measures included refusal to admit
patients without a history of PPV vaccine.

–           What
special advantage does the capsule confer on the pathogen
Streptococcus pneumoniae?

–           Why are
those who have been vaccinated against Streptococcus pneumoniae
more resistant to infection by this agent?