ASPEN, COLO. — Although the conjugate heptavalent pneumococcal vaccine has decreased penicillin resistance rates in the vaccine's bacterial serotypes, there is some evidence replacement serotypes are emerging, according to a presentation at a conference on pediatric infectious diseases sponsored by Children's Hospital, Denver.
And with those replacement serotypes, penicillin resistance may again be on the increase, warned Dr. Sheldon Kaplan, chief of the infectious disease service at Texas Children's Hospital, Houston.
Serotypes 15 and 33 seem to be the more common serotypes in the vaccine decline, he said.
According to a pneumococci surveillance project of eight children's hospitals, there was a mean of five cases of invasive disease caused by serotype 15 in 1994–2000. In 2002, there were 14 cases.
For serotype 33, the mean number of cases was less than one during the 1994–2000 period. In 2002, there were nine cases, said Dr. Kaplan, whose hospital is part of the surveillance project (Pediatrics 2004;113:443–9).
Specific isolates of serotype 15 collected at the hospitals participating in the project have the same blot pattern on a pulse-field electrophoresis gel about 60% of the time. That suggests that different isolates taken from various children are the same clone of the bacteria.
About 80% of the serotype 33 isolates seem to be the same clone.
Serotype 19A also seems to be on the increase and seems specifically to be replacing 19F, a serotype in the vaccine. According to one report, the annual incidence of invasive disease in children less than 2 years of age caused by serotype 19A has increased from 1 case per 100,000 population in 2001 to more than 6 cases per 100,000 in 2004 (J. Infect. Dis. 2005;192:1988–95). There also has been a 2.5-fold increase in cases in children older than 5 years of age.
“We're not the only people who are seeing this,” Dr. Kaplan said. “The CDC is actually reporting increases in these serotypes as well.”
Data from a number of surveys have suggested there has been a decrease in antibiotic resistance since the introduction of the conjugate vaccine, but, said Dr. Kaplan, that may no longer be true.
The rate of penicillin nonsusceptible infections may actually be increasing again. Although the number of cases caused by serotypes in the vaccine has declined, the number of cases caused by serotypes not in the vaccine has increased, and it is those serotypes that seem to be acquiring more resistance.
The incidence of invasive disease caused by penicillin nonsusceptible pneumococci in children younger than 2 years has increased since 2002. And, considering only isolates that are not in the vaccine, it has increased from 51% in 1999 to 68% in 2004, Dr. Kaplan said.
“It looks like these nonvaccine serotypes are more likely to be penicillin nonsusceptible today than they were 5 years ago,” he said.
A group from Salt Lake City has seen an increase in pediatric cases of pneumococcal pneumonia complicated with empyema since the introduction of the vaccine. Moreover, the serotypes associated with these cases tend to be serotypes 1, 3, and 19A, which are not in the vaccine.
The Salt Lake City group reported that for the 4 years before the vaccine was introduced, the medical center saw an average of 38 cases of empyema, compared with an average of 72 cases in the first 4 years after its introduction.
Also, pneumococcal parapneumonic empyema represented only 17% of the cases of identified invasive pneumococcal disease seen at that center in the years before the vaccine was introduced, but 32% of the cases afterward (Pediatr. Infect. Dis. J. 2006;25:250–4). Serotype 1 was the most common serotype associated with the empyema both before (46%) and after (34%) the vaccine was introduced. Serotypes 3 and 19A became common after the vaccine (20% and 14%, respectively).
“I can't explain this, but they are seeing more cases with more nonvaccine types,” Dr. Kaplan said, noting that the vaccine may have to be updated with at least some of these emerging strains. “How we will address that down the road will have to be seen. It is an expensive vaccine.”
'We're not the only people who are seeing this. The CDC is reporting increases in these serotypes as well.' DR. KAPLAN