Acute otitis media (AOM) is caused by Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. Since the introduction of pneumococcal conjugate vaccines (PCVs) shifts in the proportion of these three bacteria as causes of AOM and their antibiotic susceptibility profiles and strain diversity have occurred due to multiple factors including the PCVs and antibiotic selection pressure.
The 7-valent PCV (PCV7) was introduced in 2000 and was proven to be efficacious in preventing AOM, but no subsequent PCV has received an indication for prevention of AOM because the FDA required a tympanocentesis study to prove efficacy and that approval was not achieved for PCV13, PCV15, or PCV20. This is a little known fact. After introduction of PCV7, replacement pneumococcal strains expressing serotypes not in PCV7 emerged and antibiotic non-susceptible strains became predominant causes of AOM, especially antibiotic-resistant serotype 19A. To address the phenomena of pneumococcal serotype replacement, PCV13 was introduced in 2010. But serotype replacement continued to occur under PCV13 pressure, replacement serotypes increasingly caused AOM, and antibiotic-resistant serotype 35B emerged. Now we have two new higher valency PCVs: PCV15 (Merck) where serotypes 22F and 33F were added to the PCV13 serotypes and PCV20 (Pfizer) where 22F, 33F, 8, 10A, 11A, 12F, 15B were added to PCV13. Note that neither PCV15 nor PCV20 includes the most common serotype causing AOM – serotype 35B.1
While PCV15 and PCV20 should provide protection against more pneumococcal serotypes, increasing serotypes in both vaccines decreased immunogenicity of certain shared serotypes, more so with the addition of seven more in PCV20 than two more in PCV15, compared with PCV13. Whether lower antibody concentrations will make a difference clinically in terms of vaccine failure to prevent nasopharyngeal colonization, AOM, and/or invasive pneumococcal infections is currently unknown.
Our group from greater Rochester, New York, is the only one in the United States performing tympanocentesis to determine the etiology of AOM infections. Children between ages 6 and 36 months are studied. We recently reported our results for the time span September 2021 to September 2023, the immediate 2 years prior to recommendations for use of PCV15 and PCV20 in young children.2 Tympanocentesis was performed in 139 (78%) of 179 episodes of AOM, yielding 216 middle ear fluid samples (the higher number of middle ear fluids was due to bilateral tympanocentesis in some children). H. influenzae (40%) was the most common bacterial isolate, followed by S. pneumonia (19%) and M. catarrhalis (17%), with the remainder no growth. Polymerase chain reactions (PCR) was positive in many of those culture negative samples, suggesting prior use of antibiotics before tympanocentesis was performed. Among the pneumococcal isolates, 46% were oxacillin non-susceptible. Among the H. influenzae isolates, 27% were beta-lactamase producing and all M. catarrhalis were beta-lactamase-producing.
As we previously reported,1 we once again found that serotype 35B was the most frequent non-PCV15, non-PCV20, serotype. Other frequently detected non-PCV20 pneumococcal serotypes were 23A, 23B, 35D, 35F and 15C.2
Projected Pneumococcal Serotype Coverage by PCV15 and PCV20
PCV13 serotypes were identified in 9% of middle ear fluids, consistent with vaccine failure. Assuming 100% vaccine-type effectiveness, PCV15 will provide about 11% coverage of pneumococci causing AOM, the same PCV13 and PCV20 will provide 30% coverage, leaving 70% of pneumococci causing AOM in young children uncovered (Figure).
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Thus, the high proportion of pneumococcal serotype 35B and other non-PCV15 or non-PCV20 serotypes will result in a relatively small incremental benefit over PCV13 in young children for AOM.
AOM is the most common cause of pediatric outpatient visits and antibiotic prescriptions in the United States that contributes to selection of antibiotic-resistant microbes.3 The economic burden of AOM is high, estimated at about $3 billion annually in the United States, when direct and indirect costs are calculated,4 thereby making AOM a major factor in calculations of cost effectiveness analyses of PCV immunizations in children.
While PCV15 and PCV20 include common serotypes associated with invasive pneumococcal diseases, their effectiveness in preventing AOM, acute sinusitis, and non-bacteremic community-acquired pneumonia is currently unknown because these vaccines were licensed based on safety and immunogenicity data, not proven efficacy.
The data on antibiotic susceptibility of pneumococci and H. influenza and M. catarrhalis isolated in the late post PCV13 era from young children in a pediatric primary-care setting raise a question about empiric antibiotic choice for AOM today. For penicillin non-susceptible pneumococcal strains, higher dosages of amoxicillin can improve eradication. However, higher dosages of amoxicillin cannot overcome beta-lactamase production by H. influenza and M. catarrhalis. Based on the mix of pathogens causing AOM and the antibiotic susceptibility of those bacteria, high-dose amoxicillin/clavulanate or alternative cephalosporin drugs active against pneumococci and beta-lactamase producing H. influenza and M. catarrhalis would be a better empiric choice over high-dose amoxicillin.
Dr. Michael E. Pichichero
Limitations of our study include that it occurred in one center in New York, although we have previously shown results of tympanocentesis at our center are similar to those in Virginia and Pennsylvania5 and our study population was composed of children living in urban, suburban, and rural households of all economic levels. Because this study was conducted during a relatively short time frame (2021-2023), the numbers of subjects and samples were sometimes insufficient to identify statistically significant differences in some comparisons. Some children were lost to follow-up, and not every participant was consented for tympanocentesis. Some participants received antibiotics prior to middle ear fluid specimen collection.
Dr. Pichichero is a specialist in pediatric infectious diseases, Center for Infectious Diseases and Immunology, and director of the Research Institute, at Rochester (N.Y.) General Hospital. He has no conflicts of interest to declare.
References
1. Kaur R et al. Dynamic Changes in Otopathogens Colonizing the Nasopharynx and Causing Acute Otitis Media in Children After 13-Valent (PCV13) Pneumococcal Conjugate Vaccination During 2015-2019. Eur J Clin Microbiol Infect Dis. 2022 Jan;41(1):37-44. doi: 10.1007/s10096-021-04324-0.
2. Kaur R et al. Anticipated Effects of Higher-valency Pneumococcal Conjugate Vaccines on Colonization and Acute Otitis Media. Pediatr Infect Dis J. 2024 Oct 1;43(10):1004-1010. doi: 10.1097/INF.0000000000004413.
3. King LM et al. Pediatric Outpatient Visits and Antibiotic Use Attributable to Higher Valency Pneumococcal Conjugate Vaccine Serotypes. medRxiv [Preprint]. 2023 Aug 25:2023.08.24.23294570. doi: 10.1101/2023.08.24.23294570.
4. Ahmed S et al. Incremental Health Care Utilization and Costs for Acute Otitis Media in Children. Laryngoscope. 2014 Jan;124(1):301-5. doi: 10.1002/lary.24190.
5. Pichichero ME et al. Pathogens Causing Recurrent and Difficult-to-Treat Acute Otitis Media, 2003-2006. Clin Pediatr (Phila). 2008 Nov;47(9):901-6. doi: 10.1177/0009922808319966.