Dr. Robert Cohen is a distinguished Professor of Pediatrics and Pediatric Infectious Diseases specialist at the Intercommunal Hospital of Créteil, France. A graduate of the University of Paris VII Faculty of Medicine Lariboisière, Prof. Cohen has built an exceptional career dedicated to advancing the understanding, prevention, and treatment of communauty acquired infectious diseases in children. He currently is ambulatory pediatric in Saint-Maur, near Paris.
A recognized leader in pediatric infectious diseases, Prof. Cohen serves as President of the French Group of Pediatric Infectious Diseases (GPIP), President of the Conseil National Professionnel de Pédiatrie, and Scientific Director of the Association Clinique et Thérapeutique Infantile du Val-de-Marne (ACTIV), a renowned institute for research on community-acquired infections in children. He also directs InfoVac-France, a national vaccine information network for healthcare professionals, and acts as an expert for both the French and European medicinal agencies.
Prof. Cohen’s scientific leadership extends across Europe. He is a former board member of the European Society of Pediatric Infectious Diseases (ESPID) and served as President of the ESPID Congress in Nice in 2010. His work has earned him international recognition, including the prestigious Bill Marshall Lecture Award from ESPID in 2020.
An accomplished researcher, Prof. Cohen has authored more than 1000 scientific publications, including over 450 in English, in the field of pediatric infectious diseases. His research focuses on the epidemiology and clinical management of acute otitis media, group A streptococcal pharyngitis, rhinosinusitis, meningitis, and vaccine development, as well as on host-pathogen interactions in pediatric infections. His studies have significantly influenced pediatric vaccination strategies and infectious disease management across Europe.
Beyond his prolific academic output, Prof. Cohen serves on editorial boards and as a peer reviewer for numerous national and international medical journals. His ongoing commitment to research, education, and public health policy has made him one of Europe’s foremost voices in pediatric infectious diseases.
Through his multifaceted roles as clinician, researcher, and leader, Prof. Robert Cohen continues to advance the frontiers of pediatric infectious disease care and to mentor the next generation of pediatricians in France and beyond.
First steps
Prof. Cohen was the first French pediatric infectious diseases specialist to work in a microbiology laboratory that also served as the National Reference Center for pneumococci. In the late 1990s, pneumococcal resistance became a major concern, particularly resistance to beta-lactams, with high minimum inhibitory concentrations (MICs).
He and his team began conducting studies on the impact of antibiotics on the nasopharyngeal flora and particularly on pneumococcal resistance. However, when the first pneumococcal protein-conjugate vaccine (PCV) became available, they shifted the focus of their research to examining the vaccine’s effect on the nasopharyngeal flora, particularly pneumococcus and vaccine-serotypes.
They then went on to conduct large-scale studies on meningitis in France, establishing a network that included approximately 70% of pediatric wards nationwide, covering nearly two-thirds of all pneumococcal meningitis cases.
Prof. Cohen believes that the success of the research program lies in a combination of imagination, scientific rigor, strong relationships leading to large collaboration.
ACTIV
As the Scientific Director of ACTIV, an independent non-profit research organization, Prof. Cohen collaborates on numerous studies with a broad range of sponsors, including industry partners as well as national and European health authorities. He and his team manage approximately 40 to 70 studies each year, with a primary focus on community-acquired infections. This year, the main research priorities center on pneumococcus and Respiratory Syncytial Virus (RSV).
He emphasizes the importance of studying these two pathogens, noting that RSV is associated with approximately 20% of all infections. Collaborating closely with national reference centers for major pathogens, his team collects and analyzes both bacterial and viral samples. For example, in cases of otitis media, they have determined that RSV is the most common viral agent, while non-typable Haemophilus influenzae is the leading bacterial cause.
As an expert in nasopharyngeal colonization, pneumococcal conjugate vaccines (PCVs), and otitis media, Prof. Cohen has closely observed significant shifts in etiology, clinical presentation, and epidemiology, driven largely by advances in PCV implementation.
“I think it is important to consider this point: if you take samples from healthy children, the pneumococcal carriage rate is approximately 30%. [But], if you work on children with acute otitis media, the carriage rate [climbs to] 60 to 70%,” he says. “So to detect pneumococcus efficiently, it is better to focus on fewer samples from patients where carriage is more frequent.”
One of Prof. Cohen’s distinguishing approaches is his focus on patients at high risk for pneumococcal carriage, enabling a more accurate evaluation of antibiotic or vaccine effects. His extensive clinical network supports this strategy: around 80% of the investigators involved are clinicians and pediatricians, while 20% are microbiologists, ensuring optimal diagnostic precision.
From PCV7 to PCV21
According to Prof. Cohen, after the introduction of PCV7, only one serotype among those included in the vaccine continued to be carried: serotype 19F. When PCV13 was later introduced, noting that France did not implement PCV10, nearly all PCV13-covered serotypes disappeared, with the exception of serotypes 3 and 19A. Over time, serotypes 19F, 19A, and 3 continued to be associated with invasive disease in older children and adults, as their carriage did not fully disappear.
This suggests that when a serotype remains carried in children, it may still cause disease in non-vaccinated groups, including adults. Therefore, while PCVs have had a significant positive effect on reducing nasopharyngeal carriage, this impact has not been uniform across all serotypes. For this reason, it is crucial to monitor which serotypes emerge in the nasopharyngeal flora of children when designing new vaccines.
These emerging serotypes may become responsible for infections not only in unvaccinated populations but also in adults. Monitoring this evolution can inform serotype selection for next-generation vaccines while also helping detect newly appearing serotypes or variants within existing serotypes. With the progressive rollout of PCV15, PCV20, and soon PCV21, and even more extensive formulations expected within a few years, establishing robust baseline data is essential to understand epidemiologic shifts.
“Their surveillance system offers a unique opportunity, covering meningitis, pneumonia, bacteremia, and otitis media. This allows the team to evaluate and compare the disease potential of each strain within the same age groups,” Prof. Cohen emphasizes. “If a serotype is carried, what is its probability of causing disease?”
Many experts now agree that determining which serotypes to include in future vaccines should consider not only their frequency in invasive pneumococcal disease (IPD) or carriage, but also their capacity to cause severe disease.
After 25 years of PCV use, it is clear that nasopharyngeal carriage of pneumococcus will continue, but the goal is to favor carriage by strains with lower pathogenicity, less likely to cause meningitis or other forms of IPD. Understanding the relationship between carriage and disease is therefore key to anticipating epidemiological trends and evaluating the ongoing impact of PCVs.
Another essential consideration is that increasing the number of serotypes in a PCV formulation often reduces immunogenicity per serotype. Currently, vaccines are assessed primarily based on immunogenicity, but evidence shows a trade-off:
- PCV10 is less immunogenic than PCV7
- PCV13 is less immunogenic than PCV7
- PCV15 shows reduced immunogenicity for several serotypes compared with PCV13
- PCV20 demonstrates further reduced immunogenicity
The balance between more serotype and less immunogenicity for serotype included in the vaccine is very important to consider. And the only fashion, the only manner to understand is to have very quickly the impact on invasive disease, invasive disease certainly, but also in the rhinopharyngeal flora. And the assessment by FDA or EMA at the beginning is only immunogenicity.
Thus, balancing broader serotype coverage with maintained immunogenicity is critical. While regulatory evaluations by agencies such as the FDA and EMA begin with immunogenicity data, immunogenicity levels for individual serotypes are not yet fully defined. Early immunogenicity data remain essential, but rapid real-world evaluation of impact on both IPD and nasopharyngeal carriage is equally necessary.
Prof. Cohen concludes that having a surveillance system that simultaneously tracks IPD and nasopharyngeal flora provides a crucial advantage, ensuring that when new vaccines are introduced, pre-existing baseline data enable accurate interpretation of epidemiological changes.
Newly added serotypes for vaccines
Since the conjugation process between polysaccharides and carrier proteins becomes less efficient as the number of serotypes increases, leading to reduced immunogenicity, and because pneumococcal conjugate vaccine (PCV) development is largely guided by correlates of protection, an important question arises: Are PCVs still the best long-term strategy for pneumococcal vaccination, or should alternative approaches be pursued, such as improved conjugation methods or protein-based vaccines?
One potential strategy for future vaccines is to use different carrier proteins tailored to specific serotypes, including pneumococcal protein carriers. Prof. Cohen anticipates that within the coming years, vaccines may include multiple carrier proteins without compromising immunogenicity.
Another challenge relates to the pneumococcus’s ability to undergo genetic modifications. Protection is primarily conferred against the capsular polysaccharide of each serotype, but genetic changes within a serotype may alter disease potential. Additionally, capsular switching can occur when the genetic background of the bacterium changes while maintaining the same capsule, or vice versa.
Prof. Cohen emphasizes that pneumococcal vaccines are among the most complex pediatric vaccines, largely because epidemiological dynamics are unpredictable. Continuous surveillance is therefore critical, as trends vary significantly between countries, primarily due to differences in vaccine programs and serotype distribution.
Antibiotic resistance
Group A streptococcus (GAS) and pharyngitis remain leading drivers of antibiotic use, making the development of a vaccine a significant priority. GAS can cause a wide spectrum of illnesses, from pharyngitis and impetigo to severe invasive infections and rheumatic heart disease. With antibiotic resistance continuing to be a major concern, the two primary strategies to reduce antibiotic use are immunization and accurate diagnosis.
“And one way to reduce antibiotic prescription is to have a vaccine against the main bacterial species. For this reason, I believe group B streptococcus and group A streptococcus vaccines are very important to consider. But, if you look at the portfolios of all the big manufacturers of vaccines, GAS is not at the top,” Prof. Cohen notes, adding that the introduction of PCV7 and PCV13 led to a 20–25% reduction in antibiotic use among young children.
Prof. Cohen explains that traditional large-scale clinical trials, once used for meningococcal C and early pneumococcal vaccines, have become increasingly difficult or even unethical due to low disease incidence and the need to avoid leaving control groups unvaccinated. Consequently, regulatory agencies now rely more heavily on immunological surrogate markers rather than direct clinical efficacy trials.
“However, this is complicated for pneumococcal vaccines because there are around 100 serotypes, each potentially requiring different protective antibody levels. However, if you considere a period of 10 years and one region or country 30 to 40 serotypes are responsible of more than 95% of IPD cases. While surrogate markers are generally useful, they are imprecise for certain serotypes, and unexpected variations in vaccine effectiveness, for example serotypes 19A, 19F, and 3, remain difficult to fully explain,” he adds.
Prof. Cohen emphasizes that vaccine safety, demonstrated immune response, and real-world effectiveness data collected shortly after implementation are essential for regulatory decision-making. He highlights that protection before the booster dose depends not only on the number of doses but also on the age at which vaccination begins, as even a one-month difference in infancy can significantly affect immune response. After booster doses, protection becomes robust, and with high vaccine coverage, herd immunity benefits the broader population.
“Given the complexity of introducing new vaccines, the ability to [introduce and] implement them depends largely on [our ability to] design vaccination schedules with the fewest doses possible, guided by immunogenicity data. For example, the optimal pneumococcal schedule is at three, five, and twelve months of age,” he explains.
Overall, Prof. Cohen suggests that policy frameworks should support reliance on surrogate immune markers, rapid post-implementation monitoring, and careful consideration of vaccination schedules to optimize vaccine performance and enhance disease control, including reductions in antimicrobial resistance.
Immunity debt
Almost 99% of children are infected with RSV, rhinovirus, and influenza before the age of two. Prof. Cohen observed that, in the months following the onset of the COVID-19 pandemic, there was a dramatic reduction in the incidence of these infections. However, once non-pharmaceutical interventions (NPIs) were lifted, a rebound of several pathogens occurred in infants, including during periods that are normally considered ‘out-of-season.’
“While the term of ‘immunity debt’ remains debated, the resulting epidemiological impact with dramatic decline and rebumd is undeniable,” he shares. “And there are only two ways to [achieve] immunity: [through] natural immunization or vaccination. There is no other option.” He adds that immunity debt is not only pathogen-specific but also reflects interactions between multiple pathogens circulating simultaneously.
Prof. Cohen notes that some vaccines do not prevent carriage, they protect against disease and its consequences.
“For example, the pertussis vaccine is critically important because infection during the first month of life can be extremely dangerous. Currently, acellular pertussis vaccines provide no adequate mucosal immunity and therefore prevent disease but do not prevent bacterial carriage,” he concludes.
