The increase in the Incidence of Haemophilus influenzae infection against the Background of Immunization in the Russian Federation in 2019 –2025

Abstract

Background . Haemophilus influenzae infection remains a significant cause of infectious morbidity in the Russian Federation, particularly among the pediatric population. Aim . To characterize the features of the epidemic process of Haemophilus influenzae infection in 2019–2025 and to substantiate directions for optimizing surveillance measures. Materials and Methods. Data were collected using specially designed tables that included information on vaccination history, age structure, clinical forms, social groups of cases, diagnostic methods, and serotyping results. Based on the information obtained, a retrospective epidemiological analysis was performed. The study covered the period from 2019 to 2025. To assess morbidity dynamics and preventive vaccination coverage, monthly and annual statistical reporting Forms No. 2 ("Data on Infectious and Parasitic Diseases") and annual Forms No. 6 ("Data on Populations of Children and Adults Vaccinated Against Infectious Diseases") for the specified period were used [30, 32]. To study population immunity to Haemophilus influenzae infection, serological testing of blood serum samples was performed. Results and Discussion. It was established that, despite achieving a high level of vaccination coverage against Hib infection in the pediatric population (over 95%), a steady increase in morbidity has been observed in 2022–2025: in 2025, it reached 4.25 per 100,000 population, exceeding the long-term average level by a factor of  19.3. A shift in morbidity toward older age groups (50–65 years) was revealed, along with a change in the clinical structure characterized by a predominance of pneumonia (up to 94.5%) and the registration of fatal outcomes among the adult population. At the same time, against the background of routine immunization of children in the first years of life against serotype b, initiated in 2022 within the National Calendar of Preventive Vaccinations, the proportion of purulent bacterial meningitis (PBM), which had previously reached 20% among the clinical forms of Haemophilus influenzae infection, decreased to 1.5% over the past three years. The current surveillance system does not provide for serotyping of H. influenzae strains, which makes it impossible to conduct full-scale epidemiological surveillance of this infection. The results of serological analysis confirm extensive circulation of the pathogen in the population. Conclusion . The increase in the incidence of Haemophilus influenzae infection is presumably attributable to the loss of natural population immunity resulting from social distancing measures during the COVID-19 pandemic; to an increase in registered morbidity due to the introduction of highly sensitive PCR systems; and to the biological process of serotype replacement, whereby non-vaccine H. influenzae strains have come to occupy a dominant position in the etiological structure. At the same time, the high level of coverage and timeliness of vaccination of children in the first years of life against Hib infection made it possible to reduce the proportion of invasive forms of the disease caused by this pathogen. Priority directions for the development of epidemiological surveillance include the introduction of PCR panels for pathogen detection, standardization of serotyping methods, and the establishment of serological monitoring of postvaccination immunity. In addition, it is advisable to consider revaccination against Hib infection for persons in risk groups, including those over 50 years of age.