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Relevance. To achieve the goals of measles and rubella elimination and poliomyelitis eradication programs, immunization coverage of at least 95% of the target population is required. Objective data on the state of specific herd immunity are provided only by the results of serosurveys. In the Republic of Tajikistan, such monitoring is not carried out regularly. Therefore, the purpose of the study was to assess the actual state of the specific herd immunity to measles, rubella, and poliomyelitis viruses. Materials and methods. The blood sera of 563 children and adults collected in 7 cities and 13 districts of Tajikistan in 2020 were investigated. The level of antibodies (ABs) to measles and rubella viruses was determined using enzyme immunoassay. Test systems VectoKor-IgG (VECTOR-BEST, Russia) and Ecolab, Russia were used to determine ABs to measles and rubella virus, respectively. Neutralizing antibodies (nABs) to the 3 types of poliovirus (PV) were determined in 359 sera using a neutralization reaction with Sabin strains of types 1, 2, 3. Results. The conducted serosurvey showed the level of the specific herd immunity to rubella to be 87.9% in total population, including 86.2% in children, 93.1% in adolescents, and 93.5% and adults, that is sufficient to prevent transmission of the rubella virus. The proportion of individuals seropositive to measles was 54.5%, which is not enough to prevent sustained secondary transmission of infection and the resumption of circulation of the endemic strain of measles virus. The children under 15 years of age should be considered a population at risk of the infection, since children accounted for 38% among seronegative individuals. In general, less than 95% of the examined patients had nABs to PV: 94.4% to PV1, 86.1% to PV2, 83.6% to PV3; 3.3% did not have antibodies to all three types of PV. The level of herd immunity varied in the examined groups depending on the vaccination schedule and the composition of the poliovirus vaccines used: nABs to PV2 had 59.6% of children born during the period when vaccines containing PV2 were not used, and 85.7% of children born after the introduction of trivalent IPV. Deficiency in immunity to PV2 was the cause of a polio outbreak in 2021 caused by circulating vaccine-derived PV type 2. Conclusion. A high level of humoral immunity to the rubella virus was determined. Shortcomings of routine immunization against measles and polio associated with insufficient coverage and lack of IPV have been identified. Conducting regular serological monitoring in the Republic of Tajikistan is advisable to obtain objective information about the level of herd immunity, identify vulnerable groups of the population, and plan additional immunization activities.

More than 100 types of non-polio enteroviruses (NPEVs) are ubiquitous in the human population and cause a variety of symptoms ranging from very mild to meningitis and acute flaccid paralysis (AFP). Much of the information regarding diverse pathogenic properties of NPEVs comes from the surveillance of poliovirus, which also yields NPEV. The analysis of 265 NPEV isolations from 10,433 AFP cases over 24 years of surveillance and more than 2500 NPEV findings in patients without severe neurological lesions suggests that types EV-A71, E13, and E25 were significantly associated with AFP. EV-A71 was also significantly more common among AFP patients who had fever at the onset and residual paralysis compared to all AFP cases. In addition, a significant disparity was noticed between types that were common in humans (CV-A2, CVA9, EV-A71, E9, and E30) or in sewage (CVA7, E3, E7, E11, E12, and E19). Therefore, there is significant evidence of non-polio viruses being implicated in severe neurological lesions, but further multicenter studies using uniform methodology are needed for a definitive conclusion.

Significantly divergent polioviruses (VDPV) derived from the oral poliovirus vaccine (OPV) from Sabin strains, like wild polioviruses, are capable of prolonged transmission and neuropathology. This is mainly shown for VDPV type 2. Here we describe a molecular-epidemiological investigation of a case of VDPV type 3 circulation leading to paralytic poliomyelitis in a child in an orphanage, where OPV has not been used. Samples of feces and blood serum from the patient and 52 contacts from the same orphanage were collected twice and investigated. The complete genome sequencing was performed for five polioviruses isolated from the patient and three contact children. The level of divergence of the genomes of the isolates corresponded to approximately 9–10 months of evolution. The presence of 61 common substitutions in all isolates indicated a common intermediate progenitor. The possibility of VDPV3 transmission from the excretor to susceptible recipients (unvaccinated against polio or vaccinated with inactivated poliovirus vaccine, IPV) with subsequent circulation in a closed children’s group was demonstrated. The study of the blood sera of orphanage residents at least twice vaccinated with IPV revealed the absence of neutralizing antibodies against at least two poliovirus serotypes in almost 20% of children. Therefore, a complete rejection of OPV vaccination can lead to a critical decrease in collective immunity level. The development of new poliovirus vaccines that create mucosal immunity for the adequate replacement of OPV from Sabin strains is necessary.

Polio and enterovirus surveillance may include a number of approaches, including incidence-based observation, a sentinel physician system, environmental monitoring and acute flaccid paralysis (AFP) surveillance. The relative value of these methods is widely debated. Here we summarized the results of 14 years of environmental surveillance at four sewage treatment plants of various capacities in Moscow, Russia. A total of 5450 samples were screened, yielding 1089 (20.0%) positive samples. There were 1168 viruses isolated including types 1−3 polioviruses (43%) and 29 different types of non-polio enteroviruses (51%). Despite using the same methodology, a significant variation in detection rates was observed between the treatment plants and within the same facility over time. The number of poliovirus isolates obtained from sewage was roughly 60 times higher than from AFP surveillance over the same time frame. All except one poliovirus isolate were Sabin-like polioviruses. The one isolate was vaccine-derived poliovirus type 2 with 17.6% difference from the corresponding Sabin strain, suggesting long-term circulation outside the scope of the surveillance. For some non-polio enterovirus types (e.g., Echovirus 6) there was a good correlation between detection in sewage and incidence of clinical cases in a given year, while other types (e.g., Echovirus 30) could cause large outbreaks and be almost absent in sewage samples. Therefore, sewage monitoring can be an important part of enterovirus surveillance, but cannot substitute other approaches.

Complete genomic sequences of a non-redundant set of 70 recombinants between three serotypes of attenuated Sabin polioviruses as well as location (based on partial sequencing) of crossover sites of 28 additional recombinants were determined and compared with the previously published data. It is demonstrated that the genomes of Sabin viruses contain distinct strain-specific segments that are eliminated by recombination. The presumed low fitness of these segments could be linked to mutations acquired upon derivation of the vaccine strains and/or may have been present in wild-type parents of Sabin viruses. These “weak” segments contribute to the propensity of these viruses to recombine with each other and with other enteroviruses as well as determine the choice of crossover sites. The knowledge of location of such segments opens additional possibilities for the design of more genetically stable and/or more attenuated variants, i.e., candidates for new oral polio vaccines. The results also suggest that the genome of wild polioviruses, and, by generalization, of other RNA viruses, may harbor hidden low-fitness segments that can be readily eliminated only by recombination.