Characteristic of herd immunity among the population of Stavropol region amid the COVID-19 epidemic

Introduction. The first pandemic in the 21st century, caused by the pathogenic representative of the coronavirus SARS-CoV-2, began in the Chinese city of Wuhan, where the first outbreak of coronavirus pneumonia was recorded in December 2019. The disease spread so quickly around the world that already on February 11, 2020, WHO was forced to declare a pandemic of the “coronavirus disease 2019” COVID-19. The first case of COVID-19 in the Stavropol Territory was registered on March 20, 2020, and three weeks later, starting from the 15th week of the year, a steady increase in the incidence began, which lasted until the 52nd week. During the study period, the incidence increased from 21.1 to 28.3 per hundred thousand of the population. Growth 1.3 times.

Purpose: to determine the dynamics of population immunity among the population of the Stavropol Territory in 2020-2021. during the period of an epidemic increase in the incidence of COVID-19. Materials and methods. The SARS-CoV-2 study was carried out according to a unified methodology within the framework of the program for assessing the population immunity of the population of the Russian Federation, developed by Rospotrebnadzor with the participation of the St. Pasteur. In total, 2688 people were examined, divided into 7 age groups. In the examined individuals, the level of specific IgG to the SARS-CoV-2 nucleocapsid was determined by the enzyme immunoassay.

Results. The level of seroprevalence among residents of the Stavropol Territory was 9.8%. The largest proportion of seropositive individuals was found in the age groups 1-6 and 7-13 years old (19.2% and 19.7%, respectively). Seroprevalence had no gender differences and ranged from 9.3% to 10.8%. When assessing the distribution of the proportion of seropositive persons in different geographic territories of the region, it was found that the maximum proportion was found in the Kochubeevsky district (23.1%), the minimum in Kislovodsk (7.7%). Among convalescents, the content of specific antibodies to SARS-CoV-2 was noted in 73.3%, which is 7.8 times higher than the average population level. When conducting seromonitoring in the 2nd half of 2020, a 10-fold increase in seroprevalence was recorded, accompanied by a decrease in incidence from the 5th week of 2021. Among asymptomatic volunteers in whom SARS-CoV-2 RNA was detected by the polymerase chain reaction, antibody titers to viruses were found in 78.6%, which corresponds to the seroprevalence of convalescents. The proportion of seropositive persons among those who have come into contact with COVID-19 patients was 16.4%, (1.8 times higher than the average for the population). Out of 262 seroprevalent volunteers, the asymptomatic form of SARS-CoV-2 was detected in 92% of the examined, which indicates a significant role of the number of asymptomatic forms of infection in the epidemic process of COVID-19.

Conclusion. The results of assessing the population immunity of the population of the Stavropol Territory indicate that it has not yet reached the threshold level at which a decrease in the intensity of the COVID-19 epidemic process can be expected. 

Measures counteracting 2016 spread of vaccine-derived poliomyelitis virus type 2 in Russian Federation

Since April 2016 after global cessation of using trivalent oral poliovirus vaccine (tOPV) and switch to bivalent OPV consisting of polioviruses types 1 and 3 (the “switch”), any isolation of type 2 poliovirus has been regarded as an event of extreme importance requiring investigation, risk assessment and decision making. In 2016, 2 cases of isolated vaccine-derived poliovirus type 2 from healthy children was registered in Russia. Our study was aimed at on the assessing a risk of further spread of vaccine-derived poliovirus type 2 and provide measures for preventing its further spread based on epidemiological investigation and genetic characteristics of the isolated viruses. The cases were revealed within the surveillance program for poliomyelitis and acute flaccid paralysis syndrome conducted in the Russian Federation. The laboratory investigation was carried out in accordance with the algorithm adopted in the Russian Federation and recommended by the WHO standards: virus isolation on RD, L20B and Hep2C cell cultures, identification in the neutralization reaction, intratyping differentiation by using RT-PCR in real-time mode, sequencing of the poliovirus genome fragments encoding the VP1 protein. A risk assessment for spread of vaccine-derived poliovirus type 2 was performed in accordance with the WHO recommendations. There was uncovered a genetic relationship between virus strains isolated in September and December from unvaccinated Moscow resident boy (1 year old) who arrived from the Chechen Republic and from unvaccinated girl resident of the Chechen Republic (1 year old) with impaired humoral and cellular immunity. The virus strains were found to bear 10 and 13 genomic nucleotide substitutions, respectively, at the site encoding the VP1 protein compared with the Sabin type 2 vaccine strain that allowed to classify them as vaccine-derived polioviruses. In particular, both virus strains were shown to originate from the type 2 strain presented in the tOPV used shortly before the “switch”. Epidemiological investigation revealed family ties and probable contact between both children in the same premises. A series of organizational and vaccination measures was undertaken, as well as polio surveillance was strengthened in the region. No new type 2 polioviruses of vaccine origin were detected in the territory of the Chechen Republic during 18-month monitoring follow-up. The risk assessment of spread for vaccine-derived poliovirus type 2 in a region, Russian Federation as well as cross-boundary spread identified it as “low,” requiring no use of type 2 monovalent OPV. Such experience for countermeasures may be taken into account to oppose the risks before and after the global certification for poliomyelitis eradication.