Post-vaccination immunity and pneumonia incidence in medical workers in response to various flu vaccination regimens between the 1st and 2nd peaks of COVID-19 morbidity

Background.In the absence of a vaccine against SARS-CoV-2, seasonal influenza vaccination during the pandemic contributed to lowered COVID-19 susceptibility and severity. The study was aimed to assess the state of post-influenza vaccination immunity, pneumonia frequency and severity in medical workers after using various flu vaccination regimens, between the 1st and 2nd peaks of COVID-19 epidemic rise.Materials and methods.Comparatively analyzed data on the levels of antibodies against influenza virus strains and pneumonia incidence in 487 medical workers was carried out: 1st group — unvaccinated in 2020–2021 (n = 281), 2nd group — vaccinated against influenza (Sovigripp), (n = 98), 3rd group — received combined vaccination against influenza and pneumococcus (Prevenar 13), (n = 108).Results.6 months after vaccination, the highest rates of influenza virus were detected in the A(H3N2), the level of seroprotection (≥ 1:40) ranged from 49.0% in unvaccinated (1st group) to 53.4–53.2% in those who received combined vaccination (group III), as well as influenza alone (2nd group), p 0.05; for strain A(H1N1) the level of seroprotection in 1st group is 24.5%, lower (p 0.04) than in 2nd group — 32.7%, but does not differ from the levels of 40.4% in group III; for strain B the level of seroprotection is the lowest ranging from 19.4% in the group of unvaccinated subjects up to 22.4% in 2nd group and 23.4% in 3rd group. The pneumonia incidence in 1st group (3.9%), 2nd group (3.1%), 3rd group (4.6%) did not differ, however, among all vaccinated subjects severity of pneumonia clinical course was mild, whereas in unvaccinated employees, except for mild course (45.4%, 5 out of 11 people) pneumonia in 36.4% (4 out of 11 people) cases was assessed as moderate and in 18.2% (2 out of 11 people) cases — severe with fatal outcomes.Conclusion.The study showed that 6 months after seasonal influenza immunization between the 1st and 2nd peaks of COVID-19 epidemic rise, the immunogenicity of the vaccine meets the CPMP criterion for the A(H3N2) strain. In vaccinated patients, the proportion of pneumonia with COVID-19 clinical picture was mild in 100% of cases, and in unvaccinated patients in 36.4% of cases — of moderate severity and in 18.2% — severe with fatal outcome.