Effectiveness of mass vaccination for prevention of hospitalization, severe disease and death due to SARS-CoV-2 omicron Ba.2 variant; A case-population study

One of the primary concerns regarding COVID-19 vaccination programs is the emergence of new virus variants and the effectiveness of the currently available vaccines against these variants. The main objective of this study was to evaluate the effectiveness of the system of vaccination of COVID-19 in Iran in preventing hospitalization, severe illness, critical illness, and death in relation to the Omicron BA.2 variant of SARS-CoV-2. The study focused on assessing vaccine effectiveness regardless of the specific vaccine administered in the community and also investigated the potential improvement in effectiveness after receiving the second dose or subsequent doses of the vaccine. The study specifically examined two age groups including individuals aged 65 years and older and individuals younger than 65 years. This study was performed using case-population method provided by World Health Organization (WHO). To conduct the study, data on vaccination coverage and vaccination status within the community were obtained from the data center of the Public Health Service in the Tehran province, Iran. Additionally, data on hospitalization, critical illness, and death related to COVID-19 were collected from hospitals in Tehran during the period when the Omicron Ba.2 variant was dominant in Iran. The results of the study indicated that vaccination with the available vaccines was effective in preventing severe illness, critical illness, and death resulting from infection with the Omicron variant in both age groups. This study found that completing the vaccination regimen was more effective in preventing adverse outcomes associated with the Omicron variant in elderly individuals compared to younger individuals.