Seroprevalence of Anti-SARS-CoV-2 Antibodies Following the Omicron BA.1 Wave.

We conducted a seroprevalence study using convenient residual sera samples from the Slovenian population collected after the end of the Omicron BA.1 pandemic wave. Serum samples were tested for spike glycoprotein (anti-S) and nucleocapsid protein (anti-N) antibodies. Participants' data regarding confirmed infection and vaccination was obtained from national registries. Anti-S antibodies were detected in 2439 (84.1%) of 2899 sera from persons aged 0-90 years, with the lowest prevalence in the 0-17 age group. The proportion of anti-N positives was the lowest in the ≥70 age group. The proportion of anti-N positives was significantly higher among participants with confirmed past infection and among those who had never been vaccinated. In participants who had not been notified as infected and who had never been vaccinated, the seroprevalence of anti-S and anti-N antibodies was 53% and 35.5%, respectively. From the time of serum collection to mid-November 2022, 445 participants (15.3%) tested positive for SARS-CoV-2, with higher odds in seronegative participants, participants in the 40-59 age group, and those without notified previous infection. Vaccination status and gender had no significant effects on infection risk. This study underlines the importance of serosurveys in understanding the development of the pandemic.

Milder outcomes of SARS-CoV-2 genetically confirmed reinfections compared to primary infections with the delta variant: A retrospective case-control study.

Background: SARS-CoV-2 infection does not confer long immunity. However, studies suggest that prior infection is associated with lower risk of reinfection and milder outcomes of recurrent infections. The aims of this retrospective observational case-control study were to describe the clinical and molecular characteristics of genetically confirmed Delta reinfection cases and to assess the potential protective role of preceding infection on the severity of reinfection. Methods: We used next generation sequencing (NGS) to explore if cases with two positive real time RT-PCR tests > 90 days apart were infected with a different SARS-CoV-2 variant. Cases with confirmed reinfection between August 1st and October 31st, 2021 (the Delta wave) in Slovenia were matched 1:4 by age, sex and timeframe (week of positive test) with individuals with primary infection. Sociodemographic and epidemiologic data, vaccination status, and data on hospitalization and outcome of infection were retrieved from several centralized and standardized national databases. Additional epidemiologic surveys were performed on a limited number of cases and controls. Results: We identified 628 cases of genetically confirmed reinfection during the study period and matched them with 2,512 control subjects with Delta primary infection. Primary infections in individuals with reinfection were mainly caused by B.1.258.17 (51.1%), followed by B.1.1.7 (15.1%) and reinfection was detected on average 271 days after primary infection (range 101-477 days). Our results show a substantially lower probability of hospitalization in cases with reinfection compared with controls (OR: 0.21, p = 0.017), but no significant difference was observed in intensive care unit admission and deaths. We observed a significantly lower proportion of vaccinated individuals among cases compared to controls (4.5% vs. 28.2%), suggesting that hybrid immunity leads to lower probability of reinfection. Detailed analysis of the temporal distribution of variants, responsible for reinfections, showed no significant differences in reinfection potential. Conclusion: Reinfection with the SARS-CoV-2 Delta variant resulted in fewer hospitalizations compared to the primary Delta infection, suggesting that primary infection may, to some extent, produce at least short lasting protective immunity. This study provides additional insight into the reinfection dynamics that may allow appropriate public health measures to be taken in subsequent waves of the COVID-19 pandemic.

mRNA vaccine effectiveness against hospitalisation due to severe acute respiratory infection (SARI) COVID-19 during Omicron variant predominance estimated from real-world surveillance data, Slovenia, February to March 2022.

For the period of predominance of SARS-CoV-2 Omicron variant in Slovenia, February to March 2022, we estimated mRNA vaccine effectiveness (VE) against severe acute respiratory infection (SARI) COVID-19 using surveillance data. In the most vulnerable age group comprising individuals aged 65 years and more, VE against SARI COVID-19 was 95% (95% CI: 95-96%) for those vaccinated with three doses, in comparison to 82% (95% CI: 79-84%) for those vaccinated with two doses. Such levels of protection were maintained for at least 6 months.

Vaccine effectiveness against severe acute respiratory infections (SARI) COVID-19 hospitalisations estimated from real-world surveillance data, Slovenia, October 2021.

We estimated vaccine effectiveness (VE) against severe COVID-19 during October 2021, using Slovenian surveillance data. For people fully vaccinated with any vaccine in age groups 18-49, 50-64, ≥ 65 years, VE was 86% (95% CI: 79-90), 89% (85-91), and 77% (74-81). Among ≥ 65 year-olds fully vaccinated with mRNA vaccines, VE decreased from 93% (95% CI: 88-96) in those vaccinated ≤ 3 months ago to 43% (95% CI: 30-54) in those vaccinated ≥ 6 months ago, suggesting the need for early boosters.

Enhanced national surveillance of severe acute respiratory infections (SARI) within COVID-19 surveillance, Slovenia, weeks 13 to 37 2021.

We monitored trends of severe COVID-19 morbidity in Slovenia during weeks 13 to 37 2021. National weekly rates of severe acute respiratory infections (SARI) cases testing positive for SARS-CoV-2 at admission in all hospitals varied between 0.2 and 16.3 cases per 100,000 population. Of those without previous COVID-19 diagnosis, SARI COVID-19 admission rates ranged between 0.3 and 17.5 per 100,000 unvaccinated, and 0.0 and 7.3 per 100,000 fully vaccinated individuals. National SARI COVID-19 surveillance is essential in informing COVID-19 response.