Prevalence of Influenza B/Yamagata Viruses From Season 2012/2013 to 2021/2022 in Italy as an Indication of a Potential Lineage Extinction.

BACKGROUND: Influenza B/Yamagata viruses exhibited weak antigenic selection in recent years, reducing their prevalence over time and requiring no update of the vaccine component since 2015. To date, no B/Yamagata viruses have been isolated or sequenced since March 2020. METHODS: The antibody prevalence against the current B/Yamagata vaccine strain in Italy was investigated: For each influenza season from 2012/2013 to 2021/2022, 100 human serum samples were tested by haemagglutination inhibition (HAI) assay against the vaccine strain B/Phuket/3073/2013. In addition, the sequences of 156 B/Yamagata strains isolated during the influenza surveillance activities were selected for analysis of the haemagglutinin genome segment. RESULTS: About 61.9% of the human samples showed HAI antibodies, and 21.7% had protective antibody levels. The prevalence of antibodies at protective levels in the seasons between the isolation of the strain and its inclusion in the vaccine was between 11% and 25%, with no significant changes observed in subsequent years. A significant increase was observed in the 2020/2021 season, in line with the increase in influenza vaccine uptake during the pandemic. Sequence analysis showed that from 2014/2015 season onward, all B/Yamagata strains circulating in Italy were closely related to the B/Phuket/2013 vaccine strain, showing only limited amino acid variation. CONCLUSIONS: A consistent prevalence of antibodies to the current B/Yamagata vaccine strain in the general population was observed. The prolonged use of a well-matched influenza vaccine and a low antigenic diversity of B/Yamagata viruses may have facilitated a strong reduction in B/Yamagata circulation, potentially contributing to the disappearance of this lineage.

Centenarians, semi and supercentenarians, COVID-19 and Spanish flu: a serological assessment to gain insight into the resilience of older centenarians to COVID-19.

BACKGROUND: Although it is well known that the older people have been the most susceptible to COVID-19, there are conflicting data on the susceptibility of centenarians. Two epidemiological study have shown that older centenarians (> 101 years old at the time of the 2020 pandemic peak) are more resilient than the remaining centenarians, suggesting that this resilience might be linked to the 1918 Spanish Flu pandemic. To gain insight into this matter, specifically whether the resilience of older centenarians to SARS-CoV-2 infection is linked to the Spanish Flu they had been affected by, we conducted a retrospective serological study. This study examined serum samples from 33 centenarians, encompassing semi- (aged > 104 < 110 years, N = 7) and supercentenarians (aged > 109 years, N = 4), born between 1905 and 1922, against both SARS-CoV-2 and 1918 H1N1 pseudotype virus. RESULTS: Anamnestic and laboratory data suggest that SARS-CoV-2 infection occurred in 8 centenarians. The infection appeared to have been asymptomatic or mild, and hospitalization was not required, despite 3 out of 8 being between 109 and 110 years old. The levels of anti-spike antibodies in centenarians infected and/or vaccinated were higher, although not significantly, than those produced by a random sample of seventy-year-old individuals used as controls. All centenarians had antibody levels against the 1918 H1N1 virus significantly higher (almost 50 times) than those observed in the quoted group of seventy-year-old subjects, confirming the key role in maintaining immunological memory from a priming that occurred over 100 years ago. Centenarians whose blood was collected prior to the pandemic outbreak demonstrated neutralising antibodies against the 1918 H1N1 virus, but all these subjects tested negative for SARS-CoV-2. CONCLUSION: This retrospective study shows that older centenarians are quite resilient to COVID-19, as they are capable of producing good levels of neutralising antibodies and experiencing mild or asymptomatic disease. This could be attributed to the 1918 Spanish flu pandemic through mechanisms other than the presence of cross-reactive antibodies between the 1918 H1N1 virus and SARS-CoV-2. Another possibility is that the association is purely temporal, solely correlated with the advanced age of resilient centenarians compared to those born after 1918, since older centenarians are known to have better control of immune-inflammatory responses.

Evaluation of monkeypox- and vaccinia-virus neutralizing antibodies before and after smallpox vaccination: A sero-epidemiological study.

Since May 2022, several countries outside of Africa experienced multiple clusters of monkeypox virus (MPXV)-associated disease. In the present study, anti-MPXV and anti-vaccinia virus (VACV) neutralizing antibody responses were evaluated in two cohorts of subjects from the general Italian population (one half born before the WHO-recommended end of smallpox vaccination in 1980, the other half born after). Higher titers (either against MPXV or VACV) were observed in the cohort of individuals born before the interruption of VACV vaccination. An association between VACV and MPXV antibody levels was observed, suggesting that the smallpox vaccination may confer some degree of cross-protection against MPXV infection. Results from this study highlight low levels of immunity toward the assessed Orthopoxviruses, especially in young adults, advocating the introduction of a VACV- or MPXV-specific vaccine in case of resurgence of monkeypox disease outbreaks.

Seroprotection against tetanus in the Italian general population.

BACKGROUND: Tetanus is a non-communicable disease, preventable with vaccination. Despite the implemented vaccination strategy, a certain number of tetanus cases per year continue to occur. The aim of the study was to evaluate the seroprevalence of anti-tetanus antibodies in the Italian population by age, sex and geographical area. METHODS: To determine the level of tetanus-specific antibodies, an immunoenzymatic assay was used. RESULTS: A total of 3,821 serum samples were collected in the years 2019-20 from healthy subjects aged 6-90 years residing in 13 Italian regions. Overall, 85 % of the tested subjects resulted positive. The rate of subjects protected against tetanus showed a gradual decrease from the younger age groups to the older ones (6-12 years: 93.6 %, 13-24 years: 91.8 %, 25-39 years: 91.0 %, 40-64 years: 78.2 %, ≥ 65 years: 45.3 %); this is particularly evident in the Southern regions and Islands. Moreover, the prevalence of subjects with low protection (<0.1 IU/ml) was significantly higher in the ≥ 65 age group (10.3 %). Males and females' prevalence showed a significant difference only in the oldest age group (M: 60.8 %, F: 30.4 %). In general, a higher prevalence was observed for Northern (90.8 %) and Central regions (87.3 %) than Southern regions and Islands (80.0 %). CONCLUSION: These data, compared with epidemiological ones which showed a high number of cases in the elderly, confirmed that the population with lower protection has a greater risk of contracting the disease, demonstrating the need for adequate immunization through both primary vaccination and boosters for all ages and both sexes, in order to provide lifelong protection.

Seroprevalence of measles antibodies in the Italian general population in 2019-2020.

In Italy, the measles elimination target has not yet been reached despite a significant reduction in cases. A multicenter study was conducted to estimate the prevalence of anti-measles (MV) IgG antibodies in the Italian population by age, sex and geographical area. To determine the level of MV-specific antibodies in sera, the immunoenzymatic assay ELISA was used (Enzygnost Anti-VZV/IgG, Siemens Healthcare Diagnostic Products GmbH, Germany). Overall, 3746 serum samples collected in the years 2019-20 from healthy subjects aged 6-64 years residing in 13 Italian regions. The overall seroprevalence of anti-MV IgG was 91.2 % (90.6 % male, 91.7 % female). Significantly higher seroprevalence values (p < 0.05) were recorded for the extreme age groups of the study population (6-9 years: 94.2 %; 40-64 years: 97.6 %). Subjects 20-39 and 40-64 years old had significantly higher antibody titers suggesting a protection against measles mainly derived from natural infection. Seroprevalence was significantly higher in the South (93.2 %) than in the Northern-Central Italy (88.9 %). The results indicate an increase in the overall seroprevalence data compared to previous investigations. However, further efforts must be made to implement and maintain high measles vaccination coverage to avoid the risk of future epidemic outbreaks.

SARS-CoV-2 and influenza virus coinfections in the Tuscan population during the 2021/2022 influenza season.

Introduction: The 2021/2022 influenza season was not characterised by a well-defined incidence peak. As reported by the Italian National Institute of Health, a high value of incidence of influenza cases was recorded in week 13, but it was still lower than in other influenza seasons. This abnormal circulation was probably due to relaxation of the COVID-19 pandemic restriction measures, such as social distancing, smart-working, home leaning and the use of masks, which greatly reduced the circulation of respiratory-transmitted viruses, including human respiratory syncytial virus (HRSV). The symptoms of SARS-CoV-2 and influenza are quite similar, sharing the human-to-human transmission route via respiratory droplets. Methods: The aim of this study was to estimate the rate of coinfection with influenza viruses and/or HRSV in SARS-CoV-2-positive subjects (N = 940) in a population of central Italy during the 2021/2022 season. Results: A total of 54 cases of coinfection were detected during the study period, 51 cases (5.4%) of SARS-CoV-2 and influenza virus and three cases (0.3%) of SARS-CoV-2 and HRSV coinfection. Conclusions: These results highlight the importance of continuous monitoring of the circulation of influenza virus and other respiratory viruses in the context of the COVID-19 pandemic.

Detection of Influenza D Antibodies in Dogs, Apulia Region, Italy, 2016 and 2023.

Dogs are known to be susceptible to influenza A viruses, although information on influenza D virus (IDV) is limited. We investigated the seroprevalence of IDV in 426 dogs in the Apulia region of Italy during 2016 and 2023. A total of 14 samples were positive for IDV antibodies, suggesting exposure to IDV in dogs.

Immunogenicity and safety of COVID-19 booster vaccination: A population-based clinical trial to identify the best vaccination strategy.

BACKGROUND: Various SARS-CoV-2 variants of concerns (VOCs) characterized by higher transmissibility and immune evasion have emerged. Despite reduced vaccine efficacy against VOCs, currently available vaccines provide protection. Population-based evidence on the humoral immune response after booster vaccination is crucial to guide future vaccination strategies and in preparation for imminent COVID-19 waves. METHODS: This multicenter, population-based cohort study included 4697 individuals ≥18 years of age who received a booster vaccination. Antibody levels against SARS-CoV-2 receptor binding domain (RBD) and neutralizing antibodies against wild-type (WT) virus and Omicron variants were assessed at baseline (day of booster vaccination) and after four weeks. Safety was evaluated daily within the first week using a participant-completed electronic diary. Antibody levels were compared across different vaccination strategies, taking into account individual host factors. RESULTS: Our main model including 3838 participants revealed that individuals who received a booster with mRNA-1273 compared to BNT162b2 vaccine had a significantly higher increase (95 %CI) in anti-RBD-antibody levels (37,707 BAU/mL [34,575-40,839] vs. 27,176 BAU/mL [26,265-28,087]), and of neutralization levels against WT (1,681 [1490-1872] vs. 1141 [1004-1278] and Omicron variant (422 [369-474] vs. 329 [284-374]). Neutralizing antibody titres highly correlated with anti-RBD antibodies, with neutralizing capacity 4.4 fold higher against WT compared to Omicron. No differences in safety were found between the two booster vaccines. CONCLUSION: Our study underlines the superiority of a booster vaccination with mRNA-1273, independent of the primary vaccination and therefore provides guidance on the vaccination strategy.

Serological Evidence for Circulation of Influenza D Virus in the Ovine Population in Italy.

Influenza D virus (IDV) is a novel orthomyxovirus initially isolated from pigs exhibiting influenza-like disease in the USA. Since then, IDV has been detected worldwide in several host species, including livestock animals, whilst specific antibodies have been identified in humans, raising concerns about interspecies transmission and zoonotic risks. Few data regarding the seroprevalence of IDV in small ruminants have been available to date. In this study, we assessed the prevalence of antibodies against IDV in ovine serum samples in Sicily, Southern Italy. Six hundred serum samples, collected from dairy sheep herds located in Sicily in 2022, were tested by haemagglutination inhibition (HI) and virus neutralization (VN) assays using reference strains, D/660 and D/OK, representative of two distinct IDV lineages circulating in Italy. Out of 600 tested samples, 168 (28.0%) tested positive to either IDV strain D/660 or D/OK or to both by HI whilst 378 (63.0%) tested positive to either IDV strain D/660 or D/OK or to both by VN. Overall, our findings demonstrate that IDV circulates in ovine dairy herds in Sicily. Since IDV seems to have a broad host range and it has zoonotic potential, it is important to collect epidemiological information on susceptible species.

Flu vaccine administration in the period before SARS-CoV-2 infection and its outcomes: An umbrella review.

Objective: The potential association between influenza vaccination and SARS-CoV-2 infection and related outcomes is still controversial. The aim of this umbrella review is to represent the impact of previous influenza vaccination and COVID-19 outcomes using evidence currently available in literature. Methods: A literature search of MEDLINE, EMBASE, Scopus, Web of Science and Cochrane Library was conducted. The paper selection was conducted using the preferred reporting items for systematic reviews and meta-analyses (PRISMA) method by two-blinded authors. The quality of meta-analyses was assessed using the AMSTAR 2 scale (A MeaSurement Tool to Assess systematic Reviews). The outcomes investigated were SARS-CoV-2 infection after influenza vaccination, hospitalization, intensive care unit admission, mechanical ventilation and mortality. Results: The literature research identified 7 ecological studies and 6 meta-analyses. All the ecological studies show a negative relationship between influenza vaccination and COVID-19. The meta-analyses suggest a protective action of influenza vaccination against SARS-CoV-2 infection. Regarding the outcomes evaluated, only two studies reported a statistically significant reduction of 12% and of 17% in hospitalization and intensive care unit admission, respectively. Regarding mechanical ventilation, three studies showed a risk reduction of 31%, 27% and 28%. A substantial reduction of mortality risk was also observed in one study. Conclusions: These results suggest that influenza vaccination could be associated with reduced susceptibility to SARS-CoV-2 infection, mechanical ventilation and mortality. Our findings highlighted how the administration of flu vaccine in subjects at risk could lead to a reduction in mortality, particularly in the over 65y.

The burden of influenza and the role of influenza vaccination in adults aged 50-64 years: A summary of available evidence.

Influenza is a vaccine-preventable disease and a global public health problem. Although most national influenza vaccination recommendations focus on subjects aged ≥65 years, an extensive burden of influenza has also been reported in those aged ≥50 years and is exacerbated by immune system aging. The main purpose of this review is to provide an overview of the burden of influenza and its potential prevention within the 50-64 age-group. These subjects account for a large proportion of the workforce, and play a central economic and social role. Individuals aged 50-64 years had a 3-times higher rate of hospitalization and a 9-fold higher mortality rate attributable to influenza than those aged 18-49-years, generating higher influenza-related hospitalization costs. Available data suggest that including healthy subjects aged 50-64 years in influenza vaccination recommendations would allow a broader population to be reached, reducing the economic and social burden of influenza.

Evaluation of immune response to SARS-CoV-2 Omicron sublineages six months after different vaccination regimens in Italy.

The Omicron variant is the most divergent, displaying more mutations than previous SARS-CoV-2 variants, particularly in the gene that encodes the spike protein. This study aimed to assess the persistence of neutralizing antibodies towards the SARS-CoV-2 Omicron sublineages (BA.2, BA.5, BQ.1, XBB and XBB1.5) six months after the third dose in different vaccination regimens. Subjects who received 3 doses of mRNA vaccine retained their neutralization activity against BA.2 and BA.5, even though 56.3% and 66.7% showed a ≥ 2-fold reduction in the neutralizing antibody titre, respectively. Subjects who had received the adenovirus-based vaccine plus a booster dose of mRNA vaccine retained their neutralization activity especially against BA.2. With regard to BQ.1, XBB and XBB.1.5, the majority of the subjects showed a ≥ 2-fold reduction in neutralizing antibody titre, with the greatest evasion being observed in the case of XBB. Overall, our results provide further evidence that triple homologous/heterologous vaccination and hybrid immunity result in detectable neutralizing antibodies against the ancestral virus; however, emerging Omicron sublineages, such as XBB and XBB.1.5, show a great evasive capacity, which compromises the effectiveness of current COVID-19 vaccines.

Evaluation of Monkeypox- and Vaccinia virus-neutralizing antibodies in human serum samples after vaccination and natural infection.

Introduction: In early to mid-2022, an unexpected outbreak of Monkeypox virus infections occurred outside the African endemic regions. Vaccines originally developed in the past to protect against smallpox are one of the available countermeasures to prevent and protect against Orthopoxvirus infections. To date, there are few studies on the cross-reactivity of neutralizing antibodies elicited by previous vaccinia virus-based vaccination and/or Monkeypox virus infection. The aim of this study was to evaluate a possible approach to performing Monkeypox and vaccinia live-virus microneutralization assays in which the read-out is based on the production of cytopathic effect in the cell monolayer. Methods: Given the complexity of Orthopoxviruses, the microneutralization assay was performed in such a way as to uncover a potential role of complement, with and without the addition of an external source of Baby Rabbit Complement. A set of human serum samples from individuals who had been naturally infected with Monkeypox virus and individuals who may have and not have undergone vaccinia virus vaccinations, was used to evaluate the performance, sensitivity, and specificity of the assay. Results and conclusions: The results of the present study confirm the presence and cross-reactivity of antibodies elicited by vaccinia-based vaccines, which proved able to neutralize the Monkeypox virus in the presence of an external source of complement.

Establishment and validation of a high-throughput micro-neutralization assay for respiratory syncytial virus (subtypes A and B).

The validation of a bioanalytical method allows us to determine its validity for a designated purpose and to guarantee the reliability of its analytical results. The virus neutralization assay has proved to be suitable for the detection and quantification of specific serum-neutralizing antibodies against respiratory syncytial virus subtypes A and B. Respiratory syncytial virus is a negative-sense RNA virus and is responsible for the majority of acute lower respiratory tract infections in infants and older adults worldwide. Owing to its widespread infection, the WHO considers it a target for the development of preventive vaccines. Despite the high impact of its infections, however, only one vaccine has been recently approved. The aim of this paper is to provide a detailed validation process for the microneutralization assay and to demonstrate that this method can effectively support the efficacy assessment of candidate vaccines and the definition of correlates of protection.

Haemagglutination inhibition and virus microneutralisation serology assays: use of harmonised protocols and biological standards in seasonal influenza serology testing and their impact on inter-laboratory variation and assay correlation: A FLUCOP collaborative study.

Introduction: The haemagglutination inhibition assay (HAI) and the virus microneutralisation assay (MN) are long-established methods for quantifying antibodies against influenza viruses. Despite their widespread use, both assays require standardisation to improve inter-laboratory agreement in testing. The FLUCOP consortium aims to develop a toolbox of standardised serology assays for seasonal influenza. Building upon previous collaborative studies to harmonise the HAI, in this study the FLUCOP consortium carried out a head-to-head comparison of harmonised HAI and MN protocols to better understand the relationship between HAI and MN titres, and the impact of assay harmonisation and standardisation on inter-laboratory variability and agreement between these methods. Methods: In this paper, we present two large international collaborative studies testing harmonised HAI and MN protocols across 10 participating laboratories. In the first, we expanded on previously published work, carrying out HAI testing using egg and cell isolated and propagated wild-type (WT) viruses in addition to high-growth reassortants typically used influenza vaccines strains using HAI. In the second we tested two MN protocols: an overnight ELISA-based format and a 3-5 day format, using reassortant viruses and a WT H3N2 cell isolated virus. As serum panels tested in both studies included many overlapping samples, we were able to look at the correlation of HAI and MN titres across different methods and for different influenza subtypes. Results: We showed that the overnight ELISA and 3-5 day MN formats are not comparable, with titre ratios varying across the dynamic range of the assay. However, the ELISA MN and HAI are comparable, and a conversion factor could possibly be calculated. In both studies, the impact of normalising using a study standard was investigated, and we showed that for almost every strain and assay format tested, normalisation significantly reduced inter-laboratory variation, supporting the continued development of antibody standards for seasonal influenza viruses. Normalisation had no impact on the correlation between overnight ELISA and 3-5 day MN formats.

An external quality assessment feasibility study; cross laboratory comparison of haemagglutination inhibition assay and microneutralization assay performance for seasonal influenza serology testing: A FLUCOP study.

Introduction: External Quality Assessment (EQA) schemes are designed to provide a snapshot of laboratory proficiency, identifying issues and providing feedback to improve laboratory performance and inter-laboratory agreement in testing. Currently there are no international EQA schemes for seasonal influenza serology testing. Here we present a feasibility study for conducting an EQA scheme for influenza serology methods. Methods: We invited participant laboratories from industry, contract research organizations (CROs), academia and public health institutions who regularly conduct hemagglutination inhibition (HAI) and microneutralization (MN) assays and have an interest in serology standardization. In total 16 laboratories returned data including 19 data sets for HAI assays and 9 data sets for MN assays. Results: Within run analysis demonstrated good laboratory performance for HAI, with intrinsically higher levels of intra-assay variation for MN assays. Between run analysis showed laboratory and strain specific issues, particularly with B strains for HAI, whilst MN testing was consistently good across labs and strains. Inter-laboratory variability was higher for MN assays than HAI, however both assays showed a significant reduction in inter-laboratory variation when a human sera pool is used as a standard for normalization. Discussion: This study has received positive feedback from participants, highlighting the benefit such an EQA scheme would have on improving laboratory performance, reducing inter laboratory variation and raising awareness of both harmonized protocol use and the benefit of biological standards for seasonal influenza serology testing.

SARS-CoV-2 epidemiological trend before vaccination era: a seroprevalence study in Apulia, Southern Italy, in 2020.

Aim: The present study aimed at assessing the prevalence of antibodies against SARS-CoV-2 in the general population in the province of Bari (Apulia region, Southern Italy) during the year 2020. Subject and methods: In this study, 1325 serum samples collected from January to December 2020 were tested for the presence of IgM and IgG antibodies against whole-virus SARS-CoV-2 antigen by commercial ELISA. Positive samples were further tested by in-house ELISA for the detection of anti-receptor binding domain (RBD) IgM and IgG antibodies and by micro-neutralization (MN) assay for the detection of neutralizing antibody. Results: One hundred (7.55%) samples had the presence of at least one antibody class against SARS-CoV-2 by commercial ELISA, of which 88 (6.6%) showed IgG and 19 (1.4%) showed IgM antibodies. The proportion of samples with IgG antibodies increased from 1.9% in January-February to 9.6% in November-December, while no significant increase was observed for IgM. When tested by in-house ELISA and MN assay, 17.0% and 31.6% were found positive to RBD IgG and RBD IgM, respectively, while 12.0% showed neutralizing antibody. Conclusion: The proportion of samples with SARS-CoV-2 IgG antibodies increased during 2020, especially in the second half of the year, consistent with data reported by the routine epidemiological surveillance of SARS-CoV-2 cases. Despite the high number of reported cases, the seroprevalence values are relatively low, and only a small proportion of samples had neutralizing antibodies. Supplementary Information: The online version contains supplementary material available at 10.1007/s10389-023-01834-3.

Neutralizing activity of African lineage Zika virus immune sera towards Asian lineage.

Genetic and phylogenetic studies indicated that Zika virus (ZIKV) has evolved into 2 major lineages, the African and Asian. However, ZIKV has been described as a single serotype. This study aimed at assessing the cross-neutralization between ZIKV African and Asian lineages strains. Sixthy-five samples collected in 2007 and 30 samples collected from the same subjects in 2011/2012 in West Africa and positive to neutralizing antibody against ZIKV MR-766 strain (African lineage) were tested against ZIKV H/PF/2013 strain (Asian lineage) by microneutralization assay. All samples showing neutralizing antibodies against MR-766 strain showed also neutralizing activity against H/PF/2013 strain, although with lower titers. This is consistent with about 120 amino acid differences between the two strains. Despite differences in the magnitude of neutralizing activity against different ZIKV strains, all samples showed neutralizing antibody titers considered to be protective.

Evaluation of antibody response to SARS-CoV-2 variants after 2 doses of mRNA COVID-19 vaccine in a correctional facility.

The SARS-CoV-2 pandemic has posed a challenge for correctional facilities worldwide. People in such settings are more vulnerable to severe forms of infection and it is impossible to completely isolate inmates from the outside world. This study aimed to assess the antibody-mediated immune response in terms of neutralizing antibodies against Alpha, Beta, Gamma and Omicron (sub-lineage BA.1) variants of concern after two doses of mRNA vaccine in correctional officers and inmates from an Italian correctional facility. Most of the correctional officers (56.5%) and inmates (52.3% and 63.6%) retained their neutralizing activity toward the Alpha and Gamma variants, respectively. By contrast, the most striking reduction in comparison with the ancestral virus was found in the antibody response toward the Beta and Omicron variants, in both correctional officers (91.2% and 93.9%) and inmates (85.1% and 92.8%). In addition, subjects who had undergone primary vaccination and had previously been naturally infected had higher neutralizing antibody titers toward the 4 variants than negative subjects. Overall, our findings indicate that primary mRNA vaccination is able to induce neutralizing antibodies toward the ancestral virus, while titers toward variants may vary, depending on the mutations harboring by the variants. Although the correctional setting is often considered distinct or isolated from the wider society and sanitary system, the health of correctional workers and prisoners is inexorably linked to the public health of the country as a whole and it is of paramount importance to monitor the antibody response in these settings.

The neutralizing response to SARS-CoV-2 Omicron variants BA.1 and BA.2 in COVID-19 patients and homologous and heterologous vaccinees.

The rapid replacement of Omicron BA.1 by BA.2 sublineage is very alarming, raising the question of whether BA.2 can escape the immunity acquired after BA.1 infection. We compared the neutralizing activity toward the Omicron BA.1 and BA.2 sub-lineages in five groups: COVID-19 patients; subjects who had received two doses of mRNA vaccine; subjects naturally infected with SARS-CoV-2 who had received two doses of mRNA; and subjects who had received three doses of homologous or heterologous vaccine. The results obtained highlight the importance of vaccine boosters in eliciting neutralizing antibody responses against Omicron sub-lineages, and suggest that the adenovirus vectored vaccine elicits a lower response against BA.1 than against BA.2 sub-lineage.