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The option of vaccinating poultry against avian influenza (AI) as a control tool is gaining greater acceptance by governments and the poultry industry worldwide. One disadvantage about vaccination with killed whole-virus vaccines is the resulting inability to use common serologic diagnostic tests for surveillance to identify infected flocks. There has been considerable effort to develop a reliable test for the differentiation of infected from vaccinated animals (DIVA). The heterologous neuraminidase (NA) subtype DIVA approach has been used with some success in the field accompanied by an ad hoc serologic test. The traditional NA inhibition (NI) test can be used for all nine NA subtypes, but it is time consuming, and it is not designed to screen large numbers of samples. In this study, a quantitative NI test using MUN (2′-[4-methylumbelliferyl]-α-D-Nacetylneuraminic acid sodium salt hydrate) as an NA substrate was investigated as an alternative to the traditional fetuin-based NI test in a heterologous neuraminidase DIVA strategy. Serum NI activity was determined in chickens administered different vaccines containing different H5 and NA subtypes and challenged with a highly pathogenic avian influenza (HPAI) H5N2 virus. Prior to challenge, the NI DIVA test clearly discriminated between chickens receiving vaccines containing different antigens (e.g., N8 or N9) from control birds that had no NA antibody. Some birds began to seroconvert 1 wk postchallenge, and 100% of the vaccinated birds had significant levels of N2 NI activity. This activity did not interfere with the presence of vaccine-induced NI activity against N8 or N9 subtypes. The level of N2-specific NI activity continued to increase to the last sampling date, 4 wk postchallenge, indicating the potential use for the heterologous NA-based DIVA strategy in the field.

Influenza viruses remain a severe threat to human health, causing up to 650,000 deaths annually1,2. Seasonal influenza virus vaccines can prevent infection, but are rendered ineffective by antigenic drift. To provide improved protection from infection, novel influenza virus vaccines that target the conserved epitopes of influenza viruses, specifically those in the hemagglutinin stalk and neuraminidase, are currently being developed3. Antibodies against the hemagglutinin stalk confer protection in animal studies4–6. However, no data exist on natural infections in humans, and these antibodies do not show activity in the hemagglutination inhibition assay, the hemagglutination inhibition titer being the current correlate of protection against influenza virus infection7–9. While previous studies have investigated the protective effect of cellular immune responses and neuraminidase-inhibiting antibodies, additional serological correlates of protection from infection could aid the development of broadly protective or universal influenza virus vaccines10–13. To address this gap, we performed a household transmission study to identify alternative correlates of protection from infection and disease in naturally exposed individuals. Using this study, we determined 50% protective titers and levels for hemagglutination inhibition, full-length hemagglutinin, neuraminidase and hemagglutinin stalk-specific antibodies. Further, we found that hemagglutinin stalk antibodies independently correlated with protection from influenza virus infection.Study of influenza virus transmission in humans provides evidence that hemagglutinin stalk-specific antibodies correlate with protection from infection.

While it remains impossible to predict the timing of the next influenza pandemic, novel avian influenza A viruses continue to be considered a significant threat. A Phase II study was conducted in healthy adults aged 18–64 years to assess the safety and immunogenicity of two intramuscular doses of pre-pandemic 2017 influenza A(H7N9) inactivated vaccine administered 21 days apart. Participants were randomized (n = 105 in each of Arms 1–3) to receive 3.75 μg, 7.5 μg or 15 μg of hemagglutinin (HA) with MF59® adjuvant, or 15 μg of HA unadjuvanted vaccine (n = 57, Arm 4). The three MF59 adjuvanted vaccines and the 15 μg unadjuvanted vaccine were safe and well-tolerated. Little antibody activity was detected against the A(H7N9) vaccine antigen after the first vaccination across study Arms. After second vaccination, the three adjuvanted Arms showed increases in hemagglutination inhibition (HAI), neutralizing (Neut), and neuraminidase inhibition (NAI) geometric mean titers (GMT), peaking at 21 days post second vaccination. The percentage of participants with titer ≥1:40 and seroconversion rates for HAI were 30–43 % and 0 for the adjuvanted Arms and the unadjuvanted Arm, respectively. Antibody responses against antigenically drifted A(H7N9) strains A/Shanghai/2/2013 and A/Guangdong/17SF003/2016 showed similar trends. Exploratory linear modeling of HAI and Neut responses post second vaccination revealed significantly lower log antibody titers among older participants (aged 35–49 and 50–64 years) compared to participants aged 18–34 years after adjusting for study vaccination, BMI, sex, and prior seasonal influenza vaccination. Post second vaccination, participants who received seasonal influenza vaccination in at least one of the two previous seasons had significantly lower log antibody titers than participants who did not. Adjuvanted doses of vaccine provided higher antibody responses, on average, than the 15 μg unadjuvanted vaccine. Proportion of participants achieving seroconversion and antibody titers ≥40 remained below 50 % in all study Arm. •The three dosages of MF59 adjuvanted A(H7N9) vaccine were safe and well-tolerated.•There were similar influenza antibody responses across the adjuvanted Arms.•MF59 adjuvant increased the magnitude and breadth of antibody responses.•Most participants in each Arm did not seroconvert against the vaccine strain.•There was little antibody response in the unadjuvanted 15 μg Arm.

•Influenza infection was less frequent in children after QIV versus control vaccine.•Efficacy against any circulating and antigenically matching strains was 54% and 68%.•QIV induced antibodies against HA and NA as well as virus neutralizing antibodies.•Revaccination with QIV elicited strong antibody responses for all strains.•The safety and reactogenicity of QIV was comparable with control vaccine. Children are an important target group for influenza vaccination, but few studies have prospectively evaluated influenza vaccine efficacy (VE) in children under 3 years of age. This was a randomized Phase III trial to assess the efficacy, immunogenicity, and safety of an inactivated quadrivalent influenza vaccine (QIV) in young children (EudraCT: 2016–004904–74). Influenza-naïve children aged 6–35 months were randomized during three influenza seasons to receive vaccination with QIV or a non-influenza control vaccine. One group of participants was revaccinated with QIV in the subsequent influenza season. The primary efficacy endpoint was the absolute VE of QIV against influenza caused by any circulating strain. Key secondary efficacy endpoints included the absolute VE of QIV against influenza due to antigenically matching strains and immunogenicity. Safety and reactogenicity were also evaluated. In total, 1005 children received QIV and 995 received control vaccine. Influenza A/B infection due to any circulating influenza strain occurred less frequently in children who received QIV versus children receiving a control vaccine. The absolute VE of QIV against any circulating influenza strain was 54% (95% confidence interval [CI]: 37%, 66%). The absolute VE of QIV against antigenically matching influenza strains was 68% (95% CI: 45%, 81%). Mean hemagglutination inhibition titers for all influenza strains in the QIV group increased post-vaccination, whereas increases were minimal in the control vaccine group; results from virus neutralization and neuraminidase-inhibition assays were generally consistent with the hemagglutination inhibition assay findings. Approximately 12 months after primary vaccination with QIV, antibody titers remained higher than pre-vaccination titers for most strains. In participants who were revaccinated, QIV elicited strong antibody responses. The overall safety profile and reactogenicity of QIV was comparable with control vaccine. Primary vaccination with QIV was well tolerated and effective in protecting children aged 6–35 months against influenza.

•High-dose trivalent influenza vaccine is licensed for adults ≥65 years of age.•A quadrivalent formulation with antigen from both B lineages has been developed.•Adding the second B strain improved immunogenicity against the added strain.•Immunogenicity against the other strains remained the same.•The vaccine’s tolerability was unaffected by adding the second B strain. A high-dose, split-virion inactivated trivalent influenza vaccine (IIV3-HD; Fluzone® High-Dose, Sanofi Pasteur) is available for adults ≥65 years of age. This study examined the safety and immunogenicity of a quadrivalent high-dose split-virion inactivated influenza vaccine (IIV4-HD). This was a randomized, modified double-blind, active-controlled, multi-center trial in healthy adults ≥65 years of age. Subjects were randomized in a 4:1:1 ratio to receive a single intramuscular injection of IIV4-HD, the licensed IIV3-HD, or an IIV3-HD containing the alternate B-lineage strain. Hemagglutination inhibition (HAI), seroneutralisation, and anti-neuraminidase antibody titers were measured at baseline and day 28. Solicited reactions were collected for up to 7 days, unsolicited adverse events up to 28 days, and serious adverse events up to 180 days. The primary immunogenicity objective was to demonstrate that IIV4-HD induces HAI geometric mean titers (GMTs) and seroconversion rates that are non-inferior to those induced by IIV3-HD. Secondary objectives were to describe the safety of IIV4-HD and IIV3-HD and to demonstrate that IIV4-HD induces HAI GMTs and seroconversion rates that are superior to those induced by IIV3-HD not containing the same B-lineage strain. The study included 2670 adults ≥65 years of age. For all four strains, HAI GMTs and seroconversion rates induced by IIV4-HD were non-inferior to those induced by IIV3-HDs containing the same strains. For both B strains, HAI GMTs and seroconversion rates induced by IIV4-HD were superior to those induced by IIV3-HD not containing the same B–lineage strain. Seroneutralisation and anti-neuraminidase antibody responses, measured in a subset of subjects, were similar. No new safety concerns were identified, and the safety profiles of IIV4-HD and IIV3-HD were similar. Adding a second B strain in IIV4-HD resulted in improved immunogenicity against the added strain without compromising the immunogenicity of the other strains or the vaccine’s tolerability. Clinical trial registration: NCT03282240.

Abstract Background Understanding the attack rate of influenza infection and the proportion who become ill by risk group is key to implementing prevention measures. While population-based studies of antihemagglutinin antibody responses have been described previously, studies examining both antihemagglutinin and antineuraminidase antibodies are lacking. Methods In 2015, we conducted a seroepidemiologic cohort study of individuals randomly selected from a population in New Zealand. We tested paired sera for hemagglutination inhibition (HAI) or neuraminidase inhibition (NAI) titers for seroconversion. We followed participants weekly and performed influenza polymerase chain reaction (PCR) for those reporting influenza-like illness (ILI). Results Influenza infection (either HAI or NAI seroconversion) was found in 321 (35% [95% confidence interval, 32%–38%]) of 911 unvaccinated participants, of whom 100 (31%) seroconverted to NAI alone. Young children and Pacific peoples experienced the highest influenza infection attack rates, but overall only a quarter of all infected reported influenza PCR–confirmed ILI, and one-quarter of these sought medical attention. Seroconversion to NAI alone was higher among children aged <5 years vs those aged ≥5 years (14% vs 4%; P < .001) and among those with influenza B vs A(H3N2) virus infections (7% vs 0.3%; P < .001). Conclusions Measurement of antineuraminidase antibodies in addition to antihemagglutinin antibodies may be important in capturing the true influenza infection rates. New Zealand’s seroepidemiological cohort study found that neuraminidase inhibition assay identified more influenza virus infections than hemagglutination inhibition assay. This result highlights the importance to measure serologically defined infections against not just hemagglutinin but also neuraminidase antigens in future seroepidemiologic cohort studies.

Seasonal influenza continues to cause significant morbidity and mortality, particularly for the elderly and immunocompromised. Current licensed influenza vaccines provide only partial protection even for immunocompetent hosts. Vaccine adjuvants can improve the magnitude and breadth of immune responses and there is considerable interest in identifying new adjuvants that can improve immune responses to seasonal influenza vaccines. This phase I, randomized, double-blind trial evaluated the safety and immunogenicity of one dose of 2018/2019 quadrivalent influenza vaccine (either Fluzone® or Flublok®) administered intramuscularly with or without one of two adjuvants (AF03 or Advax-CpG55.2). A total of 241 healthy adults aged 18–45 years were enrolled and randomized to 1 of 6 groups. Groups 1–3 received one dose of Fluzone® QIV 2018/2019 administered alone or with AF03 or Advax-CpG55.2 and Groups 4–6 received one dose of Flublok® QIV 2018/2019 alone or with one of these two adjuvants. All participants received Fluzone® or Flublok® QIV 2019/2020 ninety days later. Primary objectives were to evaluate safety and reactogenicity along with changes in hemagglutinin inhibition (HAI), neuraminidase inhibition (NAI) and neutralizing antibodies to 2018/2019 seasonal influenza antigens, comparing Day 1 and Day 29 titers. Secondary objectives evaluated the impact of adjuvants on immune responses after subsequent doses of unadjuvanted seasonal influenza vaccine and immunologic responses to heterologous influenza H1 and H3 antigens. Overall, the adjuvanted vaccines were safe and generated robust immune responses against both homologous and heterologous strains. Similar responses were seen across all six study arms. Both adjuvants were associated with qualitatively improved immune responses against some strains at varying timepoints, but results were inconsistent. There were no substantial differences in safety or reactogenicity identified between the study groups and all vaccine formulations were well tolerated. In this highly immunologically-experienced cohort, neither AF03 nor Advax-CpG55.2 demonstrated notable benefit when added to the seasonal influenza vaccine. (ClinicalTrials.gov ID# NCT03945825). •First human study to use the combination of Advax and CpG55.2 as an adjuvant.•Vaccine candidates induced robust antibody responses to multiple influenza strains.•Each of the vaccine candidates were safe and well-tolerated.•High baseline immunity may have overshadowed potential adjuvant effects.•Not adequately powered to identify significant differences between adjuvants.

•Not all influenza-infected people generate hemagglutination inhibition antibodies.•A subset of individuals respond exclusively to alternate viral targets of infection.•Including neuraminidase and hemagglutinin stalk could improve the influenza vaccine. We investigated humoral immune response to influenza A(H1N1)pdm infection and found 32 (22%) of the infected individuals identified by PCR failed to produce a ≥ 4-fold hemagglutinin inhibition assay (HAI) response; a subset of 18 (56%) produced an alternate antibody response (against full-length HA, HA stalk, or neuraminidase). These individuals had lower pre-existing HAI antibody titers and showed a pattern of milder illness. An additional subset of 14 (44%) did not produce an alternate antibody response, had higher pre-existing antibody titers against full-length & stalk HA, and were less sick. These findings demonstrate that some individuals mount an alternate antibody response to influenza infection. In order to design more broadly protective influenza vaccines it may be useful to target these alternate sites. These findings support that there are influenza cases currently being missed by solely implementing HAI assays, resulting in an underestimation of the global burden of influenza infection.

Background Influenza A virus (IAV) causes respiratory disease in pigs and is a major concern for public health. Vaccination of pigs is the most successful measure to mitigate the impact of the disease in the herds. Influenza-based virosome is an effective immunomodulating carrier that replicates the natural antigen presentation pathway and has tolerability profile due to their purity and biocompatibility. Methods This study aimed to develop a polyvalent virosome influenza vaccine containing the hemagglutinin and neuraminidase proteins derived from the swine IAVs (swIAVs) H1N1, H1N2 and H3N2 subtypes, and to investigate its effectiveness in mice as a potential vaccine for swine. Mice were immunized with two vaccine doses (1 and 15 days), intramuscularly and intranasally. At 21 days and eight months later after the second vaccine dose, mice were euthanized. The humoral and cellular immune responses in mice vaccinated intranasally or intramuscularly with a polyvalent influenza virosomal vaccine were investigated. Results Only intramuscular vaccination induced high hemagglutination inhibition (HI) titers. Seroconversion and seroprotection (> 4-fold rise in HI antibody titers, reaching a titer of ≥ 1:40) were achieved in 80% of mice (intramuscularly vaccinated group) at 21 days after booster immunization. Virus-neutralizing antibody titers against IAV were detected at 8 months after vaccination, indicating long-lasting immunity. Overall, mice immunized with the virosome displayed greater ability for B, effector-T and memory-T cells from the spleen to respond to H1N1, H1N2 and H3N2 antigens. Conclusions All findings showed an efficient immune response against IAVs in mice vaccinated with a polyvalent virosome-based influenza vaccine.

The emergence of drug resistant variants of the influenza virus has led to a need to identify novel and effective antiviral agents. As an alternative to synthetic drugs, the consolidation of empirical knowledge with ethnopharmacological evidence of medicinal plants offers a novel platform for the development of antiviral drugs. The aim of this study was to identify plant extracts with proven activity against the influenza virus. Extracts of fifty medicinal plants, originating from the tropical rainforests of Borneo used as herbal medicines by traditional healers to treat flu-like symptoms, were tested against the H1N1 and H3N1 subtypes of the virus. In the initial phase, in vitro micro-inhibition assays along with cytotoxicity screening were performed on MDCK cells. Most plant extracts were found to be minimally cytotoxic, indicating that the compounds linked to an ethnomedical framework were relatively innocuous, and eleven crude extracts exhibited viral inhibition against both the strains. All extracts inhibited the enzymatic activity of viral neuraminidase and four extracts were also shown to act through the hemagglutination inhibition (HI) pathway. Moreover, the samples that acted through both HI and neuraminidase inhibition (NI) evidenced more than 90% reduction in virus adsorption and penetration, thereby indicating potent action in the early stages of viral replication. Concurrent studies involving Receptor Destroying Enzyme treatments of HI extracts indicated the presence of sialic acid-like component(s) that could be responsible for hemagglutination inhibition. The manifestation of both modes of viral inhibition in a single extract suggests that there may be a synergistic effect implicating more than one active component. Overall, our results provide substantive support for the use of Borneo traditional plants as promising sources of novel anti-influenza drug candidates. Furthermore, the pathways involving inhibition of hemagglutination could be a solution to the global occurrence of viral strains resistant to neuraminidase drugs.