Octavalent intranasal flu vaccine proves effective in neutralizing both existing and emerging strains

By Dr. Liji Thomas, MDReviewed by Benedette Cuffari, M.Sc.Aug 25 2024

Study: Intranasal administration of octavalent next-generation influenza vaccine elicits protective immune responses against seasonal and pre-pandemic viruses. Image Credit: Mongkolchon Akesin / Shutterstock.com

A recent study published in the Journal of Virology discusses a recent vaccine candidate that has the potential to neutralize both existing and emerging flu viruses effectively.

Modern flu vaccines

The three human influenza viruses, A, B, and C are represented as IAV, IBV and ICV, respectively. Each type has multiple 18 and 11 subtypes distinguished by their hemagglutin (HA) and neuraminidase (NA) surface glycoproteins, respectively.

H1N1 and H3N2 IAV strains and IBV circulate extensively in humans. Comparatively, H2, H5, and H7 subtypes are endemic in birds.

Current flu vaccines are based on three or four seasonal subtypes of circulating human strains and range in their effectiveness from 10% to 60%

The high mutation rate and selective immune pressure of influenza cause frequent antigenic variation to occur, enabling immune evasion. Thus, updated vaccine formulations are necessitated every year. Emerging viral variants could also cause a new pandemic if they escape pre-existing immunity.

What are recombinant antigens?

The current study sought to develop a universal recombinant vaccine candidate that could protect against multiple influenza virus strains over multiple seasons.

Recombinant antigens can overcome the limitations of standard split-inactivated or attenuated vaccines. Conventional flu vaccines are produced in embryonated chicken eggs, and they contain egg-specific adaptive mutations that limit the induction of virus-specific immunity.

In contrast, recombinant antigens target specific antigenic sites that elicit neutralizing antibodies. These work effectively or better than conventional vaccines in normal and high-risk groups, reducing their risk of hospitalization.

About the vaccine

In the current study, researchers utilized the computationally optimized broadly reactive antigen (COBRA) methodology to isolate numerous sequences from thousands of isolates. Thereafter, broadly reactive immunogens that elicit immune responses to multiple strains circulating over many seasons were identified.

In a previous mouse experiment, an intramuscular heptavalent vaccine containing COBRA HA and NA immunogens protected mice against both seasonal and potential pandemic IAV strains. However, intramuscular vaccines fail to induce local mucosal immunity.

Comparatively, intranasal vaccines stimulate local and systemic immune responses against respiratory pathogens. This prevents viral colonization at the entry site, thereby reducing the likelihood of viral transmission and severe illness. Furthermore, intranasal vaccines are less painful, more convenient, require less skill, and are associated with fewer complications as compared to intramuscular vaccines.

Octavalent COBRA immunogen

In the current experiment, octavalent combinations of COBRA HA antigens H1, H2, H3, H5, and H7, IBV, and NA N1 and N2 recombinant antigens were studied.

Bis-(3,5)-cyclic dimeric adenosine monophosphate (c-di-AMP) was used as an adjuvant to stimulate type 1 interferon (IFN) and tumor necrotic factor α (TNF-α) antiviral immune responses. Previously, c-di-AMP has been shown to promote both mucosal and systemic immunity and overcome immune tolerance while preventing vaccine breakdown.

The vaccine candidate was administered intranasally to either naïve or pre-immune ferrets, whereas a control vaccine was administered to another group of pre-immune animals. Ferrets are the model of choice for studying influenza infections, as they exhibit similarities to humans in many key aspects of infection and immunologic response.

Vaccine immunogenicity

In the pre-immune group, immunoglobulin G (IgG) antibodies to H1, N1, and N2 rose by 60 days from infection. Following vaccination, all eight antigenic components induced IgG binding antibodies.

In the control group, antibody titers were unchanged, whereas H1 antibody titers decreased.

Anti-stem antibodies

Pre-immune but not naïve ferrets had anti-HA and anti-NA antibody titers at baseline. After vaccination, antibody titers rose in the pre-immune group, whereas naïve ferrets exhibited high binding antibody levels against all eight components of the vaccine. In the control group, titers remained unchanged, except for a reduction in anti-H1 antibodies.

Baseline anti-group 1 HA stem antibody titers were high in pre-immune ferrets but not in group 2 stem antibodies. The vaccine elicited a strong rise in cross-reactive antibody titers against both group 1 and 2 stem antigens in pre-immune and naïve ferrets.

HAI activity

In the pre-immune group, hemagglutination inhibition (HAI) titers to the post-pandemic H1N1 strains isolated in 2009, 2015, 2018, and 2019 were significantly higher than baseline levels. This immune response is significant, as HAI activity in earlier strains was very low.  

Pre-immune ferrets lacked detectable HAI antibodies to H3N2 strains at baseline. After vaccination, HAI titers against four of the six H3N2 strains significantly increased.

The lowest titers were observed in the pre-immune control ferrets. HAI activity against the 2015 and 2018 strains decreased fourfold by day 56 post-vaccination, compared to an eightfold reduction against the 2019 strain.

Anti-IBV antibodies

Cross-reactive anti-IBV HAI antibodies were produced against both IBV strains following infection with one IBV lineage.

In pre-immune ferrets, post-vaccination HAI titers were observed against all IBV strains, irrespective of lineage. The naïve vaccinated group exhibited a small increase in HAI titers against the infecting lineage only.

Anti-pre-pandemic antibodies

Baseline HAI activity was not observed against any pre-pandemic strains. After vaccination, pre-immune and naïve groups exhibited detectable HAI activity against H2, H5 and H7 antigens.

Higher anti-H5 HAI titers were achieved in pre-immune ferrets with baseline antibodies to H1N1, H3N2, and IBV strains after vaccination as compared to naïve ferrets.

NAI activity

NA inhibition (NAI) titers to N1, but not N2, were detected at baseline in pre-immune ferrets. Following vaccination, NAI activity against both N1 and N2 increased.

Protection against infection

The vaccine candidate protected naïve and pre-immune ferrets against illness following infection with H1N1, IBV, and H5N1. The greatest protection was observed in the pre-immune vaccinated group.

Mucosal immunity was also highest in this group; however, both pre-immune and naïve ferrets exhibited undetectable viral titers in their nasal wash fluid by day five following immunization.

Conclusions

The mixture of eight COBRA HA/NA proteins mixed with an intranasal adjuvant is a promising candidate for a universal influenza vaccine.”

The novel vaccine formulation developed in the current study provided a balanced helper/cytotoxic/inflammatory T-cell response while also inducing the production of a broad range of IgG antibodies against virus-specific epitopes. Moreover, this octavalent vaccine elicited a cross-reactive response that was effective against new strains, which is a crucial aspect of a potential universal flu vaccine in the future.

All three ferret groups exhibited cross-reactive protection against lethal H5N1 infection, which may be due to the presence of antibodies to the conserved HA stem. These may neutralize this strain by inhibiting fusion or producing Fc-receptor-mediated antibody-dependent cellular cytotoxicity.

Future studies are needed to examine the role of immune imprinting in these post-vaccination responses. The use of additional doses to counteract the immunodominance of H1 has the potential to enhance HAI responses to IBV or H3N2 strains in naïve ferrets.