Researchers may be one step closer to creating a universal influenza vaccine after successfully testing a new sequential chimeric hemagglutinin (cHA) vaccination strategy in humans.

The immunodominant head domain of the viral hemagglutinin protein, which is targeted by typical flu vaccines, is variable and constantly changes as a result of antigenic drift. By administering two cHA constructs, spaced months apart, that shared the same hemagglutinin stalk but had divergent head domains, the research team was able to redirect the immune response from the head to the stalk.

Vaccination with these constructs, especially with adjuvanted inactivated chimeric hemagglutinin vaccines, induced a broad, strong, durable and functional immune response targeting the conserved, usually immunosubdominant, stalk domain of the hemagglutinin, Dr. Florian Krammer of the Icahn School of Medicine at Mount Sinai, in New York City, and colleagues report.

“The results suggest that chimeric hemagglutinins have the potential to be developed as universal influenza virus vaccines that protect broadly against influenza viruses,” they write in Nature Medicine.

In a press release, Dr. Krammer said the new vaccine candidate “is a major advance over conventional vaccines which are often mismatched to the circulating strains of virus, impacting their effectiveness. In addition, revaccinating individuals annually is a huge and expensive undertaking.”

He and his colleagues tested a regimen of two immunizations, the first with a cH8/1 construct followed 84 days later by a second with a cH5/1 construct, against saline placebos. They also tested different vaccination platforms (live-attenuated influenza vaccine, or LAIV, followed by an intramuscular booster of inactivated influenza virus vaccine, or IIV, vs. IIV both times) as well as the effect of GlaxoSmithKline’s AS03 adjuvant.

The researchers randomly assigned 65 young and healthy volunteers at multiple research sites to one of five experimental groups. The main immunological readout for the study was the level of anti-HA stalk antibodies as measured by enzyme-linked immunosorbent assays (ELISA) with a chimeric 6/1 HA (cH6/1) protein as the substrate.

“This substrate should be recognized by anti-H1 stalk antibodies, but since humans are naive to the H6 head domain, few anti-head antibodies will be detected,” the researchers explain, adding that this readout has been found to be an independent correlate of protection from H1N1 infection.

A single LAIV vaccination did not increase serum IgG titers against the stalk, but when LAIV-primed participants were boosted with IIV5/AS03, they induced a robust anti-stalk response. When the booster dose was given without the adjuvant, a lower induction of anti-HA stalk antibodies was observed and vaccinees had a more heterogeneous response.

Priming with IIV8/AS03 produced a “very strong” antibody response against the stalk. Serum-antibody titers decreased slightly between day 29 and day 85, the researchers report, but then rose again after the IIV5/AS03 booster to levels similar to day 29.

“Importantly, the new long-term follow up data show a slight decline between day 113 (28 days post boost) and day 252, but no further decrease in the IIV8/AS03-IIV5/AS03 group between day 252 and day 420 (titers persisting at approximately 2.25-fold above baseline),” Dr. Krammer and his colleagues note.

On day 588, the team used a different assay protocol that was previously found to correlate with protection in humans, yielding similar results to the other protocol.

“Since antibody longevity is highly important for a supra-seasonal influenza virus vaccine, the long term persistence data up to 1.5 years post vaccination measured in this assay were encouraging (approximately 4.3-fold above baseline at day 588 in the IIV8/AS03-IIV5/AS03 group),” the researchers note.

The team also tested antibody titers against H2, H9 and H18, all group 1 HAs, and for the group-2 H3. As described in a previous interim report (https://bit.ly/2JKSz0D), the responses against group-1 HAs “roughly followed the patterns observed for HA stalk-specific antibodies, with lower responses for the antigenically more distantly related HAs H9 and H18. Importantly, the additional longevity data shows that the broad responses are also maintained at least up to day 420 post vaccination.”

There was no increase in reactivity to H3 HAs except for a small increase after the prime in one group.

There were three unrelated serious adverse events, all in the placebo group. Solicited local and systemic reactions were common.

“Even though reactogenicity findings were relatively common and somewhat confounded by concurrent unrelated adverse events, all the study vaccines were found to have acceptable solicited adverse event profiles comparable to similar approved vaccines,” the researchers write.

They add, “Due to theoretical concerns over vaccine associated enhanced respiratory disease, we monitored subjects for ILI in 2 consecutive winters during the study. There was no increase in the frequency or severity among study subjects. Throughout the 18- to 21-months of follow-up, there were no observed safety concerns associated with the administration of cH8/1N1 LAIV, cH8/1N1 IIV adjuvanted with AS03, or cH5/1N1 IIV with or without AS03 adjuvant.”

The team concludes, “Development of group 2 cHA and influenza B mosaic HA vaccine candidates is currently ongoing. These constructs need to be combined into a trivalent vaccine that will potentially protect against all drifted seasonal, zoonotic and emerging pandemic influenza viruses. While much more work is needed to further develop this vaccine concept, this first-in-human study has opened up a new avenue for the development of truly universal influenza virus vaccines.”

The study was supported by the Bill and Melinda Gates Foundation, GlaxoSmithKline and others. The Icahn School of Medicine at Mount Sinai has issued patents and filed patent applications covering the use of chimeric hemagglutinin antigens as vaccines, and several authors are named as inventors on these.