Why H2N2 flu is back on the pandemic preparedness radar

by · News-Medical

Decades after H2N2 vanished from humans, declining immunity and persistent animal reservoirs are renewing calls for stockpile-ready vaccines against a historically pandemic-capable flu subtype.

Review: Seven decades after the Asian influenza pandemic: A historical review about immunity and vaccines against H2N2. Image Credit: pedrosek / Shutterstock

Review findings highlight how modern research is now pivoting toward advanced vaccine platforms, including ferritin nanoparticles and DNA-based delivery, to mitigate the risk of a reintroduction from animal reservoirs where H2 viruses continue to circulate.

1957 Asian Influenza Pandemic Background

In February 1957, a new strain of influenza A (IAV) emerged in China, marking the start of the "Asian influenza" pandemic. Subsequent research has found that the 1957 H2N2 pandemic was triggered by a genetic reassortment, a process where two different influenza viruses infect the same host and swap genomic segments, between a circulating human H1N1 virus and an avian H2N2 subtype.

This rare chance event resulted in a virus carrying three new genomic segments from the avian source: the hemagglutinin (HA), the neuraminidase (NA), and the polymerase basic (PB) protein 1. Because most of the global human population was immunologically naïve to the H2 HA, this novel virus spread globally within months, resulting in an estimated 1 to 4 million deaths.

H2N2 Review Scope and Vaccine Evidence

This review evaluates 7 decades of virological and immunological evidence to assess global pandemic preparedness, particularly regarding a potential resurgence of H2N2. The review analyzed the evolution of the H2N2 virus during its 11-year human tenure, identifying at least 25 amino acid changes within the HA protein, including 12 HA1-domain changes considered important for antigenicity, that contributed to antigenic drift, the gradual accumulation of mutations that allow a virus to evade the immune system.

Review analyses further focused on the virus's shift in receptor specificity: avian H2N2 viruses typically bind to α2,3-linked sialic acids in the bird intestine, but pandemic and later human-adapted strains acquired substitutions that shifted receptor preference toward α2,6-linked receptors in the human respiratory tract.

The analyses also synthesize data from contemporary clinical trials of H2-specific vaccines. Inactivated and live-attenuated vaccines include a randomized bioequivalence study of monovalent H2N2 vaccines (A/Singapore/1/57) and Phase I trials of cold-adapted, temperature-sensitive strains. Advanced nanoparticle platforms include the first-in-human trial (NCT03186781), which evaluated a ferritin nanoparticle vaccine in which the HA ectodomain, the outer part of the protein, was fused to a bacterial ferritin protein to enhance immune recognition. Seroprevalence and immunological studies span diverse groups, including H2-naïve individuals born after 1968 and H2-exposed individuals born before 1968, to assess the effectiveness of immune priming.

H2 Immunity Gap and Vaccine Trial Findings

This review’s demographic calculation based on birth cohorts identifies an immunity gap wherein the percentage of the population with H2 immunity is rapidly decreasing: from 100% in 1968 to an estimated 15.3% in 2025. Furthermore, these calculations project that these declines will worsen to just 2.9% by 2055.

These findings are corroborated by historical data from the 1957 pandemic, which showed an exceptionally high attack rate of up to 60%, particularly among younger age groups.

Recent clinical trials demonstrate the challenges and successes of modern vaccination. In terms of immunogenicity, in a study of H2-naïve individuals, two doses of an inactivated whole-virus vaccine achieved a seroconversion rate (the development of detectable antibodies) of 88% to 100%.

Regarding nanoparticle efficacy, a study of a novel ferritin nanoparticle vaccine found that it induced a 13- to 25-fold increase in neutralizing antibodies in naïve participants after a prime-boost regimen.

In terms of cellular immunity, a Phase I trial of a live-attenuated influenza virus showed that while 85% of participants developed detectable antibodies, 56% also showed detectable cellular immune responses, which may help limit disease severity.

Overall, the review highlights that the H2N2 subtype represents a unique threat because it has already demonstrated pandemic capability in humans and remains active in animal reservoirs. While individuals born before 1968 retain high-affinity memory B cells capable of rapid responses, as evidenced by 4.4- to 6.6-fold increases in titers after vaccination, the majority of the current population may require complex prime-boost schedules to achieve similar protection.

H2 Vaccine Preparedness and Public Health Implications

The review synthesizes historical, virological, immunological, and vaccine evidence highlighting the potential dangers of a global H2N2 outbreak and the need to strengthen global readiness of vaccination and public health facilities.

Its findings emphasize that while some limitations remain, for example, most current seasonal vaccines do not provide cross-reactive H2 immunity, the authors conclude that the development of a "stockpile-ready" H2 vaccine, potentially utilizing modern platforms such as mRNA, viral vectors, adjuvanted vaccines, nanoparticle-based designs, or broader HA- and NA-targeted strategies, is an essential pillar of 21st-century pandemic preparedness against this historically pandemic-capable influenza subtype.

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