A study published in the journal Vaccine warns that the global population is rapidly losing immunity to the H2N2 influenza subtype, a virus that caused a devastating pandemic in 1957.
The study notes that while individuals born before 1968 retain long-lasting immune memory, the majority of the global population now lacks H2-specific immunity. Cross-reactive responses from N2 and T-cells may offer only limited protection against severe disease.
The 1957 Pandemic: A Historical Warning
The 1957 pandemic, triggered by a reassortment between human H1N1 and avian H2N2, caused an estimated 1 to 4 million deaths. H2N2 circulated in humans for 11 years before being replaced by H3N2 in 1968. However, the virus has not disappeared; H2 viruses continue to circulate in animal reservoirs, maintaining the risk of reintroduction.
A Declining Shield of Immunity
Seroprevalence data reveals a stark and accelerating decline in population immunity:
The proportion of the population with H2 immunity is falling, from 100% in 1968 to an estimated 15.3% in 2025, and is projected to drop to just 2.9% by 2055.
This means an overwhelming majority of the world is now immunologically naive to this potential pandemic threat.
Promising Vaccine Platforms
Clinical trials of H2-specific vaccines have shown significant promise. Key findings include:
- Inactivated whole-virus vaccines achieved seroconversion rates of 88-100% in H2-naïve individuals after just two doses.
- Ferritin nanoparticle vaccines induced a 13- to 25-fold increase in neutralizing antibodies.
- Some vaccine candidates also induced cross-reactive antibodies against other influenza subtypes, offering broader protection.
A Call for Preparedness
The review concludes with a clear recommendation for global health authorities:
Development of a stockpile-ready H2 vaccine, utilizing modern platforms such as mRNA, viral vectors, or nanoparticle-based designs, is important for pandemic preparedness.