The Northern Hemisphere faces an early and potentially challenging influenza season, marked by the widespread dominance of the A/H3N2 K subclade. Health authorities across Europe, Asia, and North America report an unusual surge in flu activity several weeks ahead of typical patterns. This specific strain, while not exhibiting increased virulence, is raising concerns due to its rapid spread and implications for population immunity and healthcare systems already under pressure from various respiratory viruses.

Reports from Japan, where an unusually high number of cases emerged in October, signaled the early onset. Spain has also exceeded its epidemic threshold sooner than anticipated, reflecting a broader trend across the European Union. Similar patterns are now emerging in the United Kingdom, the United States, and Canada, where the A/H3N2 K subclade accounts for nearly 90% of flu samples in some regions, establishing its clear predominance.

This subclade, previously known as J.2.4.1, has been detected on all continents and represented a significant proportion of A/H3N2 viruses analyzed globally between May and November. Genetic analysis confirms that subclade K is a result of expected mutations within the long-circulating H3N2 virus, which has been present in humans since 1968. Scientists affirm these genetic changes do not stem from recombination or major structural alterations.

Crucially, experts have found no evidence linking the mutations in subclade K to increased virulence or more severe disease outcomes. Countries in East Asia, which have already experienced a decline in subclade K cases, did not report unusual illness severity. Furthermore, analyses indicate that the subclade K strains circulating in these regions are consistent with those currently identified in Europe, offering reassurance that this particular variant is not inherently more dangerous.

However, the dominance of A/H3N2 poses a distinct challenge this season. In recent years, A/H1N1 has been the prevalent influenza A strain, potentially leading to lower population immunity against A/H3N2 due to reduced recent exposure. Historically, seasons where A/H3N2 predominates tend to be more severe, often resulting in higher rates of hospitalisation and more serious complications, particularly among older adults.

The current seasonal flu vaccine was formulated based on predictions from the previous year, designed to protect against a mix of H1N1, influenza B, and an A/H3N2 type that predates the emergence of the K subclade. While some analyses suggest a divergence between the subclade K and the vaccine’s H3N2 component, real-world data on vaccine effectiveness remains limited. Monitoring this divergence closely is vital to assess the level of protection offered this season.

Despite potential mismatches, health authorities universally stress that vaccination remains the most effective tool against severe illness, hospitalisation, and death. Even in seasons with reduced vaccine efficacy against specific circulating strains, immunisation still provides substantial protection. Therefore, the vaccine is strongly recommended for all eligible individuals, especially given the early start of the epidemic and the time required for full immune response to develop post-vaccination.

The risk to the general population currently appears moderate, but it is significantly higher for vulnerable groups. This includes individuals over 65, those with underlying health conditions, pregnant women, and immunocompromised people. Alongside vaccination, maintaining rigorous hand hygiene and wearing masks when experiencing symptoms are crucial preventative measures to help curb transmission and protect those most susceptible to severe outcomes, mitigating pressure on healthcare services.

Read Original

Keywords: A/H3N2 K, flu season 2024, influenza A virus, vaccine efficacy, respiratory viruses, public health advisory, early flu surge, healthcare system pressure