#19,187
If there is one constant with influenza A viruses, it is that they are constantly changing; meaning that things that we may have assumed to be true for years or even decades can abruptly change.
One such `truism' has been that H1N1 viruses pose more of a risk to younger individuals - those born after the 1957 H2N2 pandemic emerged - than to the elderly (see PLoS Path.: Childhood Immune Imprinting to Influenza A).
While older individuals weren't immune to H1N1, they have tended to be less susceptible to infection - and typically saw milder disease - than from the H3N2 subtype.
Although it was overshadowed by the outbreaks of early May (hantavirus & Ebola), the JID reported a similar finding (see Effects of Age and Birth Cohort on Influenza A Virus Subtype-Specific Hospitalization Rates, United States, 2010–2025) in late April.At least - according to a preprint published last week - until 2022, when a new clade of H1N1 emerged (6B.1A.5a.2a) after the COVID-19 induced lull, which appears to have become more impactful on the elderly.
Assuming these reports are correct, the notion that the elderly might enjoy a bit of a respite during H1N1 centric flu seasons may no longer be true, and that could substantially increase the burden on healthcare facilities.
An additional caveat: it has not been established whether (or how strongly) this pattern will persist as H1N1 continues to evolve. Clade 5a.2a has already been largely replaced by newer subclades (see week 20 FluView), so ongoing surveillance and analysis is needed.
First, the link and some excerpts from the preprint, but the full report is very much worth reading.
Increased burden of influenza A/H1N1pdm09 in older adults following the COVID-19 pandemic
Simon P. J. de Jong, Colin A. Russell
doi: https://doi.org/10.64898/2026.05.20.26353664
This article is a preprint and has not been peer-reviewed
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Abstract
Of the two influenza A virus (IAV) subtypes circulating endemically in humans, A/H3N2 and A/H1N1pdm09, A/H3N2 has historically been the dominant driver of disease burden in older adults. Based on an analysis of publicly available global surveillance data from 2015 to 2025 (>300,000 subtyped, age-stratified infections), we report a substantially increased contribution of A/H1N1pdm09 to influenza morbidity in older adults since approximately 2022.
Birth cohort-stratified analyses suggest elevated A/H1N1pdm09 burden among individuals born before 1955-1959, consistent with erosion of pre-existing immunity originally generated by exposure to historical A/H1N1 strains. Pooled estimates across datasets and analytical approaches indicate the increase in A/H1N1pdm09 burden rises with earlier birth year, ranging from 1.22-fold (95% CI 1.08-1.37) for the 1955-1959 birth cohort to 3.10-fold (95% CI 2.58-3.72) for the 1930-1934 cohort. These findings point to a substantial rise in the overall influenza burden among the most vulnerable age groups, with implications for vaccine policy, clinical management, and public health planning.
Main text
Older adults are at markedly elevated risk of severe outcomes from influenza. Of the two influenza A virus (IAV) subtypes currently circulating in humans, A/H3N2 and A/H1N1pdm09, A/H3N2 has historically disproportionately contributed to influenza burden in older adults1,2. However, emerging evidence suggests possible shifts in this epidemiological landscape. A recent US-based study reported increasing contributions of A/H1N1pdm09 virus infections to influenza hospitalizations among older adults since the COVID-19 pandemic3.
(SNIP)
The apparent increases in A/H1N1pdm09 risk are greatest in the earliest birth cohorts (Fig. 2). This could have substantial clinical and public health implications, as risk of severe influenza disease and mortality increases strongly with age5,6. As such, our results have implications for healthcare capacity planning, with likely substantially elevated acute and post-acute healthcare demand due to influenza compared to a counterfactual in which no immune erosion had occurred. The severity of the 2024/2025 US influenza season potentially reflects the erosion of protection in early birth cohorts3, with adults aged 75 and over experiencing the highest rates of A/H1N1pdm09-associated hospitalization since the 2009 pandemic16.
Vaccines have demonstrated effectiveness against A/H1N1pdm09 among adults aged 65 and over in the post-pandemic period17,18, but new targeted analyses in the earliest birth cohorts, for which estimated increase in burden is greatest (Fig. 2), would be valuable. A potential concern is that individuals whose immunity was shaped predominantly by early-life responses to historical A/H1N1 epitopes may, through repeated boosting of those same responses, lack immunity to epitopes on strains circulating since 2022, leaving them with little cross-reactive protection to recall upon vaccination or infection. These uncertainties notwithstanding, the elevated A/H1N1pdm09 risk suggested here further strengthens the case for vaccination in these birth cohorts and has implications for vaccination strategies, including the value and cost-effectiveness of enhanced formulations which have demonstrated superior immunogenicity and effectiveness in older adults19.
The US and Brazil hospitalization data indicate increased burden of severe disease due to A/H1N1pdm09, but whether this is due to changes in infection risk and/or changes in clinical severity warrants further investigation, particularly given that A/H1N1pdm09 was associated with relatively severe outcomes among hospitalized adults before the COVID-19 pandemic20,21.
Our analyses rely on observational data and are susceptible to reporting biases; however, consistency across four surveillance systems with distinct ascertainment mechanisms and varying analysis methods makes a surveillance artifact implausible. Replication with additional age- and subtype-resolved surveillance data would further substantiate our findings.
Together, our results indicate that the earliest birth cohorts, who are the most clinically vulnerable and have historically been considered relatively protected from influenza A/H1N1pdm09, likely now face substantially elevated risk from these viruses.
This carries potential implications for vaccine policy and healthcare capacity planning. Ensuring that surveillance infrastructure captures age- and subtype-resolved influenza data globally will be essential for tracking this shift and quantifying its consequences for the populations most at risk for severe influenza.(Continue . . . )
We tend to think that next year's seasonal flu will be pretty much like last year's, and the year before. But even non-pandemic flu viruses can throw us curves.
- In 2014, we saw a late summer mutation appear in the H3N2 virus (see HAN Advisory On `Drifted’ H3N2 Seasonal Flu Virus) which greatly reduced the effectiveness of that year's flu vaccine.
- in 2016, we saw warnings over mutated (D225G) H1N1 viruses in Russia and in Europe (see ECDC Risk Assessment : Reports Of Severe A(H1N1)pdm09 In Europe);
- Again in 2025, a drifted (Subclade K) H3N2 emerged which impacted vaccine effectiveness
- And while it has had only limited impact so far, we are now watching another slow rise in antiviral resistance in H1N1.
