#19,073
Last week Spain reported a rare human infection with H1N1v swine influenza in a patient in Catalonia with no known history of exposure to pigs, or to a contaminated environment.
While reports like this are not common, it is assumed that these cases are largely underreported, since symptoms are likely to be mild-to-moderate, and most people don't get tested for uncomplicated flu.
We've seen estimates that perhaps 1 in 100 swine variant cases here in the United States are actually picked up by surveillance (see CID Journal: Estimates Of Human Infection From H3N2v (Jul 2011-Apr 2012).
Results. We estimate that the median multiplier for children was 200 (90% range, 115–369) and for adults was 255 (90% range, 152–479) and that 2055 (90% range, 1187–3800) illnesses from H3N2v virus infections may have occurred from August 2011 to April 2012, suggesting that the new virus was more widespread than previously thought.
H1N2 variant [A/California/62/2018] Jul 2019 5.8 5.7 Moderate
H3N2 variant [A/Ohio/13/2017] Jul 2019 6.6 5.8 Moderate
H3N2 variant [A/Indiana/08/2011] Dec 2012 6.0 4.5 Moderate
But there is much diversity among swine flu viruses around the globe, with China's EA H1N1 `G4' virus often cited as the biggest pandemic threat. We've also followed repeated spillovers in Brazil, and last year the Eurasian 1C Swine Influenza A Virus was labeled a `high pandemic risk'.
The reality is, surveillance and testing for swine influenza A viruses is notoriously sub-optimal, and many strains circulate under the radar.
While swine influenza A viruses share the same HA and NA types as seasonal influenza (H1N1v, H1N2v, H3N2v, H3N1v, etc.) they are often antigenically distinct, making the value of deploying existing seasonal flu vaccines in the opening months of a swine flu pandemic unknown.
In today's study, researchers from the Francis Crick Institute in London, and Italy's WOAH Reference Laboratory for Swine in Influenza tested 5 European lineages of swine influenza (see table below) against components of recent seasonal flu vaccines.
What they found was seasonal flu boosters offered moderate protection against some European swine flu strains (notably, 1C.2.1), but little or no protection against others, like 1A.3.3.2.
Investigation of Potential Cross-Protection Conferred by the Seasonal Influenza Vaccine Against Swine Influenza A Viruses of Pandemic Potential
Alice Lilley1,*, Chiara Chiapponi2, Alice Prosperi2, Ana Moreno2, Laura Soliani2, Nicola Lewis1 and Ruth Harvey1Vaccines2026, 14(3), 211;https://doi.org/10.3390/vaccines14030211
Academic Editor
Published:25 February 2026Abstract
Background/Objectives: Influenza A viruses cause seasonal epidemics of respiratory infections in humans, the severity of which can be mitigated by influenza vaccine use. Influenza A viruses circulating in pigs continue to pose a pandemic threat, as evidenced by the influenza virus that caused the 2009 pandemic, which originated in pigs. To understand the relative risk of emergence of influenza A viruses from pigs and to assess the potential role of the seasonal influenza vaccine in mitigating this risk, we evaluate the potential cross-protection afforded by the seasonal influenza vaccine against different clades of recently circulating swine influenza A viruses.
Methods: The presence of cross-reactive antibodies in pre- and post-vaccination human serum samples was measured in haemagglutination and microneutralisation assays. Representative H1 swine influenza A viruses from different genetic lineages were tested against sera collected after administration of the seasonal influenza vaccine in healthy adult volunteers over a 6-year time-period.
Results: Although a clade-dependent boosting of post-vaccination antibody titres was observed, protective titres often failed to be reached. There was heterogeneity in recognition by sera for the contemporary swine influenza A viruses, with the 1C.2.1 clade virus being well recognised in both assays, whilst very low pre- and post-vaccination antibody titres were observed against the 1A.3.3.2 clade (which emerged in pigs following the reverse zoonotic introduction from humans of the A/H1N1 pdm09 virus) by both assays.
Conclusions: Seasonal influenza vaccines produce cross-reactive antibodies against some clades of influenza A viruses circulating in pigs, but not all. Depending on the lineage and clade of the virus, the seasonal influenza vaccine might have utility in the event of a swine variant outbreak in humans, whilst a specific vaccine against the outbreak strain is developed.
A systematic review found that post-influenza vaccination antibody levels waned within six months but remained higher than pre-vaccination titres [31]. The administration of a booster dose of vaccine in the event of a swine influenza outbreak in humans is likely to help boost antibody titres in those who have previously been vaccinated, except for the 1A.3.3.2 virus.
However, only two time points (almost one year apart) are represented here, and therefore, conclusions about antibody titres at other time points cannot be made. Additionally, volunteers did not report influenza infections occurring during this time period, which could reduce the appearance of waning immunity in the cohort.
These results suggest that the seasonal vaccine does not induce cross-reactive antibodies against all lineages, or even all clades within a lineage, of swine H1 influenza A viruses.
The value of the seasonal vaccine in the event of a swine influenza outbreak will be dependent on the strain of virus, as will the benefit of administering a booster dose to those who have already been vaccinated. However, due to the availability of the seasonal vaccine, its use should be considered in response to a swine influenza outbreak while a strain-specific vaccine is produced.
Continued surveillance of SwIAV and development of candidate vaccine viruses is vital due to low population immunity in humans. Moreover, these results highlight the need for the continued promotion of influenza vaccination due to its ability to induce cross-reactive antibodies against some H1 swine influenza viruses.
While not a home run, there is enough evidence here to consider seasonal flu boosters in the opening months of a swine flu pandemic, at least against some European lineages.
Whether the same benefits might be expected against other strains - from North America, Asia, or South America - isn't clear.
But as the authors point out, this is yet another reason to get the seasonal flu vaccine each year, as it might provide some degree of baseline immunity against a swine flu pandemic.
And any amount of protection in a pandemic beats none at all.
