On 27 November 2023, the United Kingdom Health Security Agency (UKHSA) reported the first known human case of influenza A (H1N2)v in the UK, a variant of the influenza virus previously not known to cause infections in humans. Following detection, public health agencies in the UK are now working rapidly towards characterizing the pathogen and assessing the risk it may present to human health.
Several subtypes of the influenza virus are assigned according to combinations of mutations in the proteins on the surface of the virus - hemagglutinin (H) and neuraminidase (N). The former binds to sialic acid receptors in the host. New influenza viruses can emerge through a process called reassortment where the co-infecting viruses could swap genomic fragments. Influenza A H1N2 is a subtype of influenza that is endemic in pigs and is rarely reported in humans. Swine influenza viruses normally do not infect humans, however, in rare cases, sporadic infections can occur especially with close contact with animals carrying the virus. In such occurrences, the infecting influenza strain is known as a “variant virus” denoted by adding “v” after the subtype.
The case of influenza A (H1N2)v was detected as part of the comprehensive national influenza surveillance in the UK led by UKHSA. The patient, a 75-year-old individual, who has now fully recovered, presented with mild respiratory and flu-like symptoms. Genome sequencing revealed that the virus belonged to a distinct clade 1B.1.1 of influenza viruses. While distinct from recent human cases of A(H1N2), its genetic makeup showed similarity to swine influenza viruses found in the UK and no other mutations of concern have been detected in this variant. Although the patient resides in an area in proximity to pig farms, no direct contact with animals could be established. Instances of human-to-human transmission of the virus have also not yet been detected, however, the detection of the virus in a case with no contact with animals could possibly mean that a limited and undetected human-to-human transmission may have occurred. “But there is no definitive evidence,” says the WHO.
Sporadic human infections with A(H1N2)v have been detected previously. Since 2005, almost 50 cases of human A(H1N2)v infections have been reported from across the world, however, this is the first time that a case has been reported from the UK. Recently in August 2023, a human infection was reported in the United States of America linked to an agricultural fair. Cases have also been reported in Austria, Denmark, France, Netherlands, Brazil, Canada, China and Australia. Infections with H1N2 variant viruses generally are similar in symptoms and severity to seasonal influenza viruses and there is limited evidence of human-to-human transmission. “Current evidence suggests that these swine-origin influenza viruses have limited ability for sustained transmission among humans,” the WHO says.
According to the UKHSA, the variant influenza virus presents a low risk to the general public. However, people with direct regular exposure to pigs may be at a higher risk of acquiring this virus. Influenza viruses have the potential to spill over into human populations and cause severe illnesses although the variant virus detected in the UK does not show any characteristics suggestive of pandemic potential. As public health authorities in the UK continue to follow up on the case, it is important to underscore comprehensive surveillance of pathogens, particularly rapidly evolving respiratory viruses like influenza and ability to rapidly characterize viral variants using genome sequencing forms the cornerstone of public health action and preparedness and our ability to mitigate the risk of a global outbreak.
(Bani Jolly is a senior scientist at Karkinos Healthcare and Vinod Scaria is a senior consultant at Vishwanath Cancer Care Foundation)
