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.
