Influenza A(H1N2) Virus in Humans

Influenza A viruses are enzootic in swine populations in most regions of the world. When an influenza virus that normally circulates in swine is detected in a person, it is called a 'variant influenza virus'. A(H1N1), A(H1N2) and A(H3N2) are major subtypes of Influenza A viruses circulating in pigs and occasionally do infect humans.

Sporadic human infections with swine Influenza A viruses have been reported since the late 1950s, usually after direct or indirect exposure to pigs or contaminated environments. Since 2018, sporadic influenza A(H1N2)v cases among humans have been detected in other countries in Europe (Austria, Denmark, France and the Netherlands), the Americas (Brazil, Canada and the United States of America), Asia (China) and in Australia. Infections in humans have primarily been acquired through direct contact with infected animals or contaminated environments. Risk factors include proximity to infected pigs or visiting locations where pigs are exhibited. However, in some cases, no known exposure to swine was reported.

On 25 November 2023, the United Kingdom notified the WHO of a human case of Influenza A(H1N2)v virus infection. The case reported onset of mild symptoms on 5 November 2023. The male patient visited his general practitioner on 9 November 2023. During this visit, a respiratory sample was collected and further analyzed as part of the national routine Influenza surveillance programme. On 23 November 2023 the sample was tested positive for Influenza A(H1N2)v virus. On 25 November, a follow-up RT-PCR test was performed and tested negative for influenza. The patient now fully recovered. Thus far we've seen no indication of additional cases, although contract tracing is apparently still ongoing.

Further analysis indicated that this Influenza A(H1N2)v virus belongs to the swine influenza virus genetic clade 1B.1.1. Similar A(H1N2) viruses from this genetic clade have been previously detected in pigs in the United Kingdom. However, this is the first time a virus from this swine genetic clade has been detected in a human in the United Kingdom.

It is still a mystery how the patient got infected - it has since been confirmed that the patient does not work with pigs or keep any as pets.