Everything you need to know about H1N2 “swine flu”
New swine flu variants are worrying, but there’s no evidence of pandemic potential.
- 4 December 2023
- 4 min read
- by Linda Geddes
In December 2023, a man in northern England was infected with a flu virus related to that is circulating in British pigs – yet they’d had no known contact with these animals.
What is swine flu?
Swine flu is a respiratory disease of pigs caused by type A influenza viruses similar to those that cause winter flu outbreaks in humans, although human infections with swine influenza viruses are rare. When these do occur, it is usually in people who have had direct contact with infected pigs, such as farm workers.
Human infections with swine influenza viruses do occur sporadically, and between 2005 and December 2023, 50 human cases of H1N2 were reported globally – although the December infection appears to be genetically unrelated to other recent human cases.
Transmission typically occurs when an infected pig coughs or sneezes and a human either breathes in some of their respiratory droplets or the droplets land in their mouth or nose.
Three subtypes of influenza A virus have been identified in pigs: N1N1, H1N2 and H3N2. They are different to human influenza viruses with the same names, because they are adapted to pigs' bodies, and infected pigs usually recover within seven to ten days.
The subtype identified in the British patient was H1N2.
What do we know about the British case so far?
The case was detected after the individual went to their doctor complaining of respiratory symptoms. The UK has a routine national surveillance system for respiratory viruses, and if someone visits their doctor complaining of flu-like symptoms, the doctor is encouraged to take a swab from them and send it off for analysis. In this case, the patient was offered a polymerase chain reaction (PCR) test, and the virus was further characterised using genome sequencing, which revealed that it was a swine H1N2 virus.
The patient, who lives in North Yorkshire, experienced a mild illness and has since recovered. The UK Health Security Agency (UKHSA) is currently investigating how they acquired the infection and whether there are any further associated cases. Close contacts of the patient are also being followed up. UKHSA says it is monitoring the situation closely and taking steps to increase surveillance within existing programmes.
Have you read?
Human infections with swine influenza viruses do occur sporadically, and between 2005 and December 2023, 50 human cases of H1N2 were reported globally – although the December infection appears to be genetically unrelated to other recent human cases. It is similar to viruses currently circulating in British pigs, though.
In March 2024, the US Centers for Disease Control and Prevention (CDC) reported another human infection with an H1N2 virus: a child living near a pig farm in Pennsylvania who had been in contact with pigs. They were hospitalised and subsequently recovered. This case is not thought to have been related to the British case.
Is this the same virus that triggered the swine flu pandemic in 2009/10?
The 2009 swine flu pandemic was caused by an influenza H1N1 virus and is the third known flu pandemic to have been caused by an N1N1 virus – the others being the 1918–20 Spanish flu pandemic, and the 1977 Russian flu pandemic.
The 2009 virus was known as H1N1(pdm09) and is thought to have arisen from a mixture of bird, pig and human viruses that further combined with a different pig H1N1 virus that was circulating in Europe at that time, leading to the term "swine flu". A total of 18,631 deaths were reported among laboratory-confirmed cases – although the true number of deaths is likely to have been much higher. The pandemic was declared over in August 2010.
Although this demonstrates the potential for swine influenza viruses to spill over into humans and cause serious illness, the recent infection detected in the UK was caused by a completely different virus, and there is nothing to suggest that it has pandemic potential at the current time.
The H1N1(pdm09) virus still circulates in humans and causes seasonal outbreaks of flu, but it is no longer referred to as swine flu and is distinct from the viruses currently circulating in pigs. To help protect people against severe illness caused by H1N1 influenza, the seasonal flu vaccine usually contains an H1N1(pdm09)-like virus.
What health risks does the latest H1N2 virus pose?
This is currently unclear, but the infected man was not particularly unwell and was not admitted to hospital. Contact tracing identified two further probable cases, both of whom experienced mild illness and quickly recovered. There have been no further reports of unexplained respiratory illness in large numbers of people in northern England or in British pigs. However, the fact that the man appears to have had no contact with pigs before he became ill raises the possibility that this virus is circulating more widely, even though there is no evidence of this yet.
The case highlights the importance of routine surveillance for respiratory viruses, and prompt investigation of apparent spillovers of animal viruses to humans.
Updated as of 5 July 2024
