Everything you need to know about H1N2 swine flu – Gavi, the Vaccine Alliance

A man in northern England has been infected with a flu virus related to one currently circulating in British pigs yet they had no known contact with these animals.

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.

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 that has been identified in the British patient is H1N2.

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.

Human infections with swine influenza viruses do occur sporadically, and since 2005, 50 human cases of H1N2 have reported globally although the latest infection appears to be genetically unrelated to other recent human cases. It is similar to viruses currently circulating in British pigs, though.

The 2009 swine flu pandemic was caused by an influenza a H1N1 virus and is the third known flu pandemic to have been caused by an N1N1 virus the others being the 191820 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.

This is currently unclear, but the infected man was not particularly unwell and was not admitted to hospital. Also reassuring is that there have been no 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 recent contact with pigs 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. Over the coming days and weeks, scientists will be working to understand more about how this infection occurred and how the genetic make-up of this virus relates to other influenza viruses.

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Everything you need to know about H1N2 swine flu - Gavi, the Vaccine Alliance