The first human case of swine flu strain H1N2 has been detected in the UK, health officials have said.
Routine surveillance in GP surgeries picked up the case after the person suffered a mild illness, the UK Health Security Agency (UKHSA) said, adding that contact tracing is now underway to prevent further spread of the virus. It is not known at this stage how transmissible the strain is or if there could be other cases in the UK.
It is also too early to say if the strain could have pandemic potential. The UKHSA has notified the World Health Organisation (WHO) of the latest case.
Swine flu is a respiratory disease and for most people it is mild. It comes on quickly and usually lasts for around a week, the NHS explained. It causes fever, tiredness, a cough and a sore throat.
Other symptoms can include a headache, aching muscles, chills, sneezing, a runny nose, loss of appetite, vomiting and diarrhoea. Some people are more likely to become seriously ill with swine flu, including pregnant women. According to the NHS, the risk of complications - including miscarriage and premature labour - is highest during the later stages of pregnancy.
People with swine flu can have symptoms that range from mild fever and cough to acute pneumonia and death. However, the WHO has said that nausea and sickness are not the most common in the H1N2 strain.
The WHO states: "Avian, swine and other zoonotic influenza infections in humans may cause disease ranging from mild upper respiratory infection (fever and cough) to rapid progression to severe pneumonia, acute respiratory distress syndrome, shock and even death.
"Gastrointestinal symptoms such as nausea, vomiting and diarrhea has been reported more frequently in A(H5N1) infection. Conjunctivitis has also been reported in influenza A(H7). Disease features such as the incubation period, severity of symptoms and clinical outcome varies by the virus causing infection but mainly manifests with respiratory symptoms."
There have been about 50 reported human cases worldwide of the H1N2 virus since 2005, none of them related genetically to this strain. The person involved is not known to have worked with pigs and has fully recovered.
The case was detected as part of routine national flu surveillance undertaken by UKHSA and the Royal College of GPs, which was in place even before the Covid-19 pandemic. The patient was tested by their GP in North Yorkshire after experiencing respiratory symptoms. The strain was identified via polymerase chain reaction (PCR) testing and genome sequencing.
The UKHSA said people with respiratory symptoms should continue to follow the existing guidance - avoiding contact with other people while suffering symptoms and taking particular care around vulnerable people and the elderly. It said it was "monitoring the situation closely and is taking steps to increase surveillance within existing programmes involving GP surgeries and hospitals in parts of North Yorkshire. To assist in the detection of cases and assessment of transmission, those people who are contacted and asked to test are encouraged to do so."
Meera Chand, incident director at the UKHSA, said: "It is thanks to routine flu surveillance and genome sequencing that we have been able to detect this virus. This is the first time we have detected this virus in humans in the UK, though it is very similar to viruses that have been detected in pigs.
"We are working rapidly to trace close contacts and reduce any potential spread. In accordance with established protocols, investigations are underway to learn how the individual acquired the infection and to assess whether there are any further associated cases."
Chief veterinary officer, Christine Middlemiss, said: "We know that some diseases of animals can be transferred to humans, which is why high standards of animal health, welfare and biosecurity are so important. Through our animal and human surveillance systems we work together to protect everyone.
"In this case we are providing specialist veterinary and scientific knowledge to support the UKHSA investigation. Pig-keepers must also report any suspicion of swine flu in their herds to their local vet immediately."
H1N1, H1N2 and H3N2 are major subtypes of swine flu A viruses in pigs and occasionally infect humans. Based on early information, the UKHSA said the strain detected in the UK differs from recent human cases of H1N2 elsewhere in the world, but is similar to viruses in UK pigs.
In 2009, there was a pandemic in humans caused by flu strain H1N1, commonly referred to as swine flu. This now circulates in humans seasonally. Ian Jones, professor of virology at the University of Reading, said: "It's very unlikely the single case of H1N2 swine flu reported today represents anything more than has been seen in the past.
"And although a single case may not be representative, the fact the individual concerned had a mild infection that resolved without hospitalisation is also in keeping with previous experience. We should remember that surveillance programmes can discover infections that might never have been noticed based on case numbers or severity, so a level of reasoned tolerance has to be applied. Not every new agent is a threat."
Paul Hunter, professor of medicine at the University of East Anglia, said: "Overall the evidence is that influenza A(H1N2) does not cause any more severe disease than other more commonly circulating types of influenza. Also person-to-person transmission does not appear to be very efficient and sustained person-to-person transmission has not been reported so far."