UK human swine flu case: Why now, and what's next?

Earlier this week, UK health officials confirmed a person in North Yorkshire had been infected by a new genetic strain of flu that's similar to one pigs get.

The government agency in charge of protecting public health - the UKHSA - has now shared the full timeline of how events unfolded.

It begins on Thursday 9 November, with a trip to see a GP.

But the person seeking help will have caught it days before that.

It takes about one to four days for symptoms to emerge.

The patient went to their doctor complaining of "typical" flu symptoms, meaning they were probably feeling exhausted, with a temperature, cough and sore throat.

It is still a mystery how they caught it - they have since confirmed that they do not work with pigs or keep any as pets.

During the visit, the GP did a swab test. These are not routine in every surgery - only some practices do them and they are to help the UKHSA monitor population health rather than to diagnose individual patients.

The results help experts keep checks on what diseases are circulating and how well vaccines are doing against them. That sample was sent off and processed in the usual way, which takes about two weeks.

So, it was on the evening of Thursday 23 November - a fortnight after the GP visit - that experts in a UKHSA lab discovered it was a new virus.

Officials are calling it influenza A(H1N2)v rather than swine flu.

It is a type of flu - like the winter illness many people get vaccinated against - but it is a bit different.

It is almost identical to one circulating in pigs in the UK at the moment, rather than the type humans normally get.

Pigs can sometimes spread flu viruses to people, but that is rare. It is usually the other way round.

The UKHSA's chief scientific officer, Prof Isabel Oliver, told the media earlier this week: "We do know that occasionally we identify viruses like this one that are present in pigs, so often referred to as swine flu viruses. Occasionally there are cases detected in humans."

There has been one similar but not identical human "swine flu" case in the US this year, and about 50 cases reported globally since 2005.

According to experts, although it could take off in humans, this is unlikely. They are keeping a close watch to see what risk, if any, this latest case may pose to the wider population.

This first and - so far - only UK case was a mild one, although the person's symptoms were nasty enough for them to seek medical help.

They have since made a full recovery, but that fact alone does not really tell us anything about the transmission potential of the virus or the risk to vulnerable groups, says Dr Andrew Catchpole, a virus expert who works for a research organisation called hVIVO.

"Details on the age and the overall health status of the infected person have not been released, so we do not know if this was an otherwise healthy individual whom we would expect to recover from flu without intervention," he said.

As soon as UKHSA got the test results back, it contacted the individual concerned and their close contacts. Fortunately, the person had not been out at any big events, so did not have a vast number of contacts to trace.

These contacts were advised that they could potentially be infectious and should self-isolate pending a negative test result, the UKHSA told BBC News.

However, there is no sign yet that the virus has or will spread easily between people. Typically viruses acquired from pigs do not spread easily from person to person. But the UKHSA says it will continue to investigate.

Dr Will Welfare, UKHSA Incident Director, said: "This is the first time we have detected this virus in humans in the UK. As soon as we get such a result, we immediately start work with partners to learn everything we can about it. Our investigation is continuing at pace and we will publish further information in due course.

"It's thanks to our routine surveillance and genome sequencing that we have been able to detect this virus, putting us in a stronger position to understand its characteristics and reduce any potential spread."

Virus expert Dr Andrew Catchpole says pigs are a common source of newly emerging flu strains, because they can catch flu viruses from humans and birds which then mix and make new versions.

Generally, however, these are not able to replicate and transmit efficiently in people.

Prof John Edmunds, from the London School of Hygiene & Tropical Medicine, says it is right to be cautious: "Although these events usually result in viruses that struggle to transmit in their new hosts, there is always a danger that they can. These events need to be carefully investigated."

Prof Isabel Oliver said that while there may have been some limited spread between people, that is entirely different to a big outbreak.

"So far this is the only confirmed case. We are working to understand how this person acquired the infection... We have not identified a source of infection but our investigations continue," she said.

In 2009, there was a pandemic in humans caused by an influenza virus commonly referred to as "swine flu".

That was a different virus called influenza A H1N1(pdm09). It contained genetic material from viruses that were circulating in pigs, birds and humans in the 1990s and 2000s.

It is no longer called swine flu and is now one of the many different flu viruses which circulate in people each year. The current annual flu shots that are offered on the NHS protect against it.

The vaccines are not expected to provide much protection against the new virus that has just been found. New vaccines can be made though, if they are ever needed.