Britain should brace for an early flu outbreak this winter because cases are already rising in other countries, a top expert has warned.
The UK's flu season tends to last from September to March but the virus does not usually heap pressure on the NHS until January.
Latest surveillance suggests cases have more than doubled in a fortnight but remain below levels seen from the same point of the 2019/2020 flu season. Health officials insist the virus is only circulating at 'very low' levels.
Dr John McCauley, one of the world's most eminent flu scientists who is based at the Francis Crick Institute in London, warned surges in Croatia, India and China suggest there could be an earlier outbreak in the UK and other countries.
The outbreak in Croatia may act as a 'canary in a coalmine' because it is proof flu is already present and ticking upwards in Europe.
Outbreaks in India and China demonstrate that flu remains in circulation in large populations, and could quickly be exported to other countries by travellers.
He told MailOnline: 'It’s over the next few weeks that we will be able to tell if what we are seeing now is sustained. If it is, we might expect an early flu season.'
The official surveillance scheme — published by the UKHSA — says flu cases remain at 'inter-seasonal' levels in Europe.
Health chiefs are offering flu vaccines to 35million over-50s, healthcare workers and those in at risk groups. It is the biggest flu inoculation drive in the history of the NHS, and is happening alongside the Covid booster roll out.
Experts fear flu could kill up to 60,000 Britons this year because of waning immunity from the virus, as many have not been exposed to it for at least 12 months.
There are also mounting fears that the NHS will be struck down at the same time by Covid, with colder weather and darker evenings leading to increased social contact indoors — where viruses find it easier to spread.
Health Secretary Sajid Javid was yesterday pictured getting his flu jab, as he called on Britons to get their flu and Covid vaccines.
Flu is a seasonal menace on the NHS, with outbreaks more likely at this time of year because colder weather forces more people indoors where the virus — like Covid — finds it easier to spread.
But it virtually disappeared last winter amid lockdowns aiming to control the spread of Covid.
The above graph shows how the rate of GP appointments for influenza-like illness, such as blocked noses and coughs, has risen recently. These are not confirmed cases of flu, but give an indication of the spread of the virus in the population
The above graph shows the proportion of swabs for flu that have picked up the virus in Croatia, India, China and the UK. Scientists warn that if outbreaks are sustained then the UK could be heading for an early flu season
The above graph shows flu vaccine uptake in each group since 2015. The dotted lines represent the aim for jab uptake during this year's flu season
Asked about Britain's flu situation Dr McCauley, who heads up the WorldWide Influenza Centre, warned the UK may need to brace for an early surge in cases.
He said: 'There is flu being picked up in some places, but whether it is spreading in some of these (e.g. Norway and the Netherlands) is not known for certain.
'On the other hand, Croatia has had flu activity over the last four weeks. I don't know if it will be sustained over the coming weeks.
'There has been flu in China from January — but at somewhat lower levels than two years ago.
'India has had flu as well — but it is difficult to know how widespread it is in such a large country.'
Professor Francois Balloux, a geneticist at University College London, said that it was possible for flu to surge earlier this year than in previous seasons.
'One argument that this could happen is that so few people have been exposed to the flu over the last year, that they do not have enough protection,' he told MailOnline.
'But, looking at the figures, we are still at a very inter-seasonal level of flu.
'It could pick up, and maybe there are flickers of it starting to grow, but I would not expect it to take off just yet.'
He said that restrictions on travel between countries may change the way flu circulates this season compared to previous years.
But he added that an outbreak in a European country with open international travel with the UK could help fuel a spike in cases here.
WHO data from its FluMart system — which monitors outbreaks globally — highlights a recent outbreak in Croatia, where 35 per cent of swabs tested positive in the third week of September (50 of 140 swabs picked up the virus).
This has now dropped to 17 per cent in the latest week (14 of 79), up to October 3, but it is unclear whether the downturn will be sustained.
India has seen around 10 per cent of its almost 3,000 flu tests pick up the virus every week since mid-August.
And data submitted by China suggests cases of flu there have been rising steadily since the end of August.
In Britain flu remains at very low levels with just 0.2 per cent of swabs for flu (14 of 9,052) detecting the virus in the week to October 3, the latest available.
The latest report from the UK Health Security Agency — which replaced Public Health England — shows flu cases are ticking up but remain at very low levels.
There were 10 flu infections detected in the last week of September, it said,
