The disappearing act began as Covid-19 rolled in towards the end of our flu season in March. And just how swiftly rates have plummeted can be observed in 'surveillance' data collected by the World Health Organisation (WHO).

Patients aren't routinely tested for flu, even if it's suspected, but a number of 'sentinel' GP surgeries and hospitals do carry out diagnostic screening on those who have symptoms, and this data gives us the most accurate picture of how much flu is in circulation. And the figures provide a startling insight into what has become a creeping trend across the world.

In the Southern Hemisphere, where the flu season happens during our summer months, the WHO data suggests it never took off at all.

In Australia, just 14 positive flu cases were recorded in April, compared with 367 during the same month in 2019 - a 96 per cent drop. By June, usually the peak of its flu season, there were none. In fact, Australia has not reported a positive case to the WHO since July.

In Chile, just 12 cases of flu were detected between April and October. There were nearly 7,000 during the same period in 2019.

And in South Africa, surveillance tests picked up just two cases at the beginning of the season, which quickly dropped to zero over the following month - overall, a 99 per cent drop compared with the previous year.

In the UK, our flu season is only just beginning. But since Covid-19 began spreading in March, just 767 cases have been reported to the WHO compared with nearly 7,000 from March to October last year. And while lab-confirmed flu cases last year jumped by ten per cent between September and October, as a new season gets under way this year they've risen by just 0.7 per cent so far.

Of course, this isn't the total number of flu cases. We know from Office for National Statistics data that hundreds of people have been dying from suspected flu-related pneumonia every week throughout the year. Globally, it is estimated that rates of flu may have plunged by 98 per cent compared with the same time last year.

There are those who claim flu cases haven't vanished at all, but are instead being recorded as Covid-19. Skeptics say Covid tests are unable to distinguish between coronavirus and flu, but this is simply untrue.

Dr Elisabetta Groppelli, virologist and lecturer in global health at St George's, University of London, explains:

'Flu and Covid-19 are caused by very distinct viruses, and this is clear to see under a microscope. There's no chance of mistaking one for the other - the fragment of viral genetic material from the coronavirus looks like a bit of spaghetti, while the flu genetic material we test for looks like eight pieces of penne pasta.'

Another compelling explanation suggests the presence of SARS-CoV-2, the virus that causes Covid-19 and has run rampant throughout the world, has somehow 'crowded out' the flu virus. Viral interference may well have been the reason 2009's swine flu pandemic never took hold in the way many feared it would.

Public Health England studied samples taken from about 20,000 people during the first four months of this year, as coronavirus took hold, and found those who had flu were 58 per cent less likely to also have coronavirus. A Chinese study on two previous coronavirus outbreaks, SARS and MERS, has also shown the same effect.

Dr Ellen Foxman, who authored the Yale viral interference study, says:

"One virus can only disrupt the spread of another if enough people have them. When we're talking about common colds, the rates are astronomically high, and many people are asymptomatic. But for Covid, at present we think only 15 to 20 per cent of people in hard-hit places like New York have been exposed. Most places will be a lot lower than that. That's not enough for Covid to prevent flu by interference and certainly not enough to account for the huge drops in flu we've seen in the statistics. If coronavirus interfered with anything, it was our behaviour,"

Both viruses spread in the same way: through infected droplets. But people with Covid are thought to be more contagious. One measure of this is the much talked about reproduction, or R number - the number of people that one infected person will pass on a virus to, on average. Covid-19 has a reproduction number of about three, if no action is taken to stop it spreading. It means one person would be expected to give it to three others.

Some viruses are more contagious, for instance measles, which has an R number of roughly 15. Flu, on the other hand, has an R number of just over one. The incubation period for flu is also lower. After being infected with flu, it typically causes illness within two days, compared with five days on average for Covid-19.

The other question is whether we can actually trust the flu data at all.

• Ice Age Farmer Report: No Vax, No Food? UK Supermarkets may Require Vax Passport
• 'Vaccine passports by the back door!' UK PM admits coronavirus IDs will be needed for pubs & theatres
• Vaccine passports MUST be resisted as a solution to the Covid crisis; they'll simply encourage state-backed snooping
• Don't rely on 'vaccine passports' for travel, WHO's Ryan urges amid shortage of Covid-19 jabs
• UK foreign sec says Covid vaccine passports 'under consideration' just days after vaccine minister calls idea 'discriminatory'
• UK, US & Singapore push for vaccine passports and yearly Covid-19 vaccinations
• 'Health passports' to enter music venues are being trialled in the UK
• 'It would be discriminatory': UK vaccine minister says government is NOT planning Covid vaccine passport

• New bird flu strain H5N8 detected in humans, 7 poultry workers in Southern Russia were infected
• 31 dead in DR Congo bubonic plague outbreak
• Eight people test positive for Ebola in Guinea, first resurgence in the country since world's worst outbreak

• Severe flu epidemic predicted next winter after lockdowns block usual herd immunity
• Medieval plague outbreaks picked up speed over 300 years

• Compelling Evidence That SARS-CoV-2 Was Man-Made
• Right on cue for Biden, WHO admits high-cycle PCR tests produce COVID false positives
• WHO (finally) admits PCR tests create false positives

• 31 dead in DR Congo bubonic plague outbreak
• Bird flu hits largest poultry farm in Japan, record 1.6 million chickens to be culled
• Indonesia the latest country hit by African Swine Fever outbreak

• New Light on the Black Death: The Viral and Cosmic Connection
• Book Review: New Light on the Black Death by Mike Baillie