This, say experts, not only makes a violent rebound of the disease likely but makes predicting which strain will hit us next and – crucially – deciding which vaccine to produce much more difficult.

Decision makers are also concerned that the current pressure on the flu virus could also see a more transmissible stain emerge. 

Dr John McCauley, director of the Worldwide Influenza Centre at the Francis Crick Institute – one of five WHO collaborating centres that constantly track pandemic flu viruses and helps to update influenza vaccines – said the current situation is unprecedented.

“I don’t think we’ve been in a position with so little flu in circulation for over a century,” he said. “What that means is overall in the population there’s less experience and therefore there’s less boosting of immunity.”

Dr McCauley added that identifying which strains this year’s vaccine must protect against has proved tricky, as the number of samples taken from flu strains has fallen 20 fold globally. “Because we’re looking at fewer samples, we don’t really know how representative they are,” he said.

A WHO panel on influenza meets twice a year to tweak the flu jab to make sure it protects against circulating strains. The latest gathering took place via a “very long teleconference” on Friday and details of the adjusted vaccine will be revealed later this month.  

The degree to which social distancing has suppressed flu and other infectious diseases has startled experts around the world. In a Twitter thread Nicola Oliver, a director and public health expert at Medical Intelligence, gathered data demonstrating the striking fall of flu. 

Over the past five years the number of specimens the Global Influenza Surveillance and Response System has collected – a proxy measure of the spread of flu worldwide – has ranged from roughly 15,000 to 40,000. In the past 12 months, this has plunged to below 1,000 (see chart above).

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