Should we expect aftershocks with Covid? Some important research has found that once a flu pandemic is over, many ­nasty outbreaks follow for around 20 years, leading to even more deaths. Like earthquake aftershocks.

The study, led by researchers at the universities of Glasgow and Lancaster, used data from the 19th and 20th centuries to reveal the persistent health dangers of flu pandemics beyond the first infection period.

The main pandemic waves were always followed by multiple ­sizeable flu outbreaks over the following decade, in some cases doubling the initial death toll.

And for almost two decades after an influenza pandemic, there was a high risk any flu outbreak could cause a third of the deaths of the main wave.

Studying data from eight large cities in the UK following the 1918-19 global flu pandemic, the researchers revealed multiple recurrent influenza outbreaks from 1920 to 1929. They resulted in nearly as many deaths as the main waves and lasted almost two decades after a pandemic took hold.

Studying records from individual cities – Belfast, Birmingham, Cardiff, Glasgow, Liverpool, London, Manchester and Sheffield – the size of subsequent flu outbreaks was ­considerable with some of them reaching a mortality rate of around 500-1,000 per million. Even the three cities that had comparatively low main pandemic wave mortality (Liverpool, Glasgow and Birmingham) suffered very large ­recurrent outbreaks over the next two decades, some causing as many as around a third of the number of deaths as the main wave.

Rebecca Mancy, Research Fellow at Glasgow University, said: “Our research shows that the impacts of flu pandemics did not end once the main waves of infection subsided. Indeed, data from the UK and the US showed an unusually high risk of deaths from flu as much as two decades later. As a result, we believe omitting the effects from recurrent post-pandemic outbreaks substantially ­underestimates the impact of any one flu pandemic.”

The big question is, should we be on the lookout for these same patterns with Covid? “Whether the patterns seen for historical flu pandemics might apply for Covid-19 and other future pandemics depends on a number of factors,” Rebecca said. “We now have much more advanced public health systems and medicine, including the ability to rapidly develop vaccines, as well as institutional ­organisation, including test-and-trace systems – all factors that may impact the wider effects of a pandemic.”

Dr Spyridon Lazarakis, from Lancaster University, added: “We believe that these new findings suggest we should now view pandemics and their overall health impacts ­differently– looking beyond the initial main waves.”