As England prepares to ease coronavirus restrictions further, the messaging from ministers has changed. We have reached, it seems, a tipping point in the pandemic where rules will be replaced by personal decisions. The mantra now is about living with coronavirus, much as we do with seasonal flu.
The pandemic has invited countless comparisons between coronavirus and influenza and the diseases do have some features in common. Both are infectious, potentially lethal respiratory viruses. They can spread through aerosols, droplets and contaminated surfaces. And they share some of the same symptoms in the form of fever, cough, headaches and fatigue. In the winter ahead, one challenge the NHS faces is separating the Covid patients from the flu cases.
But there are striking differences between coronavirus and flu that matter for public health. Coronavirus spreads faster than influenza and can cause far more serious illness. The symptoms of coronavirus can take longer to show, and people tend to be infectious for longer, making them more prone to passing it on.
Seasonal influenza has been around long enough that previous infection and protection from vaccines bear down on cases and deaths. Analysis of previous influenza outbreaks suggests that the R value for seasonal flu – the number of people an infected person passes the virus on to – averages about 1.28. This means a group of four people with flu might pass the virus on to five more.
Coronavirus spreads more easily than that. For the Delta variant now surging around the world, R is estimated at about seven, so in the absence of vaccines and other interventions, a single case would infect on average seven others. As vaccination programmes push on and the virus continues to spread, immunity to coronavirus will drive R down, but how low is a moot point.
Coronavirus is more lethal than influenza, largely because vulnerability to the disease rockets in older people. Seasonal influenza killed an estimated 44,505 people in England during the three combined flu seasons from 2015-16 to 2017-18. That number died from Covid in England in the first nine weeks of 2021. A major difference is that the flu figure takes into account the protection of influenza vaccines, where 50% effectiveness is considered good.
Coronavirus vaccines should have a greater proportional impact on Covid deaths. So far, the most common Covid vaccines used in the UK – the Oxford/AstraZeneca and Pfizer/BioNTech shots – reduce the risk of hospitalisation from Covid by more than 90%. As a result, the vaccination programme has driven Covid deaths down substantially, to fewer than 20 a day in the past week.
But the vaccines do much more to prevent death than transmission of the virus, so cases of Covid are expected to rise for some time yet. The larger the epidemic grows, the more chance the virus has of finding vulnerable people who have not had their shots, or are not sufficiently protected by the vaccine. While the vaccines dramatically weaken the link between cases and deaths, they are unlikely to break it entirely.
The differences between coronavirus, influenza and the vaccines that target them can easily eclipse a more fundamental issue, though. Learning to live with coronavirus as we live with flu does not mean society can take it in its stride.
Every year, a comprehensive global surveillance network detects which variants of influenza are in circulation and most likely to pose a threat in the following season. That information determines which strains go into the annual flu vaccines that are then rolled out in established campaigns. Throughout the flu season, public health authorities issue case counts and if needed, update their advice on medical care.
There is no such global system for coronavirus yet. And despite the well-oiled machine that protects the world from flu, the death toll from the virus in a typical year is still substantial. Every winter, flu puts enormous stress on the NHS, at times pushing it to breaking point. For the health service, living with coronavirus means learning to endure a double wave each winter as coronavirus and flu arrive together.
• This article was amended on 6 July 2021. Seasonal influenza killed an estimated 44,505 people in England during the three combined flu seasons from 2015-16 to 2017-18, not “2015-16 to 2018-19” as an earlier version said.