Covid-19 and flu comparisons reappraised

8 July 2021

“We are going to have to learn to accept the existence of Covid and find ways to cope with it - just as we already do with flu”

As England looks to relax all Covid restrictions on 19 July, the new health secretary, Sajid Javid, wrote in the Mail on Sunday that we will have to learn to live with Covid as we do with the flu. 

Throughout the pandemic, comparisons between the flu and Covid have been common and controversial, and this was no exception. 

While Mr Javid didn’t say Covid itself was like the flu, drawing any comparison was deemed unacceptable by various prominent scientists. 

Falsely equating the flu and Covid has been a real and harmful trope of misinformation during the pandemic. 

Last year some who sought to argue governments across the world had been overzealous in their approach to public health restrictions often falsely suggested that the two diseases are equally deadly.

Some who believe the government is playing a nefarious conspiratorial role go as far to falsely suggest Covid is just a rebranded version of flu. That they aren’t just alike, but one and the same.

But it’s also misleading to suggest that, following the mass roll-out of vaccinations in the UK and new treatments, Covid holds the same threat for the country now as the disease which took root in early 2020. 

At the start of the pandemic we looked at how the two diseases compared in terms of spread and the risk of hospitalisation or death they posed. We’ve gone back to see whether the picture has changed.

Transmissibility

It’s clear that, in the absence of any protections, Covid is more contagious than the flu, but it remains unclear what the transmission potential is once vaccines have been fully rolled out. 

A 2014 study reviewed papers which estimated the reproduction number during various flu outbreaks over the past century, as well as for seasonal flu. 

The reproduction (R) number is the average number of people an infectious case will spread the disease to. If the R number is above 1, you would see the disease outbreak increasing, while below 1, it would decrease. 

The average R number for seasonal influenza was 1.28, while for more serious outbreaks like the 1918 pandemic (caused by what is commonly referred to as Spanish flu) and the 1968 pandemic (caused by what is commonly referred to as Hong Kong flu), the R number was more like 1.8. 

Ignoring natural and vaccine immunity, the R number for the Alpha (Kent) variant of coronavirus, without any restrictions, is estimated to be around 3 but modellers at Imperial College estimate the Delta (India) variant could be far more transmissible

The current R number for Covid in the UK is estimated to be around 1.1 to 1.3. So on a similar level to seasonal flu. But, of course, under quite different circumstances.

For one thing, we are seeing similar levels of transmission to seasonal flu, but with far more restrictions on public movement and activity, and with a significantly-progressed vaccine roll-out. (We also vaccinate against the flu, but typically only those at risk, rather than the entire adult population.)

Also, this is the level of Covid transmission in summer. Seasonal flu reproduction estimates are calculated in the cooler months when the virus spreads more easily, probably because of a number of causes such as people spending more time indoors, less ventilation, and factors related to humidity.  

As Covid and the flu spread in similar ways (aerosol and droplets being exhaled from an infected person) there’s good reason to believe Covid transmission might be higher in the winter and if restrictions are eased.

But while there are factors meaning Covid transmission might increase, there are also reasons it might fall.

Most importantly, the vaccine roll-out is not complete, and the transmission risk might fall further as it continues, given the wealth of evidence that the vaccines reduce transmission.

So it’s hard to make a direct comparison between the transmissibility of Covid, once the vaccine has been fully rolled out, and the flu.

Prognosis

Without vaccination, the risk of dying once infected with Covid-19 is estimated to be somewhere between 0.5% to 1%, on average. 

With full vaccination, Public Health England estimates the risk of hospitalisation falls by over 90%. Data from Israel suggests similar efficacy for Pfizer’s vaccine against the new Delta variant. 

There is limited data on the extent to which the vaccines prevent against death specifically from new variants, but if the vaccine protects against hospitalisation it is expected to protect against death. 

The Medical Research Council (MRC) Biostatistics Unit at the University of Cambridge estimates that, in the UK, the fatality rate of Covid is currently around 0.085%.

By comparison, it’s quite hard to estimate the fatality rate for flu, largely because it is rarely reported (for example, estimates of deaths from the flu rely on mathematical models rather than death certificate data).

Data from New Zealand (which vaccinates a similar proportion of its elderly against the flu to the UK) suggests flu’s fatality rate is around 0.04%.

Data from the US Centers for Disease Control and Prevention show that around 0.1%-0.2% of symptomatic flu cases result in death. As a number of flu infections are asymptomatic (one study from the UK estimated 23% of flu infections were symptomatic) the overall fatality rate is likely to be lower.

So the risk of death from Covid at the moment could well be above that of flu, even with vaccination, with some exceptions (there is some evidence that the flu is more harmful to young children than Covid). 

As mentioned, if Covid’s transmissibility still exceeds that of the flu, then it may cause yet more damage by infecting more people more often.

And death is not the only concerning outcome of Covid. Covid can cause long term side effects. So can the flu, but it is hard to draw comparisons between the two, given the data on the subject is so limited. 

Beyond epidemiology

Whatever the difference in terms of overall risk posed by Covid and the flu, that doesn’t solve the question of how we “should” live with Covid, which is ultimately a political decision. 

While Covid may not be the national threat it was, at an individual level, for those individuals who have not been vaccinated, perhaps because they are not be able to receive the vaccine, or for whom the vaccine has a higher chance of not working (like immunosuppressed people), Covid-19 presents more of a threat than the flu ever would.

Also, over the winter, the cases of flu, which has killed tens of thousands of people in recent years, were incredibly low, in part driven by the restrictions we employed to reduce the risk of Covid. So there may be some advantages to treating flu like Covid in the future, though whether those advantages outweigh any negatives is, again, a political decision. 

Finally, there is the matter of what the future holds. Coronaviruses tend to mutate less frequently than influenza. But if a mutation occurs that protects the virus against antibodies, thus undermining vaccination, then the way in which the two diseases compare might shift once again.

Full Fact fights bad information

Bad information ruins lives. It promotes hate, damages people’s health, and hurts democracy. You deserve better.