How reliable is our data about flu?

[73Sunglasslover]

I am not a medic or scientist, so this may be a truly naive question - apologies if so! We keep hearing comparisons of Covid-19 to flu. Some of the data seems to be hampered by our not really knowing the true rate of Covid infections. But can't the same be said about flu? I was hospitalized with pneumonia a couple of years ago. They tested me and it was caused by flu. I had caught it from my OH. But he was not too poorly so he never counted in any formal figures. I've had flu twice before in my life but was not poorly enough to need medical help so those didn't count either. How can we be so sure about the case fatality rate for any virus which can show itself in milder and more severe forms?

I'm not a virologist, so happy to be corrected by someone with greater knowledge:

Flu is monitored across the world. That's how the annual vaccine is developed- by tracking what strains are active and where they are spreading throughout the world.

We do a lot of blood tests on people for other things, so it's not hard to estimate the number of cases needing hospitalisation versus a general frequency.

That does not mean we have perfect info on every strain- flu is really many different versions of several viruses.

There are probably strains of flu out there which are as bad as this particular coronavirus, but are sufficiently similar to other strains for many people to have immunity, so they don't spread as fast.

Don, do you think the case fatality rate for flu then, is the number of people who were sick enough to need medical care who died from it? So if we wanted to compare it with Covid, comparing to people hospitalised with it (which I think is what the UK stats are) is a good comparison?

or are you saying that when they do blood tests for other things, they also just to a sneaky test for flu? I would have thought we'd have to give permission for that? (but maybe some people have??)

I'm not expert enough to know how the monitoring works, but I've had blood tests come back with influenza antibodies when testing for something else.

The CFR is always just measured against confirmed cases. Death rates (which are different) aren't usually comparable to the situation now because normally most of the population won't get it, so even a 'normal' flu which was theoretically as bad as Covid 19 wouldn't be nearly as dangerous.