Yea I was pretty rough for 4-5 days, pretty much recovered now but when I do anything exerting I can tell it's still there, seems to linger
It's always a bit of a shot in the dark isn't it, as low as 10% efficacy some years IIRC
What symptoms do they have? My other half has what I thought the same thing I had (probably this flu) but she's no better at all after 2 weeks. Keep telling her to go to the GP but she says it's just a cold. But she's normally better from a cold after 24-48 hours.