Early estimates also suggest an attack rate of around 20 per cent. That compares to seasonal flu attack rates of between 5 and 15 per cent, although I stress that in recent years seasonal flu attack rates have been lower than that, at around 2 to 4 per cent.Early experience also points to its being an illness with relatively mild symptoms although, of course, we have to be very clear that even mild flu can be unpleasant and debilitating and that, in exceptional cases, flu can cause deaths.