I think this boils down to what exactly you mean by 'feeble'. The primary meaning in Collins Dictionary is 'lacking in physical and mental strength'.
Now, just because someone lacks 'strength' (i.e. they are not muscular and able to lift heavy weights), does not, I would suggest, render them any more at risk of flu-related complications than anyone else.
I am not a medical person, so please don't place any reliance medically on this, but it could be that if the feebleness is caused by some other chronic condition such as diabetes, or is the result of old-age, then it may well make them more liable to flu-related conditions. Everyone over the age of 60 in Britain is entitled to a free anti-flu vaccination once per year, and is encouraged to have it. So it does seem that older people are at risk. But I know plenty over 60 who are certainly not 'feeble'.
So I would tend to rephrase the sentence as;
'Elderly or infirm people are at more risk of flu-related complications.'
'Weak' has exactly the same problems as 'feeble'.