Atanaskovic-Markovic et al found that cross-reactivity between cephalosporins and penicillins varied between 0.3 and 23.9%, being higher among penicillins and between first-generation and second-generation cephalosporins.
However, it has recently been shown that all penicillin allergic children can tolerate cefuroxime, presumably as it has a different side chain.
Cross-reactivity appears to be higher in immediate reactions, and when penicillins and cephalosporins are identical or similar in the R1 side chain, as happens with the first and second-generation cephalosporins.
Currently BNFc says to avoid cefalosporins if history of immediate penicillin hypersensitivity, but if use of cefalosporin is “essential” then can be used (but not cefalexin!).