  We revisit the importance of collective-mode fluctuations and gauge
invariance in the electromagnetic response of superconducting systems. In
particular, we show that order-parameter fluctuations, gapless or not, have no
contribution to the Meissner effect in both $s$- and $p$-wave superconductors.
More generally, we extend this result to uniform and nonuniform superfluids
with no external wavevector scale. To facilitate this analysis, we formulate a
path-integral-based matrix methodology for computing the electromagnetic
response of fermionic fluids in the presence of concomitantly fluctuating
collective modes. Closed-form expressions for the electromagnetic response in
different scenarios are provided, including the case of fluctuations of
electronic density and the phase and amplitude of the order parameter. All
microscopic symmetries and invariances are manifestly satisfied in our
formalism, and it can be straightforwardly extended to other scenarios.

Superconductivity;Strongly Correlated Electrons