We aren't doing shields on UNSC vessels in war time, full stop.
That being said, the UNSC will suffer massive casualties, that's just how humans are at this point in time; to offset this they have a few research items that boost ship construction time and can get a very powerful economy rolling.
i do have one issue about saying this is a "war time game" the infinity simply did not exist at war time, it was post human/covenant war. at that point the UNSC would have started to use the forerunner technologies they recovered during the war and after that they found on ONYX.
with that said i believe that after you research the Infinity you should be able to research something that allows capital ships and the heavy prowler to gain shields. i say the heavy prowler because in the HALO book "The Thursday War" it is mentioned by parangosky that she wanted to apply the shield tech to the "UNSC Port Stanley" which is a heavy prowler.
obviously giving every UNSC ship a shield would give them an unfair advantage over the covenant due to the fact the UNSC has massive firepower and superior planetary defense.
to sum up my argument, i believe it is lore friendly that large UNSC ships and some select small ships get shields. this is based off of things mentioned in the HALO book series, mainly the Kilo-Five trilogy.