I check trending words in Weblio every so often, and I always see コロナ禍. I checked the reading of the relatively rare kanji and it said わざわい which reminded me of 災い (which I believe was the kanji of 2004 and 2018). What's the difference between the two?
The common readings of 禍 are まが (as in 禍々しい) and カ. コロナ禍 is read ころなか, where this 禍 is a kind of suffix which cannot be replaced by 災い.
According to dictionaries, わざわい can be written both as 災い and 禍, but the latter is a little-known reading used almost exclusively in aesthetic writing (usually with furigana). Some novelists like to use rare kanji just to add flavor, but most people can safely stick to 災い when they write something.
Still, I personally feel a subtle difference in nuance between 災い and 禍. The kanji 禍 is often associated with supernatural ominous things like curse or demonic creatures (try an image search for 禍々しい). At least in fiction, 禍 may tend to be chosen to emphasize such "cursed", "dark" or "impure" atmosphere.
Similar examples include 哭く【なく】, 嗤う【わらう】 and 斃れる【たおれる】. These rare kanji do have subtle added nuance as compared to more mundane 泣く/笑う/倒れる, but most laypeople should not use them unless they want to be frowned upon for showing off their knowledge.
According to 新明解漢和辞典
災: 運が悪くてであうわざわい、自然のわざわいの意 「天ー」
禍: X福。ふしあわせ、思いがけのないわざわいの意 「奇ー」
災: unfortunate disaster, natural disaster
禍: opposite of 福. unhappiness, unexpected disaster
災い is for a natural disaster with no way of prevention.
禍 is for circumstances which can be prevented through artificial (human) effort