Not that convinced, but I don't mind adding it.

@@ -5,0 +6 @@
+<ke_pri>spec1</ke_pri>
@@ -8,0 +10 @@
+<re_pri>spec1</re_pri>

副作用, for comparison, is the main "side effect" entry, and is [ichi1,news1,nf06]

~0.5 ~ 2 posts/minute for 副反応. Maybe 1/3rd of 副作用
https://twitter.com/search?q="副反応"&src=typed_query&f=live

site:twitter.com "副作用" 470,000 googits <--- already [news1]
site:twitter.com "副反応" 577,000 googits <--- this entry

site:mainichi.jp "副反応" 3,060 <--- this entry
site:mainichi.jp "副作用" 4,870 [news1]

Suggest [stat1], well worth learning anyway.

I failed to recognize this term when getting a corona booster shot today in Shinjuku.　Ended up saying something embarrassing (so my judgement here is biased, if you have a better source for current statistics).  It was part of the quick Q&A with a nurse before you get the shot.  Objectively perhaps 1/3rd to 1/2 as common overall in current media vs. 副作用 as far as I can see.

Coronavirus vaccines seems to have wildly increased the commonness of this term since the ngram days(both in direct application and anti-vax movements). I imagine that contributed to the gloss improvements in 2021/2022.

@@ -13 +13 @@
-<gloss>side effect (from a vaccine)</gloss>
+<gloss>side effect (of a vaccine)</gloss>

GG5: 【化･医】 a side reaction.

Probably OK to split.

https://ja.wikipedia.org/wiki/副反応
https://dictionary.goo.ne.jp/word/副反応/
https://ja.wikipedia.org/wiki/副作用#副反応

In the medical sense, this carries the specific nuance of it being from a vaccine and not another kind of medical treatment.

@@ -11,0 +12,5 @@
+<field>&med;</field>
+<gloss>side effect (from a vaccine)</gloss>
+</sense>
+<sense>
+<pos>&n;</pos>
@@ -13 +17,0 @@
-<field>&med;</field>