anti-body precondition

mortality rate＝death rate 死亡者数、死亡率

Japan's low virus mortality rate reflects societal manners, Aso say.
mortality ﾓ タﾘﾃｨ n)死ぬ定め、大量の死
reflect ﾘﾌレｸﾄ v)反射する、反映する、表す
social manners=etiquette エﾃｨｹｯﾄ n)礼儀、作法

Bowing

Bowing instead of shaking hands?
Peoples opinion is that Japan's unique social ways
instead ｲﾝｽテｰﾄﾞ adv)その代わりに
instead of...   ...の代わりに

has something to do with...  ...と関係がある

A more plausible explanation has something to do with "prior immunity" or anti-body precondition held by some population before the arrival of covid-19.

Further inguiries will be needed while I'm opting for it.
A has something to do with B  AとBは関係がある
(somethingを省略して、A has to do with Bとも言う)
It has something to do with the weather. 天気に関係がある
My job has to do with the weather.
What's the Cold War have to do with the job market.
What's his test score have to do with his true/actual ability?
what's age have to do with social responsibility?

plausible ﾌﾟラ-ｼﾞﾌﾞﾙ adj)妥当な
explanation ｴｸｽﾌﾟﾗネｲｼｮﾝ n)説明
prior ﾌﾟラｲｱ- adj)前の、先の
immunity ｲﾑミュ-ﾆﾃｨ n)免疫、抗体
immune ｲﾑミュｰﾝ adj)免疫の、抗体の
immune system 免疫力
anti-body n)抗体、抗毒素
preconditon ﾌﾟﾚｺﾝデｨｼｮﾝ n)前提条件、必須条件
hold-held-held