RSV（呼吸器合胞体ウイルス）については、最近「妊婦ワクチンで乳児を守ろう」という議論が広がっています。しかし、この問題を考える際には、RSV感染の疫学的特徴を冷静に整理する必要があります。

まず重要なのは、RSV入院の年齢分布です。RSVによる細気管支炎や肺炎で入院する乳児は、生後6か月未満に多いことが知られていますが、実際には入院のピークは生後1〜2か月頃に存在するということです[3]。つまり、出生後まもない時期が最もリスクの高い時期ということです。

ここで次に重要になるのが、感染経路です。

小児感染症の疫学研究では、乳児のRSV感染の多くが家庭内感染によって起こることが示されています。特に、保育園や学校に通う兄姉から乳児へ感染が持ち込まれるケースが多いと報告されています [27,28]。

実際、ある研究では乳児のRSV感染の約半数以上で家庭内の年長児が感染源である可能性が示されています[28]。

つまり、RSV感染の実態を単純化すると、

・感染源の多くは家庭内（特に兄姉）
・最も危険な時期は生後1〜2か月
・ほとんどの子どもは2歳までに感染する

という構造になります[18]。

この疫学構造を踏まえると、RSV対策としてどのような介入が合理的かという問題が出てきます。

妊婦RSVワクチンは、母体に接種することで抗体を胎盤経由で胎児に移行させ、出生後の乳児を守るという仕組みです。第3相臨床試験では、出生後90日以内の重症RSV下気道感染症に対して約80％の有効性が報告されています[5]。

しかし、この効果は主に出生後数か月に限定されることが知られており、RSV感染そのものを防ぐワクチンではありません。

ここで考えるべき疑問があります。もし乳児のRSV感染の多くが家庭内の兄姉から持ち込まれる感染であるならば、対策の焦点は本来、

・兄姉の感染対策
・家庭内での感染予防
・流行期の接触管理

といった家庭内感染のコントロールにこそ向けられるべきではないでしょうか。

もちろん、妊婦ワクチンは乳児期早期の重症化リスクを一定程度低下させる可能性がある介入です。しかしRSV感染の自然史、感染経路、重症化リスクの分布などを総合的に考えると、「すべての妊婦に一律に接種すべき医療介入なのか」という点については、依然として議論の余地があると言えます。

感染症対策は、単一の医療介入だけで決まるものではありません。RSVの疫学構造を理解すれば、ワクチンだけでなく、家庭内感染という最も重要な感染経路をどう管理するかという視点も同時に考える必要があるでしょう。

参考文献⇩

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