Letter to the editor: Importance of considering high-risk behaviours in COVID-19 vaccine effectiveness estimates with observational studies Arashiro, Takeshi and Arima, Yuzo and Kuramochi, Jin and Muraoka, Hirokazu and Sato, Akihiro and Chubachi, Kumi and Oba, Kunihiro and Yanai, Atsushi and Arioka, Hiroko and Uehara, Yuki and Ihara, Genei and Kato, Yasuyuki and Yanagisawa, Naoki and Nagura, Yoshito and Yanai, Hideki and Ueda, Akihiro and Numata, Akira and Kato, Hideaki and Oka, Hideaki and Nishida, Yusuke and Ooki, Takao and Nidaira, Yuki and Stucky, Ashley and Suzuki, Tadaki and Smith, Chris and Hibberd, Martin and Ariyoshi, Koya and Suzuki, Motoi, Eurosurveillance, 28, 2300034 (2023), https://doi.org/10.2807/1560-7917.ES.2023.28.4.2300034