Table 2. Risk of bias assessment of the included cohort studies.

Author, year	Selection				Comparability	Outcome			Total score
	Exposed cohort	Non-exposed cohort	Ascertainment of exposure	Outcome of interest		Assessment of outcome	Length of follow-up	Adequacy of follow up
Okada et al., 2006	☆	☆	☆	☆	☆	☆	☆	–	7
Jiang et al., 2013	☆	☆	☆	☆	☆	☆	☆	☆	8
Maniwa et al., 2013	☆	☆	☆	☆	☆	☆	☆	–	7
Shapiro et al., 2013	☆	☆	☆	☆	☆☆	☆	–	–	7
Adachi et al., 2017	☆	☆	☆	☆	☆☆	☆	☆	–	8

Risk of bias was evaluated with use of the Newcastle-Ottawa Scale. A score of 7 or higher indicates a low risk of bias.