4.2 | Strengths and limitations of study

Our meta-analysis has several strengths. Compared with a similar meta-analysis19, 5 new articles were included in this paper. Each study sample size in this meta-analysis was larger than 100 participants and with high quality so the result can be more accurate than others with low quality and smaller sample size44. Furthermore, we followed the recommendations of the Cochrane Collaboration and PRISMA statement, including a priori protocol. Comprehensive assessment of the study quality was achieved by using GRADE approach.
As with all systematic reviews, we may have failed to identify some studies, especially those with negative results, therefore, this may have influenced our findings. Finally, the length of time that the early effective protection from BCG vaccination lasts remains unanswered. The age of participants in this review may partially explain the result of the protective effect of BCG vaccination. In Linehan’s study, it was shown that any benefits of BCG vaccine are likely to be transient.45 Therefore, a large proportion of the protection from BCG vaccination may not be attribute to a reduction in the risk of atopy. Nonetheless, it can be confirmed that BCG vaccination in early childhood does reduce the risk of developing asthma in early life.