Acknowledgments
We greatly appreciate Youyu He (Shanghai Center for Bioinformation Technology) in helping us to blast the insertion sequences against the viral sequence database.

Disclosure statement
No potential conflict of interest was reported by the author(s).

ORCID
Xiaojun Li http://orcid.org/0000-0002-5780-0880
Feng Gao http://orcid.org/0000-0001-8903-0203