Conflict of interest: BJC reports honoraria from Sanofi Pasteur and Roche not related to this work. The authors report no other potential conflicts of interest.
Authors’ contributions: Conceived study: PW, BJC, GML. Collected data: PW, XH, EHYL, KSML, QX. Analysed data: PW, XH, EHYL, BJC. Interpreted results: PW, EHYL, JYW, BJC, GML. Wrote the first draft: PW. Critically revised the draft and approved the final version: all authors.