Why were so few people infected with H7N9 influenza A viruses in China from late 2017 to 2018?

Rapid Evolution of H7N9 Highly Pathogenic Viruses that Emerged in China in 2017

(2018) Jianzhong Shi et al.   Cell Host & Microbe

Significantly elevated number of human infections with H7N9 virus in Jiangsu in eastern China, October 2016 to January 2017

(2017) Xiang Huo et al.   Eurosurveillance

Human infections with recently-emerging highly pathogenic H7N9 avian influenza virus in China

(2017) Fuchun Zhang et al.   JOURNAL OF INFECTION

Bayesian evolutionary analysis for emerging infectious disease: an exemplified application for H7N9 avian influenza viruses

(2017) Xiaoman Wei et al.   Science China-Life Sciences

CASCIRE surveillance network and work on avian influenza viruses

(2017) Yuhai Bi et al.   Science China-Life Sciences

Role of Poultry in the Spread of Novel H7N9 Influenza Virus in China

(2014) M. J. Pantin-Jackwood et al.   JOURNAL OF VIROLOGY

H7N9: a low pathogenic avian influenza A virus infecting humans

(2014) Jun Liu et al.   Current Opinion in Virology

H7N9 Influenza Viruses Are Transmissible in Ferrets by Respiratory Droplet

(2013) Q. Zhang et al.   SCIENCE