Quantitative changes cause qualitative changes, Xinghe explained in a single sentence why the vaccine was only effective against airborne viruses.
"Actually, I am surprised that you could develop a vaccine. It is key to the survival of many humans to this date. Without a sufficient population base, any talk of resisting or even counterattacking the Serpent People would be empty. It is precisely because you have developed the vaccine that Earthlings can retain hope. For this, I must express my respect to you," he said.
Xinghe bowed with hands folded in front, an ancient ritual, a Han dynasty etiquette.
The person in the white lab coat suddenly lost his speech. He was stunned for a moment, then wiped his face with his hand, and said, "I... It wasn't me, I... cough cough."
After clearing his throat, the man in the white coat looked around and then he spoke loudly, "Can I speak now? May I speak now?"