Brands closed off a difficult first quarter marked by a probe into the quality of its chicken by Chinese regulators and the avian flu scare, which conspired to push profits there down 40%.
The archaic technology for making them, which involves injecting influenza virus into chicken eggs and separating out the resulting antibodies, only allows for a few flu strains to be included and they must be picked far in advance.
Though it's unlikely to have an impact on China's chicken supply, the accident came as chicken producers were seeing sales recover after an outbreak of a deadly new strain of bird flu, H7N9, briefly scared the public in April and early May.