Before revision:  For any TTI strategy, however, a major challenge arises from pre- and asymptomatic transmission as well as TTI-avoiders, which contribute to hidden , unnoticed infection chains.
After revision:  For any TTI strategy, however, a major challenge arises from pre- and asymptomatic transmission as well as TTI-avoiders, and undetected spreaders, who strongly contribute to hidden , unnoticed infection chains.