A new wave of mobile apps that help track exposure to the coronavirus is coming to US states ahead of the holidays as public health officials bet that recently introduced features from Apple Inc and Alphabet Inc's Google will significantly boost adoption and impact, according to Reuters.

Colorado, Maryland and District of Columbia launched exposure-notification apps with the new technology in the last month, garnering over 2.3 million users combined, according to their public health departments.

California, Washington and other states expect to follow in the next month, officials said.

Complementing human contact-tracers, the smartphone apps use Bluetooth signals to track when people are in close contact and anonymously alert users when a recent contact tests positive. They emerged as promising tools early in the pandemic, but technical shortcomings, privacy concerns and dismissive attitudes in the United States toward safety measures undercut their benefits.

The tide may turning as cold weather and lockdown fatigue threaten a global surge in cases.