(CNN)Political leaders are promising that mass vaccination campaigns will see life return to normal, as a more contagious variant of the coronavirus spreads across dozens of countries, adding urgency to the race to end the pandemic.

But vaccination efforts are rolling out slower than promised, raising doubts about an imminent way out of the crisis.

In nations where vaccines have been authorized, criticism is mounting over shortages and delays to getting shots into people's arms. And solutions devised to stretch supplies -- from administering half doses to spreading out the time between two doses -- have raised concerns from some experts.

Some governments are pointing the finger at manufacturers for bottlenecks, while vaccine developers say it's an issue of supply. Others cite complications with distribution plans and a lack of trained staff to administer shots. But most public health experts say the slow pace is the inevitable consequence of creating a new vaccine rollout in real time.

Just weeks after the United Kingdom became the world's first nation to begin vaccinating its citizens with a fully vetted and authorized Covid-19 shot, its government is facing questions over how many doses are available.