CANBERRA (BLOOMBERG) - Psychological fatigue with social distancing is emerging as a major challenge for curbing a pandemic now into its eight month. That's especially so among young adults who are less fearful of the coronavirus, and suffer greater economic and social costs when they stay home.

From Japan to Spain and the United States, infections among millennials and Generation Z are driving new waves of cases which don't seem to be abating despite re-imposed restrictions.

The worrying trend reflects that social distancing curbs are proving untenable over a long period, despite their initial efficacy in flattening the virus curve across the world earlier this year.

"They are the people who are most economically and socially affected with lockdowns, but who are least affected by the disease," said Peter Collignon, a professor of clinical medicine at the Australian National University Medical School in Canberra. "The problem we've got is people we most need to change their behaviour are the 20 and 30-year-olds."

The fact that younger people are less at risk for a severe Covid-19 infection or death has emboldened them to breach the rules as job losses mount.

Young adults are stepping out for reasons that range from commuting for work or care-giving, to visiting bars and nightclubs and even disturbing instances of Covid-19 parties to deliberately get infected.

This has caused public figures like Anthony Fauci, director of US National Institute of Allergy and Infectious Diseases, to ask the younger population to be more responsible and not be a "part of the propagation of a pandemic".

For governments facing aggressive resurgences and an effective vaccine still months away, there are few options besides the plea to stay home.

"How do you maintain behaviour in that group, when the consequences for them medically are much less than 70 or 80-years olds, yet the economic consequences are much higher for them?" Collignon said, "That's a dilemma that I don't know the answer to."