Cases of depression and anxiety surged by more than a quarter globally during the first year of the pandemic, especially among women and young adults, a major study showed on Saturday.

In the first worldwide estimate of the mental health impact of Covid-19, researchers estimated that 2020 saw an additional 52 million people suffer from major depressive disorder, and an additional 76 million cases of anxiety.

These represent a 28- and 26-percent increase in the two disorders respectively, according to the study, published in The Lancet medical journal. 

Covid-19 has claimed nearly 5 million lives since it emerged in late 2019, but experts say this is likely to be a vast underestimate. 

Friday's study showed that the hardest-hit countries were saddled with the greatest mental health burden, with a strong link between high Covid-19 case levels, restrictions on movement, and elevated rates of depression and anxiety.

"Our findings highlight an urgent need to strengthen mental health systems in order to address the growing burden of major depressive disorder and anxiety disorders worldwide," said lead study author Damian Santomauro, from the University of Queensland's School of Public Health.

"Meeting the added demand for mental health services due to COVID-19 will be challenging, but taking no action should not be an option."

Analysing data collected across North America, Europe and East Asia researchers modelled the expected prevalence of depression and anxiety. 

Had the pandemic not occurred, 193 million cases of depression would have been expected. This compared with an observed 246 million cases during 2020.

Similarly, for anxiety, models predicted 298 million cases of anxiety globally without Covid-19, when in fact the actual number of cases last year was 374 million. 

The analysis showed that women suffered disproportionately, largely because pandemic measures exacerbated existing health and social inequality in most nations. 

Additional caring and domestic duties still mainly fall to women, and women are far more likely to be victims of domestic violence, which surged during the pandemic. 

School and college closures restricted young people's ability to learn, interact with peers, and gain employment, leading to outsized mental health impacts among 20-24 year olds, the study showed. 

"The COVID-19 pandemic has exacerbated many existing inequalities, and social determinants of mental health disorders, and the underpinning mechanisms to improve mental health in the context of the COVID-19 pandemic globally," said Alize Ferrari, from the University of Queensland. 

"It is crucial that policymakers take underlying factors such as these into account as part of measures to strengthen mental health services."