Due to the COVID-19 pandemic, a number of non-pharmaceutical interventions colloquially known as lockdowns (encompassing stay-at-home orders, curfews, quarantines, cordons sanitaires and similar societal restrictions) have been implemented in numerous countries and territories around the world. These restrictions were established to reduce the spread of SARS-CoV-2, the virus that causes COVID-19. By April 2020, about half of the world's population was under some form of lockdown, with more than 3.9 billion people in more than 90 countries or territories having been asked or ordered to stay at home by their governments. Although similar disease control measures have been used for hundreds of years, the scale seen in the 2020s is thought to be unprecedented.Research and case studies have shown that lockdowns are effective at reducing the spread of COVID-19, therefore flattening the curve. The World Health Organization's recommendation on curfews and lockdowns is that they should be short-term measures to reorganize, regroup, rebalance resources, and protect health workers who are exhausted. To achieve a balance between restrictions and normal life, the WHO recommends a response to the pandemic that consists of strict personal hygiene, effective contact tracing, and isolating when ill.Countries and territories around the world have enforced lockdowns of varying stringency. Some include total movement control while others have enforced restrictions based on time. In many cases, only essential businesses are allowed to remain open. Schools, universities and colleges have closed either on a nationwide or local basis in 63 countries, affecting approximately 47 percent of the world's student population.Beginning with the first lockdown in China's Hubei province in January 2020 and nationwide in Italy in March, lockdowns continued to be implemented in many countries throughout 2020 and 2021. On 24 March 2020, the entire 1.3 billion population of India was ordered to stay at home during its lockdown, making it the largest of the pandemic. The world's longest continuous lockdown lasting 234 days took place in Buenos Aires, Argentina during 2020, although as of October 2021 the city of Melbourne, Australia is expected to spend the most cumulative days in lockdown at 267 days over separate periods. Pandemic restrictions have had social and economic impacts, and have been met with protests in some territories.