For decades, thousands of migrants from India, China, Africa and elsewhere have sought an escape from poverty as temporary migrant workers in wealthier countries.

Now, one of the largest migration corridors in the world has started to flow in reverse. The effects of the COVID-19 pandemic and plunging oil prices have led to wholesale layoffs in host countries, leaving foreign workers vulnerable and often destitute.

Short of money and fearing contracting the COVID-19 virus, hundreds of thousands have returned home.

With the timing of the arrival of a vaccine uncertain and COVID-19 cases rising across the globe, the chances of things returning to normal any time soon are remote.

At last count, there are 164 million migrant workers who cross borders, according to the International Labour Organisation (ILO).

About one-third of them come from South Asia, and India is the single largest source of migrant labour in the world.

About 2.5 million people from just one Indian state, Kerala, were working overseas last year. But the pandemic is driving them home in record numbers: As many as 500,000 migrants and their families are expected to return by the end of this year, the largest such exodus ever, experts say.

Nearly 400,000 people have already returned to the state from the Gulf States since May, according to the Kerala government, with at least half of them reporting they were coming home because they had lost their jobs.

And a survey of migrants from Central America and Mexico conducted by the International Organization for Migration (IOM) found that half had lost their jobs.

Also, overseas workers from places like Ukraine, Myanmar and Venezuela have also returned home in their millions.

Migrant workers are not only critical to the countries where they work but also a vital source of income back home, sending money to family back home as remittances.

Last year this flow of cash hit a record high of $US554 billion. This year remittances will fall by 20 per cent, according to the World Bank. This would be the sharpest decline on record.

For some countries, this will amount to an economic disaster. In places like Nepal, Honduras, Haiti, Lesotho and Tajikistan, remittances represent more than 20 percent of gross domestic product.

Even for a large economies like India, remittances last year, they were greater than the total of foreign investment in the country.

According to ILO data, nearly 87 per cent of migrant workers are of prime working age, between 25 and 64 years old.

Of the 164 million migrant workers worldwide, approximately 111.2 million (67.9 per cent) live in high-income countries, 30.5 million (18.6 per cent) in upper middle-income countries, 16.6 million (10.1 per cent) in lower middle-income countries and 5.6 million (3.4 per cent) in low-income countries.

Migrant workers constitute 18.5 per cent of the workforce of high-income countries, but only 1.4 to 2.2 per cent in lower-income countries.

From 2013 to 2017, the concentration of migrant workers in high-income countries fell from 74.7 to 67.9 per cent, while their share in upper middle-income countries increased.

Nearly 61 per cent of migrant workers are found in three sub-regions; 23.0 per cent in North America, 23.9 per cent in Northern, Southern and Western Europe and 13.9 per cent in the Arab countries.

Other regions that host large numbers of migrant workers – above 5 per cent – include Eastern Europe, Sub-Saharan Africa, South-Eastern Asia and the Pacific, and Central and Western Asia.

The countries from which the highest level of remittances are sent are: United Arab Emirates, $US18.9 billion; United States, $US14.3; Saudi Arabia, $US12.5; Kuwait, $US6.3; Oman, $US6.3; Qatar, $US4.6; United Kingdom, $US4.1; Canada, $US3.5; Australia, $US2.8.